Subjects -> ASTRONOMY (Total: 94 journals)
Showing 1 - 46 of 46 Journals sorted alphabetically
Advances in Astronomy     Open Access   (Followers: 51)
Annual Review of Astronomy and Astrophysics     Full-text available via subscription   (Followers: 39)
Annual Review of Earth and Planetary Sciences     Full-text available via subscription   (Followers: 63)
Artificial Satellites     Open Access   (Followers: 23)
Astrobiology     Hybrid Journal   (Followers: 14)
Astronomical & Astrophysical Transactions: The Journal of the Eurasian Astronomical Society     Hybrid Journal   (Followers: 6)
Astronomical Journal     Full-text available via subscription   (Followers: 8)
Astronomical Review     Open Access   (Followers: 4)
Astronomische Nachrichten     Hybrid Journal   (Followers: 4)
Astronomy & Geophysics     Hybrid Journal   (Followers: 48)
Astronomy and Astrophysics     Full-text available via subscription   (Followers: 60)
Astronomy and Astrophysics     Open Access   (Followers: 32)
Astronomy and Computing     Hybrid Journal   (Followers: 2)
Astronomy Letters     Hybrid Journal   (Followers: 22)
Astronomy Reports     Hybrid Journal   (Followers: 15)
Astronomy Studies Development     Open Access   (Followers: 12)
Astroparticle Physics     Hybrid Journal   (Followers: 8)
Astrophysical Bulletin     Hybrid Journal   (Followers: 3)
Astrophysical Journal     Full-text available via subscription   (Followers: 19)
Astrophysical Journal Letters     Full-text available via subscription   (Followers: 14)
Astrophysical Journal Supplement Series     Full-text available via subscription   (Followers: 14)
Astrophysics     Hybrid Journal   (Followers: 29)
Astrophysics and Space Science     Hybrid Journal   (Followers: 46)
Astrophysics and Space Sciences Transactions (ASTRA)     Open Access   (Followers: 56)
Astropolitics: The International Journal of Space Politics & Policy     Hybrid Journal   (Followers: 12)
Celestial Mechanics and Dynamical Astronomy     Hybrid Journal   (Followers: 11)
Chinese Astronomy and Astrophysics     Full-text available via subscription   (Followers: 24)
Colloid Journal     Hybrid Journal   (Followers: 3)
Comptes Rendus Physique     Full-text available via subscription   (Followers: 2)
Computational Astrophysics and Cosmology     Open Access   (Followers: 3)
COSPAR Colloquia Series     Full-text available via subscription   (Followers: 11)
Earth, Moon, and Planets     Hybrid Journal   (Followers: 55)
Earth, Planets and Space     Open Access   (Followers: 74)
EAS Publications Series     Full-text available via subscription   (Followers: 8)
EPL Europhysics Letters     Partially Free   (Followers: 8)
Experimental Astronomy     Hybrid Journal   (Followers: 39)
Expert Opinion on Astronomy and Astrophysics     Open Access   (Followers: 7)
Extreme Life, Biospeology & Astrobiology - International Journal of the Bioflux Society     Full-text available via subscription   (Followers: 6)
Few-Body Systems     Hybrid Journal   (Followers: 1)
Foundations of Physics     Hybrid Journal   (Followers: 41)
Frontiers in Astronomy and Space Sciences     Open Access   (Followers: 12)
Galaxies     Open Access   (Followers: 6)
Globe, The     Full-text available via subscription   (Followers: 4)
Gravitation and Cosmology     Hybrid Journal   (Followers: 4)
Icarus     Hybrid Journal   (Followers: 75)
International Journal of Advanced Astronomy     Open Access   (Followers: 28)
International Journal of Astrobiology     Hybrid Journal   (Followers: 4)
International Journal of Astronomy     Open Access   (Followers: 19)
International Journal of Astronomy and Astrophysics     Open Access   (Followers: 29)
International Journal of Satellite Communications Policy and Management     Hybrid Journal   (Followers: 13)
International Letters of Chemistry, Physics and Astronomy     Open Access   (Followers: 12)
ISRN Astronomy and Astrophysics     Open Access   (Followers: 7)
Journal for the History of Astronomy     Full-text available via subscription   (Followers: 19)
Journal of Astrobiology & Outreach     Open Access   (Followers: 3)
Journal of Astronomical Instrumentation     Open Access   (Followers: 3)
Journal of Astronomical Telescopes, Instruments, and Systems     Hybrid Journal   (Followers: 5)
Journal of Astrophysics     Open Access   (Followers: 26)
Journal of Astrophysics and Astronomy     Open Access   (Followers: 52)
Journal of Atmospheric and Solar-Terrestrial Physics     Hybrid Journal   (Followers: 199)
Journal of Cosmology and Astroparticle Physics     Hybrid Journal   (Followers: 38)
Journal of Geophysical Research : Planets     Full-text available via subscription   (Followers: 178)
Journal of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 178)
Journal of High Energy Astrophysics     Full-text available via subscription   (Followers: 22)
Kinematics and Physics of Celestial Bodies     Hybrid Journal   (Followers: 10)
KronoScope     Hybrid Journal   (Followers: 1)
Macalester Journal of Physics and Astronomy     Open Access   (Followers: 4)
MNASSA : Monthly Notes of the Astronomical Society of South Africa     Full-text available via subscription   (Followers: 1)
Molecular Astrophysics     Full-text available via subscription   (Followers: 1)
Monthly Notices of the Royal Astronomical Society     Hybrid Journal   (Followers: 14)
Monthly Notices of the Royal Astronomical Society : Letters     Hybrid Journal  
Nature Astronomy     Hybrid Journal   (Followers: 8)
New Astronomy     Hybrid Journal   (Followers: 27)
New Astronomy Reviews     Full-text available via subscription   (Followers: 17)
Nonlinear Dynamics     Hybrid Journal   (Followers: 19)
NRIAG Journal of Astronomy and Geophysics     Open Access   (Followers: 5)
Open Astronomy     Open Access   (Followers: 2)
Physics of the Dark Universe     Open Access   (Followers: 4)
Planetary and Space Science     Hybrid Journal   (Followers: 101)
Planetary Science     Open Access   (Followers: 52)
Proceedings of the International Astronomical Union     Full-text available via subscription   (Followers: 2)
Publications of the Astronomical Society of Australia     Hybrid Journal   (Followers: 2)
Publications of the Astronomical Society of Japan     Hybrid Journal   (Followers: 3)
Publications of the Astronomical Society of the Pacific     Full-text available via subscription   (Followers: 4)
Research & Reviews : Journal of Space Science & Technology     Full-text available via subscription   (Followers: 17)
Research in Astronomy and Astrophysics     Full-text available via subscription   (Followers: 29)
Revista Mexicana de Astronomía y Astrofísica     Open Access   (Followers: 2)
Science China Physics, Mechanics & Astronomy     Hybrid Journal   (Followers: 4)
Solar Physics     Hybrid Journal   (Followers: 34)
Solar System Research     Hybrid Journal   (Followers: 14)
Space Science International     Open Access   (Followers: 192)
Space Science Reviews     Hybrid Journal   (Followers: 97)
Space Weather     Full-text available via subscription   (Followers: 24)
Transport and Aerospace Engineering     Open Access   (Followers: 13)
Universe     Open Access   (Followers: 5)
Similar Journals
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Number of Followers: 5  

  This is an Open Access Journal Open Access journal
ISSN (Print) 2218-1997
Published by MDPI Homepage  [233 journals]
  • Universe, Vol. 7, Pages 7: Neutrino Oscillations in Neutrino-Dominated
           Accretion Around Rotating Black Holes

    • Authors: Juan David Uribe, Eduar Antonio Becerra-Vergara, Jorge Armando Rueda
      First page: 7
      Abstract: In the binary-driven hypernova model of long gamma-ray bursts, a carbon–oxygen star explodes as a supernova in the presence of a neutron star binary companion in close orbit. Hypercritical (i.e., highly super-Eddington) accretion of the ejecta matter onto the neutron star sets in, making it reach the critical mass with consequent formation of a Kerr black hole. We have recently shown that, during the accretion process onto the neutron star, fast neutrino flavor oscillations occur. Numerical simulations of the above system show that a part of the ejecta stays bound to the newborn Kerr black hole, leading to a new process of hypercritical accretion. We address herein, also for this phase of the binary-driven hypernova, the occurrence of neutrino flavor oscillations given the extreme conditions of high density (up to 1012 g cm−3) and temperatures (up to tens of MeV) inside this disk. We estimate the behavior of the electronic and non-electronic neutrino content within the two-flavor formalism (νeνx) under the action of neutrino collective effects by neutrino self-interactions. We find that in the case of inverted mass hierarchy, neutrino oscillations inside the disk have frequencies between ∼(105–109) s−1, leading the disk to achieve flavor equipartition. This implies that the energy deposition rate by neutrino annihilation (ν+ν¯→e−+e+) in the vicinity of the Kerr black hole is smaller than previous estimates in the literature not accounting for flavor oscillations inside the disk. The exact value of the reduction factor depends on the νe and νx optical depths but it can be as high as ∼5. The results of this work are a first step toward the analysis of neutrino oscillations in a novel astrophysical context, and as such, deserve further attention.
      Citation: Universe
      PubDate: 2021-01-04
      DOI: 10.3390/universe7010007
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 8: Statistical Estimates of the Pulsar Glitch

    • Authors: Alessandro Montoli, Marco Antonelli, Brynmor Haskell, Pierre Pizzochero
      First page: 8
      Abstract: A common way to calculate the glitch activity of a pulsar is an ordinary linear regression of the observed cumulative glitch history. This method however is likely to underestimate the errors on the activity, as it implicitly assumes a (long-term) linear dependence between glitch sizes and waiting times, as well as equal variance, i.e., homoscedasticity, in the fit residuals, both assumptions that are not well justified from pulsar data. In this paper, we review the extrapolation of the glitch activity parameter and explore two alternatives: the relaxation of the homoscedasticity hypothesis in the linear fit and the use of the bootstrap technique. We find a larger uncertainty in the activity with respect to that obtained by ordinary linear regression, especially for those objects in which it can be significantly affected by a single glitch. We discuss how this affects the theoretical upper bound on the moment of inertia associated with the region of a neutron star containing the superfluid reservoir of angular momentum released in a stationary sequence of glitches. We find that this upper bound is less tight if one considers the uncertainty on the activity estimated with the bootstrap method and allows for models in which the superfluid reservoir is entirely in the crust.
      Citation: Universe
      PubDate: 2021-01-05
      DOI: 10.3390/universe7010008
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 9: Gravitational Waves in
           Scalar–Tensor–Vector Gravity Theory

    • Authors: Yunqi Liu, Wei-Liang Qian, Yungui Gong, Bin Wang
      First page: 9
      Abstract: In this paper, we study the properties of gravitational waves in the scalar–tensor–vector gravity theory. The polarizations of the gravitational waves are investigated by analyzing the relative motion of the test particles. It is found that the interaction between the matter and vector field in the theory leads to two additional transverse polarization modes. By making use of the polarization content, the stress-energy pseudo-tensor is calculated by employing the perturbed equation method. Additionally, the relaxed field equation for the modified gravity in question is derived by using the Landau–Lifshitz formalism suitable to systems with non-negligible self-gravity.
      Citation: Universe
      PubDate: 2021-01-07
      DOI: 10.3390/universe7010009
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 10: New Tools for the Optimized Follow-Up of
           Imminent Impactors

    • Authors: Maddalena Mochi, Giacomo Tommei
      First page: 10
      Abstract: The solar system is populated with, other than planets, a wide variety of minor bodies, the majority of which are represented by asteroids. Most of their orbits are comprised of those between Mars and Jupiter, thus forming a population named Main Belt. However, some asteroids can run on trajectories that come close to, or even intersect, the orbit of the Earth. These objects are known as Near Earth Asteroids (NEAs) or Near Earth Objects (NEOs) and may entail a risk of collision with our planet. Predicting the occurrence of such collisions as early as possible is the task of Impact Monitoring (IM). Dedicated algorithms are in charge of orbit determination and risk assessment for any detected NEO, but their efficiency is limited in cases in which the object has been observed for a short period of time, as is the case with newly discovered asteroids and, more worryingly, imminent impactors: objects due to hit the Earth, detected only a few days or hours in advance of impacts. This timespan might be too short to take any effective safety countermeasure. For this reason, a necessary improvement of current observation capabilities is underway through the construction of dedicated telescopes, e.g., the NEO Survey Telescope (NEOSTEL), also known as “Fly-Eye”. Thanks to these developments, the number of discovered NEOs and, consequently, imminent impactors detected per year, is expected to increase, thus requiring an improvement of the methods and algorithms used to handle such cases. In this paper we present two new tools, based on the Admissible Region (AR) concept, dedicated to the observers, aiming to facilitate the planning of follow-up observations of NEOs by rapidly assessing the possibility of them being imminent impactors and the remaining visibility time from any given station.
      Citation: Universe
      PubDate: 2021-01-07
      DOI: 10.3390/universe7010010
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 11: Gravitation in Unified Scalar Field Theory

    • Authors: Alexander A. Chernitskii
      First page: 11
      Abstract: The scalar field of space-time film is considered as unified fundamental field. The field model under consideration is the space-time generalization of the model for a two-dimensional thin film. The force and metrical interactions between solitons are considered. These interactions correspond to the electromagnetic and gravitational interactions respectively. The metrical interaction and its correspondence to the gravitational one are considered in detail. The practical applications of this approach are briefly discussed.
      Citation: Universe
      PubDate: 2021-01-09
      DOI: 10.3390/universe7010011
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 12: Closed Timelike Curves, Singularities and
           Causality: A Survey from Gödel to Chronological Protection

    • Authors: Jean-Pierre Luminet
      First page: 12
      Abstract: I give a historical survey of the discussions about the existence of closed timelike curves in general relativistic models of the universe, opening the physical possibility of time travel in the past, as first recognized by K. Gödel in his rotating universe model of 1949. I emphasize that journeying into the past is intimately linked to spacetime models devoid of timelike singularities. Since such singularities arise as an inevitable consequence of the equations of general relativity given physically reasonable assumptions, time travel in the past becomes possible only when one or another of these assumptions is violated. It is the case with wormhole-type solutions. S. Hawking and other authors have tried to “save” the paradoxical consequences of time travel in the past by advocating physical mechanisms of chronological protection; however, such mechanisms remain presently unknown, even when quantum fluctuations near horizons are taken into account. I close the survey by a brief and pedestrian discussion of Causal Dynamical Triangulations, an approach to quantum gravity in which causality plays a seminal role.
      Citation: Universe
      PubDate: 2021-01-12
      DOI: 10.3390/universe7010012
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 13: Gravitational Dynamics—A Novel Shift in
           the Hamiltonian Paradigm

    • Authors: Abhay Ashtekar, Madhavan Varadarajan
      First page: 13
      Abstract: It is well known that Einstein’s equations assume a simple polynomial form in the Hamiltonian framework based on a Yang-Mills phase space. We re-examine the gravitational dynamics in this framework and show that time evolution of the gravitational field can be re-expressed as (a gauge covariant generalization of) the Lie derivative along a novel shift vector field in spatial directions. Thus, the canonical transformation generated by the Hamiltonian constraint acquires a geometrical interpretation on the Yang-Mills phase space, similar to that generated by the diffeomorphism constraint. In classical general relativity this geometrical interpretation significantly simplifies calculations and also illuminates the relation between dynamics in the ‘integrable’ (anti)self-dual sector and in the full theory. For quantum gravity, it provides a point of departure to complete the Dirac quantization program for general relativity in a more satisfactory fashion. This gauge theory perspective may also be helpful in extending the ‘double copy’ ideas relating the Einstein and Yang-Mills dynamics to a non-perturbative regime. Finally, the notion of generalized, gauge covariant Lie derivative may also be of interest to the mathematical physics community as it hints at some potentially rich structures that have not been explored.
      Citation: Universe
      PubDate: 2021-01-12
      DOI: 10.3390/universe7010013
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 14: Quantum String Cosmology

    • Authors: Maurizio Gasperini
      First page: 14
      Abstract: We present a short review of possible applications of the Wheeler-De Witt equation to cosmological models based on the low-energy string effective action, and characterised by an initial regime of asymptotically flat, low energy, weak coupling evolution. Considering in particular a class of duality-related (but classically disconnected) background solutions, we shall discuss the possibility of quantum transitions between the phases of pre-big bang and post-big bang evolution. We will show that it is possible, in such a context, to represent the birth of our Universe as a quantum process of tunneling or “anti-tunneling” from an initial state asymptotically approaching the string perturbative vacuum.
      Citation: Universe
      PubDate: 2021-01-12
      DOI: 10.3390/universe7010014
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 15: Intra-Day Variability Observations of Two
           Dozens of Blazars at 4.8 GHz

    • Authors: Xiang Liu, Xin Wang, Ning Chang, Jun Liu, Lang Cui, Xiaofeng Yang, Thomas P. Krichbaum
      First page: 15
      Abstract: Two dozens of radio loud active galactic nuclei (AGNs) have been observed with Urumqi 25 m radio telescope in order to search for intra-day variability (IDV). The target sources are blazars (namely flat spectrum radio quasars and BL Lac objects) which are mostly selected from the observing list of RadioAstron AGN monitoring campaigns. The observations were carried out at 4.8 GHz in two sessions of 8–12 February 2014 and 7–9 March respectively. We report the data reduction and the first results of observations. The results show that the majority of the blazars exhibit IDV in 99.9% confidence level, some of them show quite strong IDV. We find the strong IDV of blazar 1357 + 769 for the first time. The IDV at centimeter-wavelength is believed to be predominately caused by the scintillation of blazar emission through the local interstellar medium in a few hundreds parsecs away from Sun. No significant correlation between the IDV strength and either redshift or Galactic latitude is found in our sample. The IDV timescale along with source structure and brightness temperature analysis will be presented in a forthcoming paper.
      Citation: Universe
      PubDate: 2021-01-14
      DOI: 10.3390/universe7010015
      Issue No: Vol. 7, No. 1 (2021)
  • Universe, Vol. 7, Pages 1: Particle Acceleration Driven by Null
           Electromagnetic Fields Near a Kerr Black Hole

    • Authors: Yasufumi Kojima, Yuto Kimura
      First page: 1
      Abstract: Short timescale variability is often associated with a black hole system. The consequence of an electromagnetic outflow suddenly generated near a Kerr black hole is considered assuming that it is described by a solution of a force-free field with a null electric current. We compute charged particle acceleration induced by the burst field. We show that the particle is instantaneously accelerated to the relativistic regime by the field with a very large amplitude, which is characterized by a dimensionless number κ. Our numerical calculation demonstrates how the trajectory of the particle changes with κ. We also show that the maximum energy increases with κ2/3. The typical maximum energy attained by a proton for an event near a super massive black hole is Emax∼100 TeV, which is enough observed high-energy flares.
      Citation: Universe
      PubDate: 2020-12-22
      DOI: 10.3390/universe7010001
      Issue No: Vol. 7, No. 1 (2020)
  • Universe, Vol. 7, Pages 2: Photon Spheres, ISCOs, and OSCOs: Astrophysical
           Observables for Regular Black Holes with Asymptotically Minkowski Cores

    • Authors: Thomas Berry, Alex Simpson, Matt Visser
      First page: 2
      Abstract: Classical black holes contain a singularity at their core. This has prompted various researchers to propose a multitude of modified spacetimes that mimic the physically observable characteristics of classical black holes as best as possible, but that crucially do not contain singularities at their cores. Due to recent advances in near-horizon astronomy, the ability to observationally distinguish between a classical black hole and a potential black hole mimicker is becoming increasingly feasible. Herein, we calculate some physically observable quantities for a recently proposed regular black hole with an asymptotically Minkowski core—the radius of the photon sphere and the extremal stable timelike circular orbit (ESCO). The manner in which the photon sphere and ESCO relate to the presence (or absence) of horizons is much more complex than for the Schwarzschild black hole. We find situations in which photon spheres can approach arbitrarily close to (near extremal) horizons, situations in which some photon spheres become stable, and situations in which the locations of both photon spheres and ESCOs become multi-valued, with both ISCOs (innermost stable circular orbits) and OSCOs (outermost stable circular orbits). This provides an extremely rich phenomenology of potential astrophysical interest.
      Citation: Universe
      PubDate: 2020-12-22
      DOI: 10.3390/universe7010002
      Issue No: Vol. 7, No. 1 (2020)
  • Universe, Vol. 7, Pages 3: Comparison of Gravitational and Light Frequency
           Shifts in Rubidium Atomic Clock

    • Authors: Alexey Baranov, Sergey Ermak, Roman Lozov, Vladimir Semenov
      First page: 3
      Abstract: The article presents the results of an experimental study of the external magnetic field orientation and magnitude influence on the rubidium atomic clock, simulating the influence of the geomagnetic field on the onboard rubidium atomic clock of navigation satellites. The tensor component value of the atomic clock frequency light shift on the rubidium cell was obtained, and this value was ~2 Hz. The comparability of the relative light shift (~10−9) and the regular gravitational correction (4×10−10) to the frequency of the rubidium atomic clock was shown. The experimental results to determine the orientational shift influence on the rubidium atomic clock frequency were presented. A significant effect on the relative frequency instability of a rubidium atomic clock at a level of 10−12(10−13) for rotating external magnetic field amplitudes of 1.5 A/m and 3 A/m was demonstrated. This magnitude corresponds to the geomagnetic field in the orbit of navigation satellites. The necessity of taking into account various factors (satellite orbit parameters and atomic clock characteristics) is substantiated for correct comparison of corrections to the rubidium onboard atomic clock frequency associated with the Earth’s gravitational field action and the satellite orientation in the geomagnetic field.
      Citation: Universe
      PubDate: 2020-12-24
      DOI: 10.3390/universe7010003
      Issue No: Vol. 7, No. 1 (2020)
  • Universe, Vol. 7, Pages 4: The Uniformly Accelerated Frame as a Test Bed
           for Analysing the Gravitational Redshift

    • Authors: Don Koks
      First page: 4
      Abstract: Ever since Eddington’s analysis of the gravitational redshift a century ago, and the arguments in the relativity community that it produced, fine details of the roles of proper time and coordinate time in the redshift remain somewhat obscure. We shed light on these roles by appealing to the physics of the uniformly accelerated frame, in which coordinate time and proper time are well defined and easy to understand; and because that frame exists in flat spacetime, special relativity is sufficient to analyse it. We conclude that Eddington’s analysis was indeed correct—as was the 1980 analysis of his detractors, Earman and Glymour, who (it turns out) were following a different route. We also use the uniformly accelerated frame to pronounce invalid Schild’s old argument for spacetime curvature, which has been reproduced by many authors as a pedagogical introduction to curved spacetime. More generally, because the uniformly accelerated frame simulates a gravitational field, it can play a strong role in discussions of proper and coordinate times in advanced relativity.
      Citation: Universe
      PubDate: 2020-12-28
      DOI: 10.3390/universe7010004
      Issue No: Vol. 7, No. 1 (2020)
  • Universe, Vol. 7, Pages 5: Zeta Functions and the Cosmos—A Basic
           Brief Review

    • Authors: Emilio Elizalde
      First page: 5
      Abstract: This is a very basic and pedagogical review of the concepts of zeta function and of the associated zeta regularization method, starting from the notions of harmonic series and of divergent sums in general. By way of very simple examples, it is shown how these powerful methods are used for the regularization of physical quantities, such as quantum vacuum fluctuations in various contexts. In special, in Casimir effect setups, with a note on the dynamical Casimir effect, and mainly concerning its application in quantum theories in curved spaces, subsequently used in gravity theories and cosmology. The second part of this work starts with an essential introduction to large scale cosmology, in search of the observational foundations of the Friedmann-Lemaître-Robertson-Walker (FLRW) model, and the cosmological constant issue, with the very hard problems associated with it. In short, a concise summary of all these interrelated subjects and applications, involving zeta functions and the cosmos, and an updated list of the pioneering and more influential works (according to Google Scholar citation counts) published on all these matters to date, are provided.
      Citation: Universe
      PubDate: 2020-12-30
      DOI: 10.3390/universe7010005
      Issue No: Vol. 7, No. 1 (2020)
  • Universe, Vol. 7, Pages 6: Isospin Effect on Baryon and Charge
           Fluctuations from the pNJL Model

    • Authors: He Liu, Jun Xu
      First page: 6
      Abstract: We have studied the possible isospin corrections on the skewness and kurtosis of net-baryon and net-charge fluctuations in the isospin asymmetric matter formed in relativistic heavy-ion collisions at RHIC-BES energies, based on a 3-flavor Polyakov-looped Nambu–Jona–Lasinio model. With typical scalar–isovector and vector–isovector couplings leading to the splitting of u and d quark chiral phase transition boundaries and critical points, we have observed considerable isospin effects on the susceptibilities, especially those of net-charge fluctuations. Reliable experimental measurements at even lower collision energies are encouraged to confirm the observed isospin effects.
      Citation: Universe
      PubDate: 2020-12-31
      DOI: 10.3390/universe7010006
      Issue No: Vol. 7, No. 1 (2020)
  • Universe, Vol. 6, Pages 221: Can a Chameleon Field Be Identified with

    • Authors: A. N. Ivanov, M. Wellenzohn
      First page: 221
      Abstract: In the Einstein–Cartan gravitational theory with the chameleon field, while changing its mass independently of the density of its environment, we analyze the Friedmann–Einstein equations for the Universe’s evolution with the expansion parameter a being dependent on time only. We analyze the problem of an identification of the chameleon field with quintessence, i.e., a canonical scalar field responsible for dark energy dynamics, and for the acceleration of the Universe’s expansion. We show that since the cosmological constant related to the relic dark energy density is induced by torsion (Astrophys. J.2016, 829, 47), the chameleon field may, in principle, possess some properties of quintessence, such as an influence on the dark energy dynamics and the acceleration of the Universe’s expansion, even in the late-time acceleration, but it cannot be identified with quintessence to the full extent in the classical Einstein–Cartan gravitational theory.
      Citation: Universe
      PubDate: 2020-11-26
      DOI: 10.3390/universe6120221
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 222: Neutron Stars and Dark Matter

    • Authors: Antonino Del Popolo, Morgan Le Delliou, Maksym Deliyergiyev
      First page: 222
      Abstract: Neutron stars change their structure with accumulation of dark matter. We study how their mass is influenced from the environment. Close to the sun, the dark matter accretion from the neutron star does not have any effect on it. Moving towards the galactic center, the density increase in dark matter results in increased accretion. At distances of some fraction of a parsec, the neutron star acquire enough dark matter to have its structure changed. We show that the neutron star mass decreases going towards the galactic centre, and that dark matter accumulation beyond a critical value collapses the neutron star into a black hole. Calculations cover cases varying the dark matter particle mass, self-interaction strength, and ratio between the pressure of dark matter and ordinary matter. This allow us to constrain the interaction cross section, σdm, between nucleons and dark matter particles, as well as the dark matter self-interaction cross section.
      Citation: Universe
      PubDate: 2020-11-26
      DOI: 10.3390/universe6120222
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 223: Studying Asymptotic Giant Branch Stars in the
           JWST Era

    • Authors: Paolo Ventura, Ester Marini, Silvia Tosi, Flavia Dell’Agli
      First page: 223
      Abstract: We explore the potential offered by the incoming launch of the James Webb Space Telescope, to study the stars evolving through the asymptotic giant branch (AGB) phase. To this aim we compare data of AGB stars in the Large Magellanic Cloud, taken with the IRS spectrograph, with the results from modelling of AGB evolution and dust formation in the wind. We find that the best diagrams to study M- and C-stars are, respectively, ([F770W]−[F2500W], [F770W]) and ([F770W]−[F1800W], [F1800W]). ([F770W]−[F2500W], [F770W]) turns out to be the best way of studying the AGB population in its entirely.
      Citation: Universe
      PubDate: 2020-11-26
      DOI: 10.3390/universe6120223
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 224: Rotation Effects in Relativity

    • Authors: Matteo Luca Ruggiero
      First page: 224
      Abstract: Rotation has always been a central thread in physics and has influenced its development [...]
      Citation: Universe
      PubDate: 2020-11-27
      DOI: 10.3390/universe6120224
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 225: Neutrino-Mass Sensitivity and Nuclear Matrix
           Element for Neutrinoless Double Beta Decay

    • Authors: Hiroyasu Ejiri
      First page: 225
      Abstract: Neutrinoless double beta decay (DBD) is a useful probe to study neutrino properties such as the Majorana nature, the absolute neutrino mass, the CP phase and the others, which are beyond the standard model. The nuclear matrix element (NME) for DBD is crucial to extract the neutrino properties from the experimental transition rate. The neutrino-mass sensitivity, i.e., the minimum neutrino-mass to be measured by the DBD experiment, is very sensitive to the DBD NME. Actually, the NME is one of the key elements for designing the DBD experiment. Theoretical evaluation for the DBD NME, however, is very hard. Recently experimental studies of charge-exchange nuclear and leptonic reactions have shown to be used to get single-ββ NMEs associated with the DBD NME. Critical discussions are made on the neutrino-mass sensitivity and the NME for the DBD neutrino-mass study and on the experimental studies of the single-ββ NMEs and nuclear structures associated with DBD NMEs.
      Citation: Universe
      PubDate: 2020-11-27
      DOI: 10.3390/universe6120225
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 226: Jeans Instability of Dissipative
           Self-Gravitating Bose–Einstein Condensates with Repulsive or Attractive
           Self-Interaction: Application to Dark Matter

    • Authors: Pierre-Henri Chavanis
      First page: 226
      Abstract: We study the Jeans instability of an infinite homogeneous dissipative self-gravitating Bose–Einstein condensate described by generalized Gross–Pitaevskii–Poisson equations [Chavanis, P.H. Eur. Phys. J. Plus 2017, 132, 248]. This problem has applications in relation to the formation of dark matter halos in cosmology. We consider the case of a static and an expanding universe. We take into account an arbitrary form of repulsive or attractive self-interaction between the bosons (an attractive self-interaction being particularly relevant for the axion). We consider both gravitational and hydrodynamical (tachyonic) instabilities and determine the maximum growth rate of the instability and the corresponding wave number. We study how they depend on the scattering length of the bosons (or more generally on the squared speed of sound) and on the friction coefficient. Previously obtained results (notably in the dissipationless case) are recovered in particular limits of our study.
      Citation: Universe
      PubDate: 2020-11-27
      DOI: 10.3390/universe6120226
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 227: New Scenarios of High-Energy Particle
           Collisions Near Wormholes

    • Authors: Oleg B. Zaslavskii
      First page: 227
      Abstract: We suggest two new scenarios of high-energy particle collisions in the background of a wormhole. In scenario 1, the novelty consists of the fact that the effect does not require two particles coming from different mouths. Instead, all such scenarios of high energy collisions develop, when an experimenter sends particles towards a wormhole from the same side of the throat. For static wormholes, this approach leads to indefinitely large energy in the center of mass. For rotating wormholes, it makes possible the super-Penrose process (unbounded energies measured at infinity). In scenario 2, one of colliding particles oscillates near the wormhole throat from the very beginning. In this sense, scenario 2 is intermediate between the standard one and scenario 1 since the particle under discussion does not come from infinity at all.
      Citation: Universe
      PubDate: 2020-11-30
      DOI: 10.3390/universe6120227
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 228: Post-Editorial of the Special Issue
           “Wormholes in Space-Time: Theory and Facts”

    • Authors: Francisco S. N. Lobo, Gonzalo J. Olmo, Diego Rubiera-Garcia
      First page: 228
      Abstract: Wormholes made their first appearance in gravitational physics as soon as in 1916 but, as with their black hole cousins, it took a long time and effort for their true nature to be properly understood [...]
      Citation: Universe
      PubDate: 2020-11-30
      DOI: 10.3390/universe6120228
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 229: Toward Nonlocal Electrodynamics of
           Accelerated Systems

    • Authors: Bahram Mashhoon
      First page: 229
      Abstract: We revisit acceleration-induced nonlocal electrodynamics and the phenomenon of photon spin-rotation coupling. The kernel of the theory for the electromagnetic field tensor involves parity violation under the assumption of linearity of the field kernel in the acceleration tensor. However, we show that parity conservation can be maintained by extending the field kernel to include quadratic terms in the acceleration tensor. The field kernel must vanish in the absence of acceleration; otherwise, a general dependence of the kernel on the acceleration tensor cannot be theoretically excluded. The physical implications of the quadratic kernel are briefly discussed.
      Citation: Universe
      PubDate: 2020-12-03
      DOI: 10.3390/universe6120229
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 230: The Solution of the Cosmological Constant
           Problem: The Cosmological Constant Exponential Decrease in the Super-Early

    • Authors: Babourova, Frolov
      First page: 230
      Abstract: The stage of a super-early (primordial) scale-invariant Universe is considered on the basis of the Poincaré–Weyl gauge theory of gravity in a Cartan–Weyl space-time. An approximate solution has been found that demonstrates an inflationary behavior of the scale factor and, at the same time, a sharp exponential decrease in the effective cosmological constant from a huge value at the beginning of the Big Bang to an extremely small (but not zero) value in the modern era, which solves the well-known “cosmological constant problem.”
      Citation: Universe
      PubDate: 2020-12-04
      DOI: 10.3390/universe6120230
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 231: Constraint on Hybrid Stars with Gravitational
           Wave Events

    • Authors: Kilar Zhang, Feng-Li Lin
      First page: 231
      Abstract: Motivated by the recent discoveries of compact objects from LIGO/Virgo observations, we study the possibility of identifying some of these objects as compact stars made of dark matter called dark stars, or the mix of dark and nuclear matters called hybrid stars. In particular, in GW190814, a new compact object with 2.6 M⊙ is reported. This could be the lightest black hole, the heaviest neutron star, and a dark or hybrid star. In this work, we extend the discussion on the interpretations of the recent LIGO/Virgo events as hybrid stars made of various self-interacting dark matter (SIDM) in the isotropic limit. We pay particular attention to the saddle instability of the hybrid stars which will constrain the possible SIDM models.
      Citation: Universe
      PubDate: 2020-12-04
      DOI: 10.3390/universe6120231
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 232: Beyond General Relativity: Models for Quantum
           Gravity, Loop Quantum Cosmology and Black Holes

    • Authors: Nick E. Mavromatos
      First page: 232
      Abstract: In the past two decades, we have witnessed extraordinary progress in precision measurements in cosmology [...]
      Citation: Universe
      PubDate: 2020-12-07
      DOI: 10.3390/universe6120232
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 233: Present Status of Nuclear Shell-Model
           Calculations of 0νββ Decay Matrix Elements

    • Authors: Luigi Coraggio, Nunzio Itaco, Giovanni De Gregorio, Angela Gargano, Riccardo Mancino, Saori Pastore
      First page: 233
      Abstract: Neutrinoless double beta (0νββ) decay searches are currently among the major foci of experimental physics. The observation of such a decay will have important implications in our understanding of the intrinsic nature of neutrinos and shed light on the limitations of the Standard Model. The rate of this process depends on both the unknown neutrino effective mass and the nuclear matrix element (M0ν) associated with the given 0νββ transition. The latter can only be provided by theoretical calculations, hence the need of accurate theoretical predictions of M0ν for the success of the experimental programs. This need drives the theoretical nuclear physics community to provide the most reliable calculations of M0ν. Among the various computational models adopted to solve the many-body nuclear problem, the shell model is widely considered as the basic framework of the microscopic description of the nucleus. Here, we review the most recent and advanced shell-model calculations of M0ν considering the light-neutrino-exchange channel for nuclei of experimental interest. We report the sensitivity of the theoretical calculations with respect to variations in the model spaces and the shell-model nuclear Hamiltonians.
      Citation: Universe
      PubDate: 2020-12-07
      DOI: 10.3390/universe6120233
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 234: Dark Matter as Gravitational Solitons in the
           Weak Field Limit

    • Authors: Torsten Asselmeyer-Maluga, Jerzy Król
      First page: 234
      Abstract: In this paper, we will describe the idea that dark matter partly consists of gravitational solitons (gravisolitons). The corresponding solution is valid for weak gravitational fields (weak field limit) with respect to a background metric. The stability of this soliton is connected with the existence of a special foliation and amazingly with the smoothness properties of spacetime. Gravisolitons have many properties of dark matter, such as no interaction with light but act on matter via gravitation. In this paper, we showed that the gravitational lensing effect of gravisolitons agreed with the lensing effect of usual matter. Furthermore, we obtained the same equation of state w=0 as matter.
      Citation: Universe
      PubDate: 2020-12-09
      DOI: 10.3390/universe6120234
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 235: Editorial for the Special Issue “Quantum
           Fields—From Fundamental Concepts to Phenomenological Questions”

    • Authors: Astrid Eichhorn, Roberto Percacci, Frank Saueressig
      First page: 235
      Abstract: Quantum field theory and Einstein’s theory of general relativity are extremely successful in predicting the outcome of particle physics and gravitational experiments [...]
      Citation: Universe
      PubDate: 2020-12-10
      DOI: 10.3390/universe6120235
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 236: The Montevideo Interpretation: How the
           Inclusion of a Quantum Gravitational Notion of Time Solves the Measurement

    • Authors: Rodolfo Gambini, Jorge Pullin
      First page: 236
      Abstract: We review the Montevideo Interpretation of quantum mechanics, which is based on the use of real clocks to describe physics, using the framework that was recently introduced by Höhn, Smith, and Lock to treat the problem of time in generally covariant systems. These new methods, which solve several problems in the introduction of a notion of time in such systems, do not change the main results of the Montevideo Interpretation. The use of the new formalism makes the construction more general and valid for any system in a quantum generally covariant theory. We find that, as in the original formulation, a fundamental mechanism of decoherence emerges that allows for supplementing ordinary environmental decoherence and avoiding its criticisms. The recent results on quantum complexity provide additional support to the type of global protocols that are used to prove that within ordinary—unitary—quantum mechanics, no definite event—an outcome to which a probability can be associated—occurs. In lieu of this, states that start in a coherent superposition of possible outcomes always remain as a superposition. We show that, if one takes into account fundamental inescapable uncertainties in measuring length and time intervals due to general relativity and quantum mechanics, the previously mentioned global protocols no longer allow for distinguishing whether the state is in a superposition or not. One is left with a formulation of quantum mechanics purely defined in quantum mechanical terms without any reference to the classical world and with an intrinsic operational definition of quantum events that does not need external observers.
      Citation: Universe
      PubDate: 2020-12-11
      DOI: 10.3390/universe6120236
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 237: Relativistic Combination of Non-Collinear
           3-Velocities Using Quaternions

    • Authors: Thomas Berry, Matt Visser
      First page: 237
      Abstract: Quaternions have an (over a century-old) extensive and quite complicated interaction with special relativity. Since quaternions are intrinsically 4-dimensional, and do such a good job of handling 3-dimensional rotations, the hope has always been that the use of quaternions would simplify some of the algebra of the Lorentz transformations. Herein we report a new and relatively nice result for the relativistic combination of non-collinear 3-velocities. We work with the relativistic half-velocities w defined by v=2w1+w2, so that w=v1+1−v2=v2+O(v3), and promote them to quaternions using w=wn^, where n^ is a unit quaternion. We shall first show that the composition of relativistic half-velocities is given by w1⊕2≡w1⊕w2≡(1−w1w2)−1(w1+w2), and then show that this is also equivalent to w1⊕2=(w1+w2)(1−w2w1)−1. Here as usual we adopt units where the speed of light is set to unity. Note that all of the complicated angular dependence for relativistic combination of non-collinear 3-velocities is now encoded in the quaternion multiplication of w1 with w2. This result can furthermore be extended to obtain novel elegant and compact formulae for both the associated Wigner angle Ω and the direction of the combined velocities: eΩ=eΩΩ^=(1−w1w2)−1(1−w2w1), and w^1⊕2=eΩ/2w1+w2 w1+w2 . Finally, we use this formalism to investigate the conditions under which the relativistic composition of 3-velocities is associative. Thus, we would argue, many key results that are ultimately due to the non-commutativity of non-collinear boosts can be easily rephrased in terms of the non-commutative algebra of quaternions.
      Citation: Universe
      PubDate: 2020-12-11
      DOI: 10.3390/universe6120237
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 238: Fundamental Symmetries and Spacetime
           Geometries in Gauge Theories of Gravity—Prospects for Unified Field

    • Authors: Francisco Cabral, Francisco S. N. Lobo, Diego Rubiera-Garcia
      First page: 238
      Abstract: Gravity can be formulated as a gauge theory by combining symmetry principles and geometrical methods in a consistent mathematical framework. The gauge approach to gravity leads directly to non-Euclidean, post-Riemannian spacetime geometries, providing the adequate formalism for metric-affine theories of gravity with curvature, torsion and non-metricity. In this paper, we analyze the structure of gauge theories of gravity and consider the relation between fundamental geometrical objects and symmetry principles as well as different spacetime paradigms. Special attention is given to Poincaré gauge theories of gravity, their field equations and Noether conserved currents, which are the sources of gravity. We then discuss several topics of the gauge approach to gravitational phenomena, namely, quadratic Poincaré gauge models, the Einstein-Cartan-Sciama-Kibble theory, the teleparallel equivalent of general relativity, quadratic metric-affine Lagrangians, non-Lorentzian connections, and the breaking of Lorentz invariance in the presence of non-metricity. We also highlight the probing of post-Riemannian geometries with test matter. Finally, we briefly discuss some perspectives regarding the role of both geometrical methods and symmetry principles towards unified field theories and a new spacetime paradigm, motivated from the gauge approach to gravity.
      Citation: Universe
      PubDate: 2020-12-11
      DOI: 10.3390/universe6120238
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 239: Double Beta Decay to Excited States of
           Daughter Nuclei

    • Authors: Pierluigi Belli, Rita Bernabei, Fabio Cappella, Vincenzo Caracciolo, Riccardo Cerulli, Antonella Incicchitti, Vittorio Merlo
      First page: 239
      Abstract: In this paper we review results obtained in the searches of double beta decays to excited states of the daughter nuclei and illustrate the related experimental techniques. In particular, we describe in some detail the only two cases in which the transition has been observed; that is the 2β−(0+→01+) decay of 100Mo and 150Nd nuclides. Moreover, the most significant results in terms of lower limits on the half-life are also summarized.
      Citation: Universe
      PubDate: 2020-12-13
      DOI: 10.3390/universe6120239
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 240: Post-Editorial of the Special Issue “Estate
           Quantistica Conference—Recent Developments in Gravity, Cosmology, and
           Mathematical Physics”

    • Authors: Hermano Velten, Giuseppe Dito
      First page: 240
      Abstract: This editorial summarizes the contributions presented during the Estate Quantistica 2018 and published in a special issue of Universe [...]
      Citation: Universe
      PubDate: 2020-12-13
      DOI: 10.3390/universe6120240
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 241: Uniqueness Criteria for the Fock Quantization
           of Dirac Fields and Applications in Hybrid Loop Quantum Cosmology

    • Authors: Jerónimo Cortez, Beatriz Elizaga Navascués, Guillermo A. Mena Marugán, Santiago Prado, José M. Velhinho
      First page: 241
      Abstract: In generic curved spacetimes, the unavailability of a natural choice of vacuum state introduces a serious ambiguity in the Fock quantization of fields. In this review, we study the case of fermions described by a Dirac field in non-stationary spacetimes, and present recent results obtained by us and our collaborators about well-motivated criteria capable to ensure the uniqueness in the selection of a vacuum up to unitary transformations, at least in certain situations of interest in cosmology. These criteria are based on two reasonable requirements. First, the invariance of the vacuum under the symmetries of the Dirac equations in the considered spacetime. These symmetries include the spatial isometries. Second, the unitary implementability of the Heisenberg dynamics of the annihilation and creation operators when the curved spacetime is treated as a fixed background. This last requirement not only permits the uniqueness of the Fock quantization but, remarkably, it also allows us to determine an essentially unique splitting between the phase space variables assigned to the background and the fermionic annihilation and creation variables. We first consider Dirac fields in 2 + 1 dimensions and then discuss the more relevant case of 3 + 1 dimensions, particularizing the analysis to cosmological spacetimes with spatial sections of spherical or toroidal topology. We use this analysis to investigate the combined, hybrid quantization of the Dirac field and a flat homogeneous and isotropic background cosmology when the latter is treated as a quantum entity, and the former as a perturbation. Specifically, we focus our study on a background quantization along the lines of loop quantum cosmology. Among the Fock quantizations for the fermionic perturbations admissible according to our criteria, we discuss the possibility of further restricting the choice of a vacuum by the requisite of a finite fermionic backreaction and, moreover, by the diagonalization of the fermionic contribution to the total Hamiltonian in the asymptotic limit of large wave numbers of the Dirac modes. Finally, we argue in support of the uniqueness of the vacuum state selected by the extension of this diagonalization condition beyond the commented asymptotic region, in particular proving that it picks out the standard Poincaré and Bunch–Davies vacua for fixed flat and de Sitter background spacetimes, respectively.
      Citation: Universe
      PubDate: 2020-12-13
      DOI: 10.3390/universe6120241
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 242: Editorial for the Special Issue “Accretion
           Disks, Jets, Gamma-Ray Bursts and Related Gravitational Waves”

    • Authors: Banibrata Mukhopadhyay
      First page: 242
      Abstract: The present Editorial introduces the Special Issue dedicated by the journal Universe to the “Accretion Disks, Jets, Gamma-Ray Bursts and Related Gravitational Waves” [...]
      Citation: Universe
      PubDate: 2020-12-15
      DOI: 10.3390/universe6120242
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 243: An Improved Framework for Quantum Gravity

    • Authors: José G. Pereira, Diego F. López
      First page: 243
      Abstract: General relativity has two fundamental problems that render it unsuitable for tackling the gravitational field’s quantization. The first problem is the lack of a genuine gravitational variable representing gravitation only, inertial forces apart. The second problem is its incompatibility with quantum mechanics, a problem inherited from the more fundamental conflict of special relativity with quantum mechanics. A procedure to overcome these difficulties is outlined, which amounts to replacing general relativity with its teleparallel equivalent and the Poincaré-invariant special relativity with the de Sitter-invariant special relativity. Those replacements give rise to the de Sitter-modified teleparallel gravity, which does not have the two mentioned problems. It can thus be considered an improved alternative approach to quantum gravity.
      Citation: Universe
      PubDate: 2020-12-16
      DOI: 10.3390/universe6120243
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 244: Dynamical Stability and Geometrical
           Diagnostic of the Power Law K-Essence Dark Energy Model with Interaction

    • Authors: Bo-Hai Chen, Ya-Bo Wu, Dong-Fang Xu, Wei Dong, Nan Zhang
      First page: 244
      Abstract: We investigate the cosmological evolution of the power law k-essence dark energy (DE) model with interaction in FRWL spacetime with the Lagrangian that contains a kinetic function F(X)=−X+X. Concretely, the cosmological evolution in this model are discussed by the autonomous dynamical system and its critical points, together with the corresponding cosmological quantities, such as Ωϕ, wϕ, cs2, and q, are calculated at each critical point. The evolutionary trajectories are drawn in order to show the dynamical process on the phases plan around the critical points. The result that we obtained indicates that there are four dynamical attractors, and all of them correspond to an accelerating expansion of universe for certain potential parameter and coupling parameter. Besides that, the geometrical diagnostic by the statefinder hierarchy S3(1) and S4(1) of this scalar field model are numerically obtained by the phase components, as an extended null diagnostic for the cosmological constant. This diagnostic shows that both the potential parameter λ and interaction parameter α play important roles in the evolution of the statefinder hierarchy.
      Citation: Universe
      PubDate: 2020-12-18
      DOI: 10.3390/universe6120244
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 245: Supersymmetric HS Yang-Mills-like Models

    • Authors: Loriano Bonora, Stefano Giaccari
      First page: 245
      Abstract: We introduce the supersymmetric version of YM-like theories with infinitely many spin fields in four dimensions. The construction is carried out via the superfield method. The surprising feature of these models is that they describe, in particular, gauge and gravity in a supersymmetric form, with no need for supergravity.
      Citation: Universe
      PubDate: 2020-12-18
      DOI: 10.3390/universe6120245
      Issue No: Vol. 6, No. 12 (2020)
  • Universe, Vol. 6, Pages 189: Some Issues on the Foundations of Modern
           Cosmology, Gravitation and Quantum Physics

    • Authors: Emilio Elizalde
      First page: 189
      Abstract: After the first clear evidence of the recession—at very high speeds—of the spiral nebulae was announced by V.M. Slipher in 1914, as a result of his work started in 1912, it still took several decades to properly understand the phenomenon in terms of an expansion of the Universe. Some historical issues around that crucial discovery and the contemporary attempts at determining the scale of the visible Universe will be discussed. Presently, very important questions to answer are: What is the precise value of the expansion rate' What drives the acceleration of the Universe’s expansion' The latter is called dark energy, but what is it actually' The possibility that this could be the result of a sort of Casimir effect at the cosmological level has not been discarded, yet. One of the main technical problems in tackling this issue is constituted by the regularization and corresponding renormalization procedures. Beautiful but rather non-trivial mathematics, involving the zeta function of pseudodifferential operators (associated with physical quantities), are key in this respect. A discussion of those items is provided here.
      Citation: Universe
      PubDate: 2020-10-22
      DOI: 10.3390/universe6110189
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 190: Resonant Effect for Breit–Wheeler Process
           in the Field of an X-ray Pulsar

    • Authors: Vitalii D. Serov, Sergei P. Roshchupkin, Victor V. Dubov
      First page: 190
      Abstract: The resonant process of the creation of an ultrarelativistic electron–positron pair by two hard gamma quanta in the field of an X-ray pulsar (the Breit–Wheeler process modified by an external field) was theoretically studied. Under resonance conditions, the intermediate virtual electron (positron) in the external field becomes a real particle. As a result, there are four reaction channels for the process instead of two. For each of those channels, the initial process of the second order in the fine structure constant in the field of an X-ray pulsar effectively reduces into two successive processes of the first order: X-ray-stimulated Breit–Wheeler process and X-ray-stimulated Compton effect. The resonant kinematics of the process was also studied in detail. The process had characteristic threshold energy, and all initial and final particles had to be ultrarelativistic and propagate in a narrow cone. Furthermore, the resonant energy spectrum of the electron-positron pair significantly depended on emission angles. Clearly, there was a qualitative difference between resonant and nonresonant cases. Lastly, the resonant differential probability of studied process was obtained. The resonant differential probability significantly exceeded the nonresonant one without the external field of an X-ray pulsar.
      Citation: Universe
      PubDate: 2020-10-22
      DOI: 10.3390/universe6110190
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 191: Is OJ 287 a Single Supermassive Black

    • Authors: Marina S. Butuzova, Alexander B. Pushkarev
      First page: 191
      Abstract: Light curves for more than century optical photometric observations of the blazar OJ 287 reveals strong flares with a quasi-period of about 12 years. For a long time, this period has been interpreted by processes in a binary black hole system. We propose an alternative explanation for this period, which is based on Doppler factor periodic variations of the emitting region caused by jet helicity. Using multi-epoch very large baseline interferometry (VLBI) observations carried out in a framework of the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) program and other VLBA (Very Long Baseline Array) archival experiments at the observing frequency of 15 GHz, we derived geometrical parameters of the jet helix. To reach an agreement between the VLBI and photometric optical observation data, the jet component motion at a small angle to the radial direction is necessary. Such non-radial motion is observed and, together with the jet helical shape, can be naturally explained by the development of the Kelvin–Helmholtz instability in the parsec-scale outflow. In this case, the true precession of the OJ 287 jet may manifest itself in differences between the peak flux values of the 12-year optical flares. A possibility to create this precession due to Lense–Thirring effect of a single supermassive black hole is also discussed.
      Citation: Universe
      PubDate: 2020-10-22
      DOI: 10.3390/universe6110191
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 192: Is the Axionic Dark Matter an Equilibrium

    • Authors: Alexander B. Balakin, Amir F. Shakirzyanov
      First page: 192
      Abstract: We consider an axionic dark matter model with a modified periodic potential for the pseudoscalar field in the framework of the axionic extension of the Einstein-aether theory. The modified potential is assumed to be equipped by the guiding function, which depends on the expansion scalar constructed as the trace of the covariant derivative of the aether velocity four-vector. The equilibrium state of the axion field is defined as the state, for which the modified potential itself and its first derivative with respect to the pseudoscalar field are equal to zero. We apply the developed formalism to the homogeneous isotropic cosmological model, and find the basic function, which describes the equilibrium state of the axionic dark matter in the expanding Universe.
      Citation: Universe
      PubDate: 2020-10-22
      DOI: 10.3390/universe6110192
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 193: On a Crucial Role of Gravity in the Formation
           of Elementary Particles

    • Authors: Ahmed Alharthy, Vladimir V. Kassandrov
      First page: 193
      Abstract: We consider the model of minimally interacting electromagnetic, gravitational and massive scalar fields free of any additional nonlinearities. In the dimensionless form, the Lagranginan contains only one parameter γ=(mG/e)2 which corresponds to the ratio of gravitational and electromagnetic interactions and, for a typical elementary particle, is about 10−40 in value. However, regular (soliton-like) solutions can exist only for γ≠0, so that gravity would be necessary to form the structure of an (extended) elementary particle. Unfortunately (in the stationary spherically symmetrical case), the numerical procedure breaks in the range γ≤0.9 so that whether the particle-like solutions actually exist in the model remains unclear. Nonetheless, for γ∼1 we obtain, making use of the minimal energy requirement, a discrete set of (horizon-free) electrically charged regular solutions of the Planck’s range mass and dimensions (“maximons”, “planckeons”, etc.). In the limit γ→∞, the model reduces to the well-known coupled system of the Einstein and Klein–Gordon equations. We obtain—to our knowledge—for the first time, the discrete spectrum of neutral soliton-like solutions (“mini-boson stars”, “soliton stars”, etc.)
      Citation: Universe
      PubDate: 2020-10-23
      DOI: 10.3390/universe6110193
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 194: Making a Quantum Universe: Symmetry and

    • Authors: Houri Ziaeepour
      First page: 194
      Abstract: So far, none of attempts to quantize gravity has led to a satisfactory model that not only describe gravity in the realm of a quantum world, but also its relation to elementary particles and other fundamental forces. Here, we outline the preliminary results for a model of quantum universe, in which gravity is fundamentally and by construction quantic. The model is based on three well motivated assumptions with compelling observational and theoretical evidence: quantum mechanics is valid at all scales; quantum systems are described by their symmetries; universe has infinite independent degrees of freedom. The last assumption means that the Hilbert space of the Universe has SU(N→∞)≅areapreservingDiff.(S2) symmetry, which is parameterized by two angular variables. We show that, in the absence of a background spacetime, this Universe is trivial and static. Nonetheless, quantum fluctuations break the symmetry and divide the Universe to subsystems. When a subsystem is singled out as reference—observer—and another as clock, two more continuous parameters arise, which can be interpreted as distance and time. We identify the classical spacetime with parameter space of the Hilbert space of the Universe. Therefore, its quantization is meaningless. In this view, the Einstein equation presents the projection of quantum dynamics in the Hilbert space into its parameter space. Finite dimensional symmetries of elementary particles emerge as a consequence of symmetry breaking when the Universe is divided to subsystems/particles, without having any implication for the infinite dimensional symmetry and its associated interaction-percived as gravity. This explains why gravity is a universal force.
      Citation: Universe
      PubDate: 2020-10-23
      DOI: 10.3390/universe6110194
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 195: Qualitative Analysis of the Dynamics of a
           Two-Component Chiral Cosmological Model

    • Authors: Viktor Zhuravlev, Sergey Chervon
      First page: 195
      Abstract: We present a qualitative analysis of chiral cosmological model (CCM) dynamics with two scalar fields in the spatially flat Friedman–Robertson–Walker Universe. The asymptotic behavior of chiral models is investigated based on the characteristics of the critical points of the selfinteraction potential and zeros of the metric components of the chiral space. The classification of critical points of CCMs is proposed. The role of zeros of the metric components of the chiral space in the asymptotic dynamics is analysed. It is shown that such zeros lead to new critical points of the corresponding dynamical systems. Examples of models with different types of zeros of metric components are represented.
      Citation: Universe
      PubDate: 2020-10-24
      DOI: 10.3390/universe6110195
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 196: New Physics of Strong Interaction and Dark

    • Authors: Vitaly Beylin, Maxim Khlopov, Vladimir Kuksa, Nikolay Volchanskiy
      First page: 196
      Abstract: The history of dark universe physics can be traced from processes in the very early universe to the modern dominance of dark matter and energy. Here, we review the possible nontrivial role of strong interactions in cosmological effects of new physics. In the case of ordinary QCD interaction, the existence of new stable colored particles such as new stable quarks leads to new exotic forms of matter, some of which can be candidates for dark matter. New QCD-like strong interactions lead to new stable composite candidates bound by QCD-like confinement. We put special emphasis on the effects of interaction between new stable hadrons and ordinary matter, formation of anomalous forms of cosmic rays and exotic forms of matter, like stable fractionally charged particles. The possible correlation of these effects with high energy neutrino and cosmic ray signatures opens the way to study new physics of strong interactions by its indirect multi-messenger astrophysical probes.
      Citation: Universe
      PubDate: 2020-10-26
      DOI: 10.3390/universe6110196
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 197: Thin-Shell Wormholes in Einstein and
           Einstein–Gauss–Bonnet Theories of Gravity

    • Authors: Takafumi Kokubu, Tomohiro Harada
      First page: 197
      Abstract: We review recent works on the possibility for eternal existence of thin-shell wormholes on Einstein and Einstein–Gauss–Bonnet gravity. We introduce thin-shell wormholes that are categorized into a class of traversable wormhole solutions. After that, we discuss stable thin-shell wormholes with negative-tension branes in Reissner–Nordström–(anti) de Sitter spacetimes in d-dimensional Einstein gravity. Imposing Z2 symmetry, we construct and classify traversable static thin-shell wormholes in spherical, planar and hyperbolic symmetries. It is found that the spherical wormholes are stable against spherically symmetric perturbations. It is also found that some classes of wormholes in planar and hyperbolic symmetries with a negative cosmological constant are stable against perturbations preserving symmetries. In most cases, stable wormholes are found with the appropriate combination of an electric charge and a negative cosmological constant. However, as special cases, there are stable wormholes even with a vanishing cosmological constant in spherical symmetry and with a vanishing electric charge in hyperbolic symmetry. Subsequently, the existence and dynamical stability of traversable thin-shell wormholes with electrically neutral negative-tension branes is discussed in Einstein–Gauss–Bonnet theory of gravitation. We consider radial perturbations against the shell for the solutions, which have the Z2 symmetry. The effect of the Gauss–Bonnet term on the stability depends on the spacetime symmetry.
      Citation: Universe
      PubDate: 2020-10-26
      DOI: 10.3390/universe6110197
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 198: Black Holes and Complexity via Constructible

    • Authors: Jerzy Król, Paweł Klimasara
      First page: 198
      Abstract: The relation of randomness and classical algorithmic computational complexity is a vast and deep subject by itself. However, already, 1-randomness sequences call for quantum mechanics in their realization. Thus, we propose to approach black hole’s quantum computational complexity by classical computational classes and randomness classes. The model of a general black hole is proposed based on formal tools from Zermelo–Fraenkel set theory like random forcing or minimal countable constructible model Lα. The Bekenstein–Hawking proportionality rule is shown to hold up to a multiplicative constant. Higher degrees of randomness and algorithmic computational complexity are derived in the model. Directions for further studies are also formulated. The model is designed for exploring deep quantum regime of spacetime.
      Citation: Universe
      PubDate: 2020-10-27
      DOI: 10.3390/universe6110198
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 199: Exact and Slow-Roll Solutions for Exponential
           Power-Law Inflation Connected with Modified Gravity and Observational

    • Authors: Igor Fomin, Sergey Chervon
      First page: 199
      Abstract: We investigate the ability of the exponential power-law inflation to be a phenomenologically correct model of the early universe. We study General Relativity (GR) scalar cosmology equations in Ivanov–Salopek–Bond (or Hamilton–Jacobi like) representation where the Hubble parameter H is the function of a scalar field ϕ. Such approach admits calculation of the potential for given H(ϕ) and consequently reconstruction of f(R) gravity in parametric form. By this manner the Starobinsky potential and non-minimal Higgs potential (and consequently the corresponding f(R) gravity) were reconstructed using constraints on the model’s parameters. We also consider methods for generalising the obtained solutions to the case of chiral cosmological models and scalar-tensor gravity. Models based on the quadratic relationship between the Hubble parameter and the function of the non-minimal interaction of the scalar field and curvature are also considered. Comparison to observation (PLANCK 2018) data shows that all models under consideration give correct values for the scalar spectral index and tensor-to-scalar ratio under a wide range of exponential-power-law model’s parameters.
      Citation: Universe
      PubDate: 2020-10-29
      DOI: 10.3390/universe6110199
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 200: The Impact of Coronal Mass Ejections on the
           Seasonal Variation of the Ionospheric Critical Frequency f0F2

    • Authors: Hussein M. Farid, Ramy Mawad, Essam Ghamry, Akimasa Yoshikawa
      First page: 200
      Abstract: We investigated the relations between the monthly average values of the critical frequency (f0F2) and the physical properties of the coronal mass ejections (CMEs), then we examined the seasonal variation of f0F2 values as an impact of the several CMEs properties. Given that, f0F2 were detected by PRJ18 (Puerto Rico) ionosonde station during the period 1996–2013. We found that the monthly average values of f0F2 are varying coherently with the sunspot number (SSN). A similar trend was found for f0F2 with the CMEs parameters such as the CME energy (linear correlation coefficient R = 0.73), width (R = 0.6) and the speed (R = 0.6). The arrived CMEs cause a plasma injection into the ionosphere, in turn, increasing the electron density, and consequently, f0F2 values. This happens in the high latitudes followed by the middle and lower latitudes. By examining the seasonal variation of f0F2, we found that the higher correlation between f0F2 and CMEs parameters occurs in the summer, then the equinoxes (spring and autumn), followed by the winter. However, the faster CMEs affect the ionosphere more efficiently in the spring more than in the summer, then the winter and the autumn seasons.
      Citation: Universe
      PubDate: 2020-10-30
      DOI: 10.3390/universe6110200
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 201: Quantum Analysis of BTZ Black Hole Formation
           Due to the Collapse of a Dust Shell

    • Authors: Alexander A. Andrianov, Artem Starodubtsev, Yasser Elmahalawy
      First page: 201
      Abstract: We perform Hamiltonian reduction of a model in which 2 + 1 dimensional gravity with negative cosmological constant is coupled to a cylindrically symmetric dust shell. The resulting action contains only a finite number of degrees of freedom. The phase space consists of two copies of ADS2—both coordinate and momentum space are curved. Different regions in the Penrose diagram can be identified with different patches of ADS2 momentum space. Quantization in the momentum representation becomes particularly simple in the vicinity of the horizon, where one can neglect momentum non-commutativity. In this region, we calculate the spectrum of the shell radius. This spectrum turns out to be continuous outside the horizon and becomes discrete inside the horizon with eigenvalue spacing proportional to the square root of the black hole mass. We also calculate numerically quantum transition amplitudes between different regions of the Penrose diagram in the vicinity of the horizon. This calculation shows a possibility of quantum tunneling of the shell into classically forbidden regions of the Penrose diagram, although with an exponentially damped rate away from the horizon.
      Citation: Universe
      PubDate: 2020-10-30
      DOI: 10.3390/universe6110201
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 202: Conformal Anomaly in Yang-Mills Theory and
           Thermodynamics of Open Confining Strings

    • Authors: Maxim N. Chernodub
      First page: 202
      Abstract: We discuss thermodynamic properties of open confining strings introduced via static sources in the vacuum of Yang-Mills theory. We derive new sum rules for the chromoelectric and chromomagnetic condensates and use them to show that the presence of the confining string lowers the gluonic pressure in the bulk of the system. The pressure deficit of the gluon plasma is related to the potential energy in the system of heavy quarks and anti-quarks in the plasma.
      Citation: Universe
      PubDate: 2020-10-31
      DOI: 10.3390/universe6110202
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 203: Self-Adjoint Extension Approach to Motion of
           Spin-1/2 Particle in the Presence of External Magnetic Fields in the
           Spinning Cosmic String Spacetime

    • Authors: Márcio M. Cunha, Edilberto O. Silva
      First page: 203
      Abstract: In this work, we study the relativistic quantum motion of an electron in the presence of external magnetic fields in the spinning cosmic string spacetime. The approach takes into account the terms that explicitly depend on the particle spin in the Dirac equation. The inclusion of the spin element in the solution of the problem reveals that the energy spectrum is modified. We determine the energies and wave functions using the self-adjoint extension method. The technique used is based on boundary conditions allowed by the system. We investigate the profiles of the energies found. We also investigate some particular cases for the energies and compare them with the results in the literature.
      Citation: Universe
      PubDate: 2020-11-04
      DOI: 10.3390/universe6110203
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 204: Saturation of Energy Levels of the Hydrogen
           Atom in Strong Magnetic Field

    • Authors: Tiago C. Adorno, Dmitry M. Gitman, Anatoly E. Shabad
      First page: 204
      Abstract: We demonstrate that the finiteness of the limiting values of the lower energy levels of a hydrogen atom under an unrestricted growth of the magnetic field, into which this atom is embedded, is achieved already when the vacuum polarization (VP) is calculated in the magnetic field within the approximation of the local action of Euler–Heisenberg. We find that the mechanism for this saturation is different from the one acting, when VP is calculated via the Feynman diagram in the Furry picture. We study the effective potential that appears when the adiabatic (diagonal) approximation is exploited for solving the Schrödinger equation for the longitudinal degree of freedom of the electron on the lowest Landau level in the atom. We find that the (effective) potential of a point-like charge remains nonsingular thanks to the growing screening provided by VP. The regularizing length turns out to be α/3π¯λC, where ¯λC is the electron Compton length. The family of effective potentials, labeled by growing values of the magnetic field condenses towards a certain limiting, magnetic-field-independent potential-distance curve. The limiting values of even ground-state energies are determined for four magnetic quantum numbers using the Karnakov–Popov method.
      Citation: Universe
      PubDate: 2020-11-05
      DOI: 10.3390/universe6110204
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 205: Radiation Problems Accompanying Carrier
           Production by an Electric Field in the Graphene

    • Authors: Sergei P. Gavrilov, Dmitry M. Gitman, Vadim V. Dmitriev, Anatolii D. Panferov, Stanislav A. Smolyansky
      First page: 205
      Abstract: A number of physical processes that occur in a flat one-dimensional graphene structure under the action of strong time-dependent electric fields are considered. It is assumed that the Dirac model can be applied to the graphene as a subsystem of the general system under consideration, which includes an interaction with quantized electromagnetic field. The Dirac model itself in the external electromagnetic field (in particular, the behavior of charged carriers) is treated nonperturbatively with respect to this field within the framework of strong-field QED with unstable vacuum. This treatment is combined with a kinetic description of the radiation of photons from the electron-hole plasma created from the vacuum under the action of the electric field. An interaction with quantized electromagnetic field is described perturbatively. A significant development of the kinetic equation formalism is presented. A number of specific results are derived in the course of analytical and numerical study of the equations. We believe that some of predicted effects and properties of considered processes may be verified experimentally. Among these effects, it should be noted a characteristic spectral composition anisotropy of the quantum radiation and a possible presence of even harmonics of the external field in the latter radiation.
      Citation: Universe
      PubDate: 2020-11-06
      DOI: 10.3390/universe6110205
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 206: Comparison between the Thomas–Fermi and
           Hartree–Fock–Bogoliubov Methods in the Inner Crust of a Neutron Star:
           The Role of Pairing Correlations

    • Authors: Matthew Shelley, Alessandro Pastore
      First page: 206
      Abstract: We investigated the role of a pairing correlation in the chemical composition of the inner crust of a neutron star with the extended Thomas–Fermi method, using the Strutinsky integral correction. We compare our results with the fully self-consistent Hartree–Fock–Bogoliubov approach, showing that the resulting discrepancy, apart from the very low density region, is compatible with the typical accuracy we can achieve with standard mean-field methods.
      Citation: Universe
      PubDate: 2020-11-11
      DOI: 10.3390/universe6110206
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 207: Discerning the Nature of Neutrinos:
           Decoherence and Geometric Phases

    • Authors: Antonio Capolupo, Salvatore Marco Giampaolo, Gaetano Lambiase, Aniello Quaranta
      First page: 207
      Abstract: We present new approaches to distinguish between Dirac and Majorana neutrinos. The first is based on the analysis of the geometric phases associated to neutrinos in matter, the second on the effects of decoherence on neutrino oscillations. In the former we compute the total and geometric phase for neutrinos, and find that they depend on the Majorana phase and on the parametrization of the mixing matrix. In the latter, we show that Majorana neutrinos might violate CPT symmetry, whereas Dirac neutrinos preserve CPT. A phenomenological analysis is also reported showing the possibility to highlight the distinctions between Dirac and Majorana neutrinos.
      Citation: Universe
      PubDate: 2020-11-13
      DOI: 10.3390/universe6110207
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 208: BCS-BEC Crossover Effects and Pseudogap in
           Neutron Matter

    • Authors: David Durel, Michael Urban
      First page: 208
      Abstract: Due to the large neutron–neutron scattering length, dilute neutron matter resembles the unitary Fermi gas, which lies half-way in the crossover from the BCS phase of weakly coupled Cooper pairs to the Bose–Einstein condensate of dimers. We discuss crossover effects in analogy with the T-matrix theory used in the physics of ultracold atoms, which we generalize to the case of a non-separable finite-range interaction. A problem of the standard Nozières–Schmitt-Rink approach and different ways to solve it are discussed. It is shown that in the strong-coupling regime, the spectral function exhibits a pseudo-gap at temperatures above the critical temperature Tc. The effect of the correlated density on the density dependence of Tc is found to be rather weak, but a possibly important effect due to the reduced quasiparticle weight is identified.
      Citation: Universe
      PubDate: 2020-11-13
      DOI: 10.3390/universe6110208
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 209: Modified Newtonian Gravity, Wide Binaries and
           the Tully-Fisher Relation

    • Authors: Luis Acedo
      First page: 209
      Abstract: A recent study of a sample of wide binary star systems from the Hipparcos and Gaia catalogues has found clear evidence of a gravitational anomaly of the same kind as that appearing in galaxies and galactic clusters. Instead of a relative orbital velocity decaying as the square root of the separation, ΔV∝r−1/2, it was shown that an asymptotic constant velocity is reached for distances of order 0.1 pc. If confirmed, it would be difficult to accommodate this breakdown of Kepler’s laws within the current dark matter (DM) paradigm because DM does not aggregate in small scales, so there would be very little DM in a 0.1 pc sphere. In this paper, we propose a simple non-Newtonian model of gravity that could explain both the wide binaries anomaly and the anomalous rotation curves of galaxies as codified by the Tully-Fisher relation. The required extra potential can be understood as a Klein-Gordon field with a position-dependent mass parameter. The extra forces behave as 1/r on parsec scales and r on Solar system scales. We show that retrograde anomalous perihelion precessions are predicted for the planets. This could be tested by precision ephemerides in the near future.
      Citation: Universe
      PubDate: 2020-11-14
      DOI: 10.3390/universe6110209
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 210: Extremal Cosmological Black Holes in
           Horndeski Gravity and the Anti-Evaporation Regime

    • Authors: Ismael Ayuso, Diego Sáez-Chillón Gómez
      First page: 210
      Abstract: Extremal cosmological black holes are analysed in the framework of the most general second order scalar-tensor theory, the so-called Horndeski gravity. Such extremal black holes are a particular case of Schwarzschild-De Sitter black holes that arises when the black hole horizon and the cosmological one coincide. Such metric is induced by a particular value of the effective cosmological constant and is known as Nariai spacetime. The existence of this type of solutions is studied when considering the Horndeski Lagrangian and its stability is analysed, where the so-called anti-evaporation regime is studied. Contrary to other frameworks, the radius of the horizon remains stable for some cases of the Horndeski Lagrangian when considering perturbations at linear order.
      Citation: Universe
      PubDate: 2020-11-17
      DOI: 10.3390/universe6110210
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 211: Kinematics and Selection Rules for
           Light-by-Light Scattering in a Strong Magnetic Field

    • Authors: Anatoly Shabad
      First page: 211
      Abstract: Selection rules that follow from parity and four-momentum conservation are listed for head-on light-by-light scattering in a strong magnetic field taking into account nontrivial dispersion laws of different photon eigen-modes. The wave-length shifts occur for certain transitions between photon eigen-modes.
      Citation: Universe
      PubDate: 2020-11-17
      DOI: 10.3390/universe6110211
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 212: Einstein’s Geometrical versus Feynman’s
           Quantum-Field Approaches to Gravity Physics: Testing by Modern
           Multimessenger Astronomy

    • Authors: Yurij Baryshev
      First page: 212
      Abstract: Modern multimessenger astronomy delivers unique opportunity for performing crucial observations that allow for testing the physics of the gravitational interaction. These tests include detection of gravitational waves by advanced LIGO-Virgo antennas, Event Horizon Telescope observations of central relativistic compact objects (RCO) in active galactic nuclei (AGN), X-ray spectroscopic observations of Fe Kα line in AGN, Galactic X-ray sources measurement of masses and radiuses of neutron stars, quark stars, and other RCO. A very important task of observational cosmology is to perform large surveys of galactic distances independent on cosmological redshifts for testing the nature of the Hubble law and peculiar velocities. Forthcoming multimessenger astronomy, while using such facilities as advanced LIGO-Virgo, Event Horizon Telescope (EHT), ALMA, WALLABY, JWST, EUCLID, and THESEUS, can elucidate the relation between Einstein’s geometrical and Feynman’s quantum-field approaches to gravity physics and deliver a new possibilities for unification of gravitation with other fundamental quantum physical interactions.
      Citation: Universe
      PubDate: 2020-11-18
      DOI: 10.3390/universe6110212
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 213: The Reconstruction of Non-Minimal Derivative
           Coupling Inflationary Potentials

    • Authors: Qin Fei, Zhu Yi, Yingjie Yang
      First page: 213
      Abstract: We derive the reconstruction formulae for the inflation model with the non-minimal derivative coupling term. If reconstructing the potential from the tensor-to-scalar ratio r, we could obtain the potential without using the high friction limit. As an example, we reconstruct the potential from the parameterization r=8α/(N+β)γ, which is a general form of the α-attractor. The reconstructed potential has the same asymptotic behavior as the T- and E-model if we choose γ=2 and α≪1. We also discuss the constraints from the reheating phase by assuming the parameter wre of state equation during reheating is a constant. The scale of big-bang nucleosynthesis could put an upper limit on ns if wre=2/3 and a low limit on ns if wre=1/6.
      Citation: Universe
      PubDate: 2020-11-19
      DOI: 10.3390/universe6110213
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 214: On the Collision of Relativistic Shock Waves
           and the Large Scale Structure of the Universe

    • Authors: Alexander Golubiatnikov, Daniil Lyuboshits
      First page: 214
      Abstract: The solution to the problem of symmetric collision of two relativistic shock waves is given and limiting cases are investigated: Newtonian mechanics and ultrarelativistic mechanics. The results are correlated with the presence of known superclusters and "walls" in the Universe.
      Citation: Universe
      PubDate: 2020-11-20
      DOI: 10.3390/universe6110214
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 215: The Line-of-Sight Analysis of Spatial
           Distribution of Galaxies in the COSMOS2015 Catalogue

    • Authors: Maxim Nikonov, Mikhail Chekal, Stanislav Shirokov, Andrey Baryshev, Vladimir Gorokhov
      First page: 215
      Abstract: New observations of high-redshift objects are crucial for the improvement of the standard ΛCDM cosmological model and our understanding of the Universe. One of the main directions of modern observational cosmology is the analysis of the large-scale structure of Universe, in particular, in deep fields. We study the large-scale structure of the Universe along the line of sight using the latest version of the COSMOS2015 catalogue, which contains 518,404 high quality photometric redshifts of galaxies selected in the optical range of the COSMOS field (2×2 deg2), with depth up to the redshift z∼6. We analyze large-scale fluctuations in the number of galaxies along the line of sight and provide an estimate of the average linear sizes of the self-correlating fluctuations (structures) in independent redshift bins of Δz=0.1 along with the estimate of the standard deviation from homogeneity (the observed cosmic variance). We suggest a new method of the line-of-sight analysis based on previous works and formulate further prospects of method development. For the case of the theoretical form of approximation of homogeneity in the ΛCDM framework, the average standard deviation of detected structures from homogeneity is σmeanΛCDM=0.09±0.02, and the average characteristic size of structures is RmeanΛCDM=790±150 Mpc. For the case of the empirical approximation of homogeneity, the average standard deviation of detected structures from homogeneity is σmeanempiric=0.08±0.01, and the average characteristic size of structures is Rmeanempiric=640±140 Mpc.
      Citation: Universe
      PubDate: 2020-11-20
      DOI: 10.3390/universe6110215
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 216: Density Operator Approach to Turbulent Flows
           in Plasma and Atmospheric Fluids

    • Authors: Konstantin G. Zloshchastiev
      First page: 216
      Abstract: We formulate a statistical wave-mechanical approach to describe dissipation and instabilities in two-dimensional turbulent flows of magnetized plasmas and atmospheric fluids, such as drift and Rossby waves. This is made possible by the existence of Hilbert space, associated with the electric potential of plasma or stream function of atmospheric fluid. We therefore regard such turbulent flows as macroscopic wave-mechanical phenomena, driven by the non-Hermitian Hamiltonian operator we derive, whose anti-Hermitian component is attributed to an effect of the environment. Introducing a wave-mechanical density operator for the statistical ensembles of waves, we formulate master equations and define observables: such as the enstrophy and energy of both the waves and zonal flow as statistical averages. We establish that our open system can generally follow two types of time evolution, depending on whether the environment hinders or assists the system’s stability and integrity. We also consider a phase-space formulation of the theory, including the geometrical-optic limit and beyond, and study the conservation laws of physical observables. It is thus shown that the approach predicts various mechanisms of energy and enstrophy exchange between drift waves and zonal flow, which were hitherto overlooked in models based on wave kinetic equations.
      Citation: Universe
      PubDate: 2020-11-20
      DOI: 10.3390/universe6110216
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 217: Nuclear Response to Second-Order Isospin
           Probes in Connection to Double Beta Decay

    • Authors: Francesco Cappuzzello, Manuela Cavallaro
      First page: 217
      Abstract: One of the key ingredients needed to extract quantitative information on neutrino absolute mass scale from the possible measurement of the neutrinoless double-beta (0νββ) decay half-lives is the nuclear matrix element (NME) characterizing such transitions. NMEs are not physical observables and can only be deduced by theoretical calculations. However, since the atomic nuclei involved in the decay are many-body systems, only approximated values are available to date. In addition, the value of the coupling constants to be used for the weak interaction vertices is still an open question, which introduces a further indetermination in the calculations of NMEs. Several experimental approaches were developed in the years with the aim of providing useful information to further constrain the theory. Here we give an overview of the role of charge exchange reactions in this scenario, focusing on second-order processes, namely the double charge exchange (DCE) reactions.
      Citation: Universe
      PubDate: 2020-11-20
      DOI: 10.3390/universe6110217
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 218: String-Inspired Running Vacuum—The
           “Vacuumon”—And the Swampland Criteria

    • Authors: Nick E. Mavromatos, Joan Solà Peracaula, Spyros Basilakos
      First page: 218
      Abstract: We elaborate further on the compatibility of the “vacuumon potential” that characterises the inflationary phase of the running vacuum model (RVM) with the swampland criteria. The work is motivated by the fact that, as demonstrated recently by the authors, the RVM framework can be derived as an effective gravitational field theory stemming from underlying microscopic (critical) string theory models with gravitational anomalies, involving condensation of primordial gravitational waves. Although believed to be a classical scalar field description, not representing a fully fledged quantum field, we show here that the vacuumon potential satisfies certain swampland criteria for the relevant regime of parameters and field range. We link the criteria to the Gibbons–Hawking entropy that has been argued to characterise the RVM during the de Sitter phase. These results imply that the vacuumon may, after all, admit under certain conditions, a rôle as a quantum field during the inflationary (almost de Sitter) phase of the running vacuum. The conventional slow-roll interpretation of this field, however, fails just because it satisfies the swampland criteria. The RVM effective theory derived from the low-energy effective action of string theory does, however, successfully describe inflation thanks to the ∼H4 terms induced by the gravitational anomalous condensates. In addition, the stringy version of the RVM involves the Kalb–Ramond (KR) axion field, which, in contrast to the vacuumon, does perfectly satisfy the slow-roll condition. We conclude that the vacuumon description is not fully equivalent to the stringy formulation of the RVM. Our study provides a particularly interesting example of a successful phenomenological theory beyond the ΛCDM, such as the RVM, in which the fulfilment of the swampland criteria by the associated scalar field potential, along with its compatibility with (an appropriate form of) the weak gravity conjecture, prove to be insufficient conditions for warranting consistency of the scalar vacuum field representation as a faithful ultraviolet complete representation of the RVM at the quantum gravity level.
      Citation: Universe
      PubDate: 2020-11-20
      DOI: 10.3390/universe6110218
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 219: X-ray Properties of 3C 111: Separation of
           Primary Nuclear Emission and Jet Continuum

    • Authors: Elena Fedorova, B.I. Hnatyk, V.I. Zhdanov, A. Del Popolo
      First page: 219
      Abstract: 3C111 is BLRG with signatures of both FSRQ and Sy1 in X-ray spectrum. The significant X-ray observational dataset was collected for it by INTEGRAL, XMM-Newton, SWIFT, Suzaku and others. The overall X-ray spectrum of 3C 111 shows signs of a peculiarity with the large value of the high-energy cut-off typical rather for RQ AGN, probably due to the jet contamination. Separating the jet counterpart in the X-ray spectrum of 3C 111 from the primary nuclear counterpart can answer the question is this nucleus truly peculiar or this is a fake “peculiarity” due to a significant jet contribution. In view of this question, our aim is to estimate separately the accretion disk/corona and non-thermal jet emission in the 3C 111 X-ray spectra within different observational periods. To separate the disk/corona and jet contributions in total continuum, we use the idea that radio and X-ray spectra of jet emission can be described by a simple power-law model with the same photon index. This additional information allows us to derive rather accurate values of these contributions. In order to test these results, we also consider relations between the nuclear continuum and the line emission.
      Citation: Universe
      PubDate: 2020-11-21
      DOI: 10.3390/universe6110219
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 220: Neutron Star Properties: Quantifying the
           Effect of the Crust–Core Matching Procedure

    • Authors: Márcio Ferreira, Constança Providência
      First page: 220
      Abstract: The impact of the equation of state (EoS) crust-core matching procedure on neutron star (NS) properties is analyzed within a meta-modeling approach. Using a Taylor expansion to parametrize the core equation of state (EoS) and the SLy4 crust EoS, we create two distinct EoS datasets employing two matching procedures. Each EoS describes cold NS matter in a β equilibrium that is thermodynamically stable and causal. It is shown that the crust-core matching procedure affects not only the crust-core transition but also the nuclear matter parameter space of the core EoS, and thus the most probable nuclear matter properties. An uncertainty of as much as 5% (8%) on the determination of low mass NS radii (tidal deformability) is attributed to the complete matching procedure, including the effect on core EoS. By restricting the analysis, imposing that the same set of core EoS is retained in both matching procedures, the uncertainty on the NS radius drops to 3.5% and below 1.5% for 1.9M⊙. Moreover, under these conditions, the crust-core matching procedure has a strong impact on the Love number k2, of almost 20% for 1.0M⊙ stars and 7% for 1.9M⊙ stars, but it shows a very small impact on the tidal deformability Λ, below 1%.
      Citation: Universe
      PubDate: 2020-11-23
      DOI: 10.3390/universe6110220
      Issue No: Vol. 6, No. 11 (2020)
  • Universe, Vol. 6, Pages 161: Predicting >10 MeV SEP Events from Solar
           Flare and Radio Burst Data

    • Authors: Marlon Núñez, Daniel Paul-Pena
      First page: 161
      Abstract: The prediction of solar energetic particle (SEP) events or solar radiation storms is one of the most important problems in the space weather field. These events may have adverse effects on technology infrastructures and humans in space; they may also irradiate passengers and flight crews in commercial aircraft flying at polar latitudes. This paper explores the use of ≥ M2 solar flares and radio burst observations as proxies for predicting >10 MeV SEP events on Earth. These observations are manifestations of the parent event at the sun associated with the SEP event. As a consequence of processing data at the beginning of the physical process that leads to the radiation storm, the model may provide its predictions with large anticipation. The main advantage of the present approach is that the model analyzes solar data that are updated every 30 min and, as such, it may be operational; however, a disadvantage is that those SEP events associated with strong well-connected flares cannot be predicted. For the period from November 1997 to February 2014, we obtained a probability of detection of 70.2%, a false alarm ratio of 40.2%, and an average anticipation time of 9 h 52 min. In this study, the prediction model was built using decision trees, an interpretable machine learning technique. This approach leads to outputs and results comparable to those derived by the Empirical model for Solar Proton Event Real Time Alert (ESPERTA) model. The obtained decision tree shows that the best criteria to differentiate pre-SEP scenarios and non-pre-SEP scenarios are the peak and integrated flux for soft X-ray flares and the radio type III bursts.
      Citation: Universe
      PubDate: 2020-09-28
      DOI: 10.3390/universe6100161
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 162: Cosmogenic Activation in Double Beta Decay

    • Authors: Susana Cebrián
      First page: 162
      Abstract: Double beta decay is a very rare nuclear process and, therefore, experiments intended to detect it must be operated deep underground and in ultra-low background conditions. Long-lived radioisotopes produced by the previous exposure of materials to cosmic rays on the Earth’s surface or even underground can become problematic for the required sensitivity. Here, the studies developed to quantify and reduce the activation yields in detectors and materials used in the set-up of these experiments will be reviewed, considering target materials like germanium, tellurium and xenon together with other ones commonly used like copper, lead, stainless steel or argon. Calculations following very different approaches and measurements from irradiation experiments using beams or directly cosmic rays will be considered for relevant radioisotopes. The effect of cosmogenic activation in present and future double beta decay projects based on different types of detectors will be analyzed too.
      Citation: Universe
      PubDate: 2020-09-29
      DOI: 10.3390/universe6100162
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 163: Non-Relativistic Limit of Embedding Gravity
           as General Relativity with Dark Matter

    • Authors: Sergey Paston
      First page: 163
      Abstract: Regge-Teitelboim embedding gravity is the modified gravity based on a simple string-inspired geometrical principle – our spacetime is considered here as a 4-dimensional surface in a flat bulk. This theory is similar to the recently popular theory of mimetic gravity – the modification of gravity appears in both theories as a result of the change of variables in the action of General Relativity. Embedding gravity, as well as mimetic gravity, can be used in explaining the dark matter mystery since, in both cases, the modified theory can be presented as General Relativity with additional fictitious matter (embedding matter or mimetic matter). For the general case, we obtain the equations of motion of embedding matter in terms of embedding function as a set of first-order dynamical equations and constraints consistent with them. Then, we construct a non-relativistic limit of these equations, in which the motion of embedding matter turns out to be slow enough so that it can play the role of cold dark matter. The non-relativistic embedding matter turns out to have a certain self-interaction, which could be useful in the context of solving the core-cusp problem that appears in the Λ-Cold Dark Matter (ΛCDM) model.
      Citation: Universe
      PubDate: 2020-09-29
      DOI: 10.3390/universe6100163
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 164: Resonant Production of an Ultrarelativistic
           Electron–Positron Pair at the Gamma Quantum Scattering by a Field of the
           X-ray Pulsar

    • Authors: Vadim A. Yelatontsev, Sergei P. Roshchupkin, Viktor V. Dubov
      First page: 164
      Abstract: The process of a resonant production of an ultrarelativistic electron–positron pair in the process of gamma-quantum scattering in the X-ray field of a pulsar is theoretically studied. This process has two reaction channels. Under resonant conditions, an intermediate electron (for a channel A) or a positron (for a channel B) enters the mass shell. As a result, the initial second-order process of the fine-structure constant in the X-ray field effectively splits into two first-order processes: the X-ray field-stimulated Breit–Wheeler process and the the X-ray field-stimulated Compton effect on an intermediate electron or a positron. The resonant kinematics of the process is studied in detail. It is shown that for the initial gamma quantum there is a threshold energy, which for the X-ray photon energy (1–102) keV has the order of magnitude (103–10) MeV. In this case, all the final particles (electron, positron, and final gamma quantum) fly in a narrow cone along the direction of the initial gamma quantum momentum. It is important to note that the energies of the electron–positron pair and the final gamma quantum depend significantly on their outgoing angles. The obtained resonant probability significantly exceeds the non-resonant one. The obtained results can be used to explain the spectrum of positrons near pulsars.
      Citation: Universe
      PubDate: 2020-10-01
      DOI: 10.3390/universe6100164
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 165: Soft Anomalous Dimensions and Resummation in

    • Authors: Nikolaos Kidonakis
      First page: 165
      Abstract: I discuss and review soft anomalous dimensions in QCD that describe soft-gluon threshold resummation for a wide range of hard-scattering processes. The factorization properties of the cross section in moment space and renormalization-group evolution are implemented to derive a general form for differential resummed cross sections. Detailed expressions are given for the soft anomalous dimensions at one, two, and three loops, including some new results, for a large number of partonic processes involving top quarks, electroweak bosons, Higgs bosons, and other particles in the standard model and beyond.
      Citation: Universe
      PubDate: 2020-10-01
      DOI: 10.3390/universe6100165
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 166: Universal Constants and Natural Systems of
           Units in a Spacetime of Arbitrary Dimension

    • Authors: Anton Sheykin, Sergey Manida
      First page: 166
      Abstract: We study the properties of fundamental physical constants using the threefold classification of dimensional constants proposed by J.-M. Lévy-Leblond: constants of objects (masses, etc.), constants of phenomena (coupling constants), and “universal constants” (such as c and ℏ). We show that all of the known “natural” systems of units contain at least one non-universal constant. We discuss the possible consequences of such non-universality, e.g., the dependence of some of these systems on the number of spatial dimensions. In the search for a “fully universal” system of units, we propose a set of constants that consists of c, ℏ, and a length parameter and discuss its origins and the connection to the possible kinematic groups discovered by Lévy-Leblond and Bacry. Finally, we give some comments about the interpretation of these constants.
      Citation: Universe
      PubDate: 2020-10-01
      DOI: 10.3390/universe6100166
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 167: Study on Anisotropic Strange Stars in f(T,T)

    • Authors: Ines G. Salako, M. Khlopov, Saibal Ray, M.Z. Arouko, Pameli Saha, Ujjal Debnath
      First page: 167
      Abstract: In this work, we study the existence of strange stars in the background of f(T,T) gravity in the Einstein spacetime geometry, where T is the torsion tensor and T is the trace of the energy-momentum tensor. The equations of motion are derived for anisotropic pressure within the spherically symmetric strange star. We explore the physical features like energy conditions, mass-radius relations, modified Tolman–Oppenheimer–Volkoff (TOV) equations, principal of causality, adiabatic index, redshift and stability analysis of our model. These features are realistic and appealing to further investigation of properties of compact objects in f(T,T) gravity as well as their observational signatures.
      Citation: Universe
      PubDate: 2020-10-03
      DOI: 10.3390/universe6100167
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 168: Using Unreal Engine to Visualize a
           Cosmological Volume

    • Authors: Christopher Marsden, Francesco Shankar
      First page: 168
      Abstract: In this work we present “Astera’’, a cosmological visualization tool that renders a mock universe in real time using Unreal Engine 4. The large scale structure of the cosmic web is hard to visualize in two dimensions, and a 3D real time projection of this distribution allows for an unprecedented view of the large scale universe, with visually accurate galaxies placed in a dynamic 3D world. The underlying data are based on empirical relations assigned using results from N-Body dark matter simulations, and are matched to galaxies with similar morphologies and sizes, images of which are extracted from the Sloan Digital Sky Survey. Within Unreal Engine 4, galaxy images are transformed into textures and dynamic materials (with appropriate transparency) that are applied to static mesh objects with appropriate sizes and locations. To ensure excellent performance, these static meshes are “instanced’’ to utilize the full capabilities of a graphics processing unit. Additional components include a dynamic system for representing accelerated-time active galactic nuclei. The end result is a visually realistic large scale universe that can be explored by a user in real time, with accurate large scale structure. Astera is not yet ready for public release, but we are exploring options to make different versions of the code available for both research and outreach applications.
      Citation: Universe
      PubDate: 2020-10-06
      DOI: 10.3390/universe6100168
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 169: On the Energy of a Non-Singular Black Hole
           Solution Satisfying the Weak Energy Condition

    • Authors: Irina Radinschi, Theophanes Grammenos, Farook Rahaman, Marius-Mihai Cazacu, Andromahi Spanou, Joydeep Chakraborty
      First page: 169
      Abstract: The energy-momentum localization for a new four-dimensional and spherically symmetric, charged black hole solution that through a coupling of general relativity with non-linear electrodynamics is everywhere non-singular while it satisfies the weak energy condition, is investigated. The Einstein and Møller energy-momentum complexes have been employed in order to calculate the energy distribution and the momenta for the aforesaid solution. It is found that the energy distribution depends explicitly on the mass and the charge of the black hole, on two parameters arising from the space-time geometry considered, and on the radial coordinate. Further, in both prescriptions all the momenta vanish. In addition, a comparison of the results obtained by the two energy-momentum complexes is made, whereby some limiting and particular cases are pointed out.
      Citation: Universe
      PubDate: 2020-10-07
      DOI: 10.3390/universe6100169
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 170: How Extra Symmetries Affect Solutions in
           General Relativity

    • Authors: Aroonkumar Beesham, Fisokuhle Makhanya
      First page: 170
      Abstract: To get exact solutions to Einstein’s field equations in general relativity, one has to impose some symmetry requirements. Otherwise, the equations are too difficult to solve. However, sometimes, the imposition of too much extra symmetry can cause the problem to become somewhat trivial. As a typical example to illustrate this, the effects of conharmonic flatness are studied and applied to Friedmann–Lemaitre–Robertson–Walker spacetime. Hence, we need to impose some symmetry to make the problem tractable, but not too much so as to make it too simple.
      Citation: Universe
      PubDate: 2020-10-09
      DOI: 10.3390/universe6100170
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 171: Singularity Theorems in the Effective Field
           Theory for Quantum Gravity at Second Order in Curvature

    • Authors: Folkert Kuipers, Xavier Calmet
      First page: 171
      Abstract: In this paper, we discuss singularity theorems in quantum gravity using effective field theory methods. To second order in curvature, the effective field theory contains two new degrees of freedom which have important implications for the derivation of these theorems: a massive spin-2 field and a massive spin-0 field. Using an explicit mapping of this theory from the Jordan frame to the Einstein frame, we show that the massive spin-2 field violates the null energy condition, while the massive spin-0 field satisfies the null energy condition, but may violate the strong energy condition. Due to this violation, classical singularity theorems are no longer applicable, indicating that singularities can be avoided, if the leading quantum corrections are taken into account.
      Citation: Universe
      PubDate: 2020-10-10
      DOI: 10.3390/universe6100171
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 172: Hybrid Metric-Palatini Gravity: Regular
           Stringlike Configurations

    • Authors: Kirill Bronnikov, Sergey Bolokhov, Milena Skvortsova
      First page: 172
      Abstract: We discuss static, cylindrically symmetric vacuum solutions of hybrid metric-Palatini gravity (HMPG), a recently proposed theory that has been shown to successfully pass the local observational tests and produce a certain progress in cosmology. We use HMPG in its well-known scalar-tensor representation. The latter coincides with general relativity containing, as a source of gravity, a conformally coupled scalar field ϕ and a self-interaction potential V(ϕ). The ϕ field can be canonical or phantom, and, accordingly, the theory splits into canonical and phantom sectors. We seek solitonic (stringlike) vacuum solutions of HMPG, that is, completely regular solutions with Minkowski metric far from the symmetry axis, with a possible angular deficit. A transition of the theory to the Einstein conformal frame is used as a tool, and many of the results apply to the general Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories as well as f(R) theories of gravity. One of these results is a one-to-one correspondence between stringlike solutions in the Einstein and Jordan frames if the conformal factor that connects them is everywhere regular. An algorithm for the construction of stringlike solutions in HMPG and scalar-tensor theories is suggested, and some examples of such solutions are obtained and discussed.
      Citation: Universe
      PubDate: 2020-10-11
      DOI: 10.3390/universe6100172
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 173: Energy–Momentum Pseudotensor and
           Superpotential for Generally Covariant Theories of Gravity of General Form

    • Authors: Roman Ilin, Sergey Paston
      First page: 173
      Abstract: The current paper is devoted to the investigation of the general form of the energy–momentum pseudotensor (pEMT) and the corresponding superpotential for the wide class of theories. The only requirement for such a theory is the general covariance of the action without any restrictions on the order of derivatives of the independent variables in it or their transformation laws. As a result of the generalized Noether procedure, we obtain a recurrent chain of the equations, which allows one to express canonical pEMT as a divergence of the superpotential. The explicit expression for this superpotential is also given. We discuss the structure of the obtained expressions and the conditions for the derived pEMT conservation laws to be satisfied independently (fully or partially) by the equations of motion. Deformations of the superpotential form for theories with a change in the independent variables in action are also considered. We apply these results to some interesting particular cases: general relativity and its modifications, particularly mimetic gravity and Regge–Teitelboim embedding gravity.
      Citation: Universe
      PubDate: 2020-10-11
      DOI: 10.3390/universe6100173
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 174: Energy Conservation Law in the Closed
           Universe and a Concept of the Proper Time

    • Authors: Natalia Gorobey, Alexander Lukyanenko, Pavel Drozdov
      First page: 174
      Abstract: To define time in the homogeneous anisotropic Bianchi-IX model of the universe, we propose a classical equation of motion of the proper time of the universe as an additional gauge condition. This equation is the law of conservation of energy. As a result, a new parameter, called a “mass” of the universe, appears. This parameter is added to the anisotropy energy and regarded as an observed quantity. The “mass” of the universe is decisive when it comes to the dynamics of its origin.
      Citation: Universe
      PubDate: 2020-10-12
      DOI: 10.3390/universe6100174
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 175: A Test of Gravitational Theories Including
           Torsion with the BepiColombo Radio Science Experiment

    • Authors: Giulia Schettino, Daniele Serra, Giacomo Tommei, Vincenzo Di Pierri
      First page: 175
      Abstract: Within the framework of the relativity experiment of the ESA/JAXA BepiColombo mission to Mercury, which was launched at the end of 2018, we describe how a test of alternative theories of gravity, including torsion can be set up. Following March et al. (2011), the effects of a non-vanishing spacetime torsion have been parameterized by three torsion parameters, t1, t2, and t3. These parameters can be estimated within a global least squares fit, together with a number of parameters of interest, such as post-Newtonian parameters γ and β, and the orbits of Mercury and the Earth. The simulations have been performed by means of the ORBIT14 orbit determination software, which was developed by the Celestial Mechanics Group of the University of Pisa for the analysis of the BepiColombo radio science experiment. We claim that the torsion parameters can be determined by means of the relativity experiment of BepiColombo at the level of some parts in 10−4, which is a significant result for constraining gravitational theories that allow spacetime torsion.
      Citation: Universe
      PubDate: 2020-10-12
      DOI: 10.3390/universe6100175
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 176: The Functional Schrödinger Equation in the
           Semiclassical Limit of Quantum Gravity with a Gaussian Clock Field

    • Authors: Marcello Rotondo
      First page: 176
      Abstract: We derive the functional Schrödinger equation for quantum fields in curved spacetime in the semiclassical limit of quantum geometrodynamics with a Gaussian incoherent dust acting as a clock field. We perform the semiclassical limit using a WKB-type expansion of the wave functional in powers of the squared Planck mass. The functional Schrödinger equation that we obtain exhibits a functional time derivative that completes the usual definition of WKB time for curved spacetime, and the usual Schrödinger-type evolution is recovered in Minkowski spacetime.
      Citation: Universe
      PubDate: 2020-10-13
      DOI: 10.3390/universe6100176
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 177: Relativistic Effects in Orbital Motion of the
           S-Stars at the Galactic Center

    • Authors: Rustam Gainutdinov, Yurij Baryshev
      First page: 177
      Abstract: The Galactic Center star cluster, known as S-stars, is a perfect source of relativistic phenomena observations. The stars are located in the strong field of relativistic compact object Sgr A* and are moving with very high velocities at pericenters of their orbits. In this work we consider motion of several S-stars by using the Parameterized Post-Newtonian (PPN) formalism of General Relativity (GR) and Post-Newtonian (PN) equations of motion of the Feynman’s quantum-field gravity theory, where the positive energy density of the gravity field can be measured via the relativistic pericenter shift. The PPN parameters β and γ are constrained using the S-stars data. The positive value of the Tg00 component of the gravity energy–momentum tensor is confirmed for condition of S-stars motion.
      Citation: Universe
      PubDate: 2020-10-14
      DOI: 10.3390/universe6100177
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 178: Matter Accretion Versus Semiclassical Bounce
           in Schwarzschild Interior

    • Authors: Kirill Bronnikov, Sergey Bolokhov, Milena Skvortsova
      First page: 178
      Abstract: We discuss the properties of the previously constructed model of a Schwarzschild black hole interior where the singularity is replaced by a regular bounce, ultimately leading to a white hole. We assume that the black hole is young enough so that the Hawking radiation may be neglected. The model is semiclassical in nature and uses as a source of gravity the effective stress-energy tensor (SET) corresponding to vacuum polarization of quantum fields, and the minimum spherical radius is a few orders of magnitude larger than the Planck length, so that the effects of quantum gravity should still be negligible. We estimate the other quantum contributions to the effective SET, caused by a nontrivial topology of spatial sections and particle production from vacuum due to a nonstationary gravitational field and show that these contributions are negligibly small as compared to the SET due to vacuum polarization. The same is shown for such classical phenomena as accretion of different kinds of matter to the black hole and its further motion to the would-be singularity. Thus, in a clear sense, our model of a semiclassical bounce instead of a Schwarzschild singularity is stable under both quantum and classical perturbations.
      Citation: Universe
      PubDate: 2020-10-14
      DOI: 10.3390/universe6100178
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 179: The Higgs Mechanism and Spacetime Symmetry

    • Authors: Irina Dymnikova
      First page: 179
      Abstract: In this review, we summarize the results of the analysis of the inherent relation between the Higgs mechanism and spacetime symmetry provided by generic incorporation of the de Sitter vacuum as a false vacuum with the equation of state p=−ρ. This relation has been verified by the application for the interpretation of the experimental results on the negative mass squares for neutrinos, and of the appearance of the minimal length in the annihilation reaction e+e−→γγ(γ). An additional verification is expected for the dark matter candidates with the interior de Sitter vacuum of the GUT scale, whose predicted observational signatures include the induced proton decay in the matter of an underground detector, such as IceCUBE.
      Citation: Universe
      PubDate: 2020-10-15
      DOI: 10.3390/universe6100179
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 180: An Alternative to Dark Matter and Dark
           Energy: Scale-Dependent Gravity in Superfluid Vacuum Theory

    • Authors: Konstantin G. Zloshchastiev
      First page: 180
      Abstract: We derive an effective gravitational potential, induced by the quantum wavefunction of a physical vacuum of a self-gravitating configuration, while the vacuum itself is viewed as the superfluid described by the logarithmic quantum wave equation. We determine that gravity has a multiple-scale pattern, to such an extent that one can distinguish sub-Newtonian, Newtonian, galactic, extragalactic and cosmological terms. The last of these dominates at the largest length scale of the model, where superfluid vacuum induces an asymptotically Friedmann–Lemaître–Robertson–Walker-type spacetime, which provides an explanation for the accelerating expansion of the Universe. The model describes different types of expansion mechanisms, which could explain the discrepancy between measurements of the Hubble constant using different methods. On a galactic scale, our model explains the non-Keplerian behaviour of galactic rotation curves, and also why their profiles can vary depending on the galaxy. It also makes a number of predictions about the behaviour of gravity at larger galactic and extragalactic scales. We demonstrate how the behaviour of rotation curves varies with distance from a gravitating center, growing from an inner galactic scale towards a metagalactic scale: A squared orbital velocity’s profile crosses over from Keplerian to flat, and then to non-flat. The asymptotic non-flat regime is thus expected to be seen in the outer regions of large spiral galaxies.
      Citation: Universe
      PubDate: 2020-10-15
      DOI: 10.3390/universe6100180
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 181: Quantum Vacuum Effects in Braneworlds on AdS

    • Authors: Aram A. Saharian
      First page: 181
      Abstract: We review the results of investigations for brane-induced effects on the local properties of quantum vacuum in background of AdS spacetime. Two geometries are considered: a brane parallel to the AdS boundary and a brane intersecting the AdS boundary. For both cases, the contribution in the vacuum expectation value (VEV) of the energy–momentum tensor is separated explicitly and its behavior in various asymptotic regions of the parameters is studied. It is shown that the influence of the gravitational field on the local properties of the quantum vacuum is essential at distance from the brane larger than the AdS curvature radius. In the geometry with a brane parallel to the AdS boundary, the VEV of the energy–momentum tensor is considered for scalar field with the Robin boundary condition, for Dirac field with the bag boundary condition and for the electromagnetic field. In the latter case, two types of boundary conditions are discussed. The first one is a generalization of the perfect conductor boundary condition and the second one corresponds to the confining boundary condition used in QCD for gluons. For the geometry of a brane intersecting the AdS boundary, the case of a scalar field is considered. The corresponding energy–momentum tensor, apart from the diagonal components, has nonzero off-diagonal component. As a consequence of the latter, in addition to the normal component, the Casimir force acquires a component parallel to the brane.
      Citation: Universe
      PubDate: 2020-10-15
      DOI: 10.3390/universe6100181
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 182: Search for Double Beta Decay of 106Cd with an
           Enriched 106CdWO4 Crystal Scintillator in Coincidence with CdWO4
           Scintillation Counters

    • Authors: Pierluigi Belli, R. Bernabei, V.B. Brudanin, F. Cappella, V. Caracciolo, R. Cerulli, F. A. Danevich, Antonella Incicchitti, D.V. Kasperovych, V.R. Klavdiienko, V.V. Kobychev, Vittorio Merlo, O.G. Polischuk, V.I. Tretyak, M.M. Zarytskyy
      First page: 182
      Abstract: Studies on double beta decay processes in 106Cd were performed by using a cadmium tungstate scintillator enriched in 106Cd at 66% (106CdWO4) with two CdWO4 scintillation counters (with natural Cd composition). No effect was observed in the data that accumulated over 26,033 h. New improved half-life limits were set on the different channels and modes of the 106Cd double beta decay at level of limT1/2∼1020−1022 yr. The limit for the two neutrino electron capture with positron emission in 106Cd to the ground state of 106Pd, T1/22νECβ+≥2.1×1021 yr, was set by the analysis of the 106CdWO4 data in coincidence with the energy release 511 keV in both CdWO4 counters. The sensitivity approaches the theoretical predictions for the decay half-life that are in the range T1/2∼1021−1022 yr. The resonant neutrinoless double-electron capture to the 2718 keV excited state of 106Pd is restricted at the level of T1/20ν2K≥2.9×1021 yr.
      Citation: Universe
      PubDate: 2020-10-16
      DOI: 10.3390/universe6100182
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 183: Null and Timelike Geodesics near the Throats
           of Phantom Scalar Field Wormholes

    • Authors: Ivan Potashov, Julia Tchemarina, Alexander Tsirulev
      First page: 183
      Abstract: We study geodesic motion near the throats of asymptotically flat, static, spherically symmetric traversable wormholes supported by a self-gravitating minimally coupled phantom scalar field with an arbitrary self-interaction potential. We assume that any such wormhole possesses the reflection symmetry with respect to the throat, and consider only its observable “right half”. It turns out that the main features of bound orbits and photon trajectories close to the throats of such wormholes are very different from those near the horizons of black holes. We distinguish between wormholes of two types, the first and second ones, depending on whether the redshift metric function has a minimum or maximum at the throat. First, it turns out that orbits located near the centre of a wormhole of any type exhibit retrograde precession, that is, the angle of pericentre precession is negative. Second, in the case of high accretion activity, wormholes of the first type have the innermost stable circular orbit at the throat while those of the second type have the resting-state stable circular orbit in which test particles are at rest at all times. In our study, we have in mind the possibility that the strongly gravitating objects in the centres of galaxies are wormholes, which can be regarded as an alternative to black holes, and the scalar field can be regarded as a realistic model of dark matter surrounding galactic centres. In this connection, we discuss qualitatively some observational aspects of results obtained in this article.
      Citation: Universe
      PubDate: 2020-10-16
      DOI: 10.3390/universe6100183
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 184: Gravitational Interaction of Cosmic String
           with Spinless Particle

    • Authors: Pavel Spirin
      First page: 184
      Abstract: We consider the gravitational interaction of spinless relativistic particle and infinitely thin cosmic string within the classical linearized-theory framework. We compute the particle’s motion in the transverse (to the unperturbed string) plane. The reciprocal action of the particle on the cosmic string is also investigated. We derive the retarded solution which includes the longitudinal (with respect to the unperturbed-particle motion) and totally-transverse string perturbations.
      Citation: Universe
      PubDate: 2020-10-16
      DOI: 10.3390/universe6100184
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 185: On the Discrete Version of the Schwarzschild

    • Authors: Vladimir Khatsymovsky
      First page: 185
      Abstract: We consider a Schwarzschild type solution in the discrete Regge calculus formulation of general relativity quantized within the path integral approach. Earlier, we found a mechanism of a loose fixation of the background scale of Regge lengths. This elementary length scale is defined by the Planck scale and some free parameter of such a quantum extension of the theory. Besides, Regge action was reduced to an expansion over metric variations between the tetrahedra and, in the main approximation, is a finite-difference form of the Hilbert–Einstein action. Using for the Schwarzschild problem a priori general non-spherically symmetrical ansatz, we get finite-difference equations for its discrete version. This defines a solution which at large distances is close to the continuum Schwarzschild geometry, and the metric and effective curvature at the center are cut off at the elementary length scale. Slow rotation can also be taken into account (Lense–Thirring-like metric). Thus, we get a general approach to the classical background in the quantum framework in zero order: it is an optimal starting point for the perturbative expansion of the theory, finite-difference equations are classical, and the elementary length scale has quantum origin. Singularities, if any, are resolved.
      Citation: Universe
      PubDate: 2020-10-17
      DOI: 10.3390/universe6100185
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 186: Collapsing Wormholes Sustained by Dustlike

    • Authors: Pavel E. Kashargin, Sergey V. Sushkov
      First page: 186
      Abstract: It is well known that static wormhole configurations in general relativity (GR) are possible only if matter threading the wormhole throat is “exotic”—i.e., violates a number of energy conditions. For this reason, it is impossible to construct static wormholes supported only by dust-like matter which satisfies all usual energy conditions. However, this is not the case for non-static configurations. In 1934, Tolman found a general solution describing the evolution of a spherical dust shell in GR. In this particular case, Tolman’s solution describes the collapsing dust ball; the inner space-time structure of the ball corresponds to the Friedmann universe filled by a dust. In the present work we use the general Tolman’s solution in order to construct a dynamic spherically symmetric wormhole solution in GR with dust-like matter. The solution constructed represents the collapsing dust ball with the inner wormhole space-time structure. It is worth noting that, with the dust-like matter, the ball is made of satisfies the usual energy conditions and cannot prevent the collapse. We discuss in detail the properties of the collapsing dust wormhole.
      Citation: Universe
      PubDate: 2020-10-18
      DOI: 10.3390/universe6100186
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 187: Gravity with Higher Derivatives in

    • Authors: Sergey G. Rubin, Arkadiy Popov, Polina M. Petriakova
      First page: 187
      Abstract: The aim of this review is to discuss the ways to obtain results based on gravity with higher derivatives in D-dimensional world. We considered the following ways: (1) reduction to scalar tensor gravity, (2) direct solution of the equations of motion, (3) derivation of approximate equations in the presence of a small parameter in the system, and (4) the method of test functions. Some applications are presented to illustrate each method. The unification of two necessary elements of a future theory is also kept in mind—the extra dimensions and the extended form of the gravity.
      Citation: Universe
      PubDate: 2020-10-20
      DOI: 10.3390/universe6100187
      Issue No: Vol. 6, No. 10 (2020)
  • Universe, Vol. 6, Pages 188: Gravitational Radiation as the Bremsstrahlung
           of Superheavy Particles in the Early Universe

    • Authors: Andrey A. Grib, Yuri V. Pavlov
      First page: 188
      Abstract: The number of superheavy particles with the mass of the Grand Unification scale with trans-Planckian energy created at the epoch of superheavy particle creation from the vacuum by the gravitation of the expanding Universe is calculated. In later collisions of these particles, gravitational radiation is radiated playing the role of bremsstrahlung for gravity. The effective background radiation of the Universe is evaluated.
      Citation: Universe
      PubDate: 2020-10-20
      DOI: 10.3390/universe6100188
      Issue No: Vol. 6, No. 10 (2020)
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