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Please help us test our new pre-print finding feature by giving the pre-print link a rating. A 5 star rating indicates the linked pre-print has the exact same content as the published article.

Abstract: We investigate the Barrow holographic dark energy (BHDE) model using IR cutoff as the Hubble horizon in the background of a flat Friedmann–Lemaître–Robertson–Walker universe. The deceleration parameter exhibits the universe evolution from decelerated to accelerated phase. The EoS parameter of the BHDE model presents a nice behavior as it lies in the phantom era ( \(\omega_{B}<-1\) ) and the quintessence era ( \(\omega_{B}\geq-1\) ) for different values of the Barrow parameter \(\Delta\) . The squared sound velocity \(v_{s}^{2}\) has been investigated for the stability of the model. In addition, a correspondence with the phantom and quintessence scalar fields has been studied for the model, which helps us to describe the accelerated expansion of the universe. PubDate: 2022-03-01

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Abstract: Based on the previously formulated mathematical model of a statistical system with scalar interaction of fermions, a cosmological model is studied, based on a one-component statistical system of doubly scalarly charged degenerate fermions interacting with an asymmetric scalar doublet of canonical and phantom scalar fields. A relationship of the presented model with previously studied models based on one-component and two-component fermion systems is investigated. The asymptotic and limiting properties of the cosmological model are investigated, it is shown that among all models there is a class of models with finite lifetime. The asymptotic behavior of models near the corresponding singularities is investigated, a qualitative analysis of the corresponding dynamical system is carried out, and numerical implementations of such models are constructed. Based on numerical integration, it is shown that in the present model there can be transitions from a stable asymptotically vacuum state with a zero canonical field and a constant phantom field, corresponding to the phase of cosmological compression, to a symmetric state corresponding to an expansion phase. The time interval of the transition between these phases is accompanied by oscillations of the canonical scalar field. PubDate: 2022-03-01

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Abstract: A scalar potential obtained from the \(D\) -term in the Supergravity models, which dominates over the \(F\) term and is mainly responsible for the inflationary phase in the early universe, is studied. The potential with canonical kinetic terms for scalar fields in the Lagrangian has a very slow roll feature in comparison with various other plateau type inflationary potentials. In this case, a much lower tensor-to-scalar ratio ( \(r\) ) of \(\mathcal{O}(10^{-3})\) is achievable. The requirement of the slow roll condition for the inflation potential implies that the up type neutral scalar and the down type neutral scalar in Supergravity models are with equal field strength at the time of inflation. If this relationship holds down to the electroweak scale for the corresponding vacuum expectation values of these fields, then it will indicate a higher SUSY breaking scale around 100 TeV. The predicted values of the inflationary observables are well within the 1- \(\sigma\) bounds of the recent constraints from Planck’18 observations. The era of reheating after the inflationary phase is also studied, and the bounds on the reheating temperature ( \(T_{\textrm{re}}\) ) is calculated for different equations of states during reheating ( \(w_{\textrm{re}}\) ) for the Planck’18 allowed values of the scalar spectral index ( \(n_{s}\) ). For our model with \(w_{\textrm{re}}=2/3\) and \(w_{\textrm{re}}=1\) , after satisfying all the bounds due to gravitino overproduction, we can have a large parameter space for \(T_{\textrm{re}}\) , which is well inside the Planck’18 1- \(\sigma\) bound on \(n_{s}\) . PubDate: 2022-03-01

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Abstract: We propose and consider cosmological models developing from the infinitely distant past ( \(t\to-\infty\) ), that is, which have always existed. The material basis that provides the fundamental properties of the Universe evolution in these cosmological models is a fundamental nonlinear Dirac spinor field \(\psi(x^{k})\) with the nonlinearity of the type \(\left(\overline{\psi}\psi\right)^{n}\) in the Lagrangian ( \(n\) is a rational number). Other possible components of matter in the form of a perfect fluid with the equation of state \(p=w\varepsilon\) ( \(-1\leq w\leq 1\) ) are also taken into account. The 3-dimensional spatial section in all proposed cosmological models is Euclidean, in accordance with the established astronomical data. PubDate: 2022-03-01

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Abstract: We study a conformally flat charged static spherically symmetric space-time for a spinning fluid within the Einstein–Cartan theory. We also discuss the physical interpretation of a conformally flat, static solution to the Einstein–Maxwell equations. We have obtained an exact interior solution and investigated the pressure, spin-torsion and energy density assuming a pure charge condition, and then for a specific charge distribution. PubDate: 2022-03-01

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Abstract: Based on the previously formulated mathematical model of a statistical system with scalar interaction of fermions and the theory of gravitational-scalar instability of a cosmological model with a two-component statistical system of scalarly charged degenerate fermions, a numerical model of the cosmological evolution of gravitational-scalar perturbations is constructed, and specific examples of the development of instability are given. Some features of the instability development are investigated, depending on the behavior of the unperturbed cosmological model. It is shown that unstable modes can appear at very early stages of cosmological expansion or contraction, and the duration of the unstable phase is comparable to tens of Planck scales. In this case, however, a very significant increase in unstable modes is possible due to redistribution of energy between the components of the scalar doublet. PubDate: 2022-03-01

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Abstract: The paper discusses the dynamics of cylindrical thin shell wormholes within the context of \(f(R)\) gravity by joining two identical copies of metric space-time using the cut-and-paste approach. The stability of cylindrical wormholes is analyzed through the linear radial perturbation about the static solution and the variable modified generalized Chaplygin gas equation of state. This formalism is applied to two specific different configurations of \(f(R)\) gravity, such as a quadratic type model and a combined quadratic-cubic type model. We reveal the presence of both stable and unstable regions, depending on the appropriate values of different parameters involved in the variable equation of state and \(f(R)\) gravity models as well as the metric space-time. Further, the existence of charge and cosmological constant parameters in the metric as well as dark source parameters seems to enlarge the stability regions. PubDate: 2022-03-01

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Abstract: The accelerated expansion of the Universe constitutes one of the biggest challenges in present-day cosmology. To understand and explain this phenomenon in the framework of general relativity, corrections and extensions to it are required, which make the so-called extended theories of gravity (ETGs). In these theories, the geometry of space-time that represents the gravitational sector at the left hand side of the Einstein field equation \({G_{\mu\nu}}=8\pi{T_{\mu\nu}}\) is necessarily modified. These theories have attracted much attention since the time the accelerated expansion was discovered. A class of these theories known as \(f(R)\) gravity, offers a potent candidacy for this purpose, in addition to matter content modifications. The gravitational sector depending on the Ricci scalar invariant \(R\) is basically replaced with some its general nonlinear function which consists of higher-order curvature terms. In this work, we attempt to realize the late-time accelerated expansion in the context of \(f(R)\) gravity using the dynamical system approach. Analyzing the dynamical system arising from a particular \(f(R)\) model, its stability is studied for the cosmological inferences. The particular model \(f(R)={R^{p}}\exp({qR})\) with \(m=\frac{{R{f_{,RR}}}}{{{f_{,R}}}}=\frac{{p(p-1)+2pqR+{q^{2}}{R^{2}}}}{{p+qR}}\) and \(r=-\frac{{R{f_{,R}}}}{f}=-(p+qR)\) and with the geometric curve \(m(r)=-\frac{{{r^{2}}-p}}{r}\) , is studied in this paper. We use the geometric approach for the curve \(m(r)\) in the plane \((r,m)\) which provides some properties of the model. In the case of a matter-dominated era the viability conditions at \(r=-1\) , \(m(r)=0\) and \(dm/dr>-1\) are investigated. On the other hand, for the late-time acceleration, however, at \(r=-2\) , either of the two conditions \(m(r)=-r-1\) with \(dm/dr<-1\) , \(1\geq m>(\sqrt{3}-1)/2\) and \(1\geq m\geq 0\) are sought to fulfill. In the first place, the cosmic content is assumed to comprise matter and radiation only in the absence of a cosmological constant \(\Lambda\) . In this case, an interaction of any kind is disregarded. Afterwards, as the second consideration, an interaction term in the presence of a cosmological constant representing dark energy is taken into account. The effects of linear and nonlinear interactions between matter and dark energy are also taken into account orderly in this case. The results are presented for each case, along with a discussion of critical points, their eigenvalues, and the equation of state parameter. PubDate: 2022-03-01

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Abstract: In their Letter [Phys. Rev. Lett. 126, 101102 (2021); arXiv: 2010.07317], J.L. Blázquez-Salcedo, C. Knoll, and E. Radu have constructed a very interesting class of wormhole solutions in general relativity (GR), supported by a pair of classical charged spinor fields obeying the Dirac equation. The main new feature of these solutions is that such Dirac spinor fields can possess exotic properties, necessary for the existence of static wormhole configurations in GR. The present note contains a few remarks clarifying some points concerning this approach. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040034

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Abstract: We study gravitational baryogenesis in an isotropic and homogeneous universe in the framework of general relativity. We investigate a new exact solution of Einstein’s field equations for the FRW metric. Our solution represents a transitioning model of the universe which was expanding in a decelerated mode and passes on to an accelerated mode after dominance of the cosmological constant \(\Lambda\) . We observe that gravitational baryogenesis occurs in the derived universe, and the derived baryon-entropy ratio is in good agreement with its observational value. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040137

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Abstract: A method has been developed for precision measurement of the gravitational frequency shift of communication radio signals between a spacecraft and a ground tracking station, based on the maximum likelihood principle, using the Rao-Cramer limit estimates for the kinematic parameters associated with orbital motion. Numerical illustrations of the efficiency of the method are presented using the example of data obtained in experiments with the Spektr-R satellite as part of the VLBI system in the Radioastron mission. A compensatory “on-line” technique for suppressing Doppler and atmospheric noise has been implemented due to the presence of two modes of communication in gravity sessions: unidirectional (1w) and looped (2w). Recipes for reducing the magnitude of systematic errors are discussed. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040022

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Abstract: The mechanism of generation of dark matter and dark radiation from the evaporation of primordial black holes is very interesting. We consider the case of Kerr black holes to generalize previous results obtained in the Schwarzschild case. For dark matter, the results do not change dramatically, and the bounds on warm dark matter apply similarly: in particular, the Kerr case cannot save the scenario of black hole domination for light dark matter. For dark radiation, the expectations for \(\Delta N_{\textrm{eff}}\) do not change significantly with respect to the Schwarzschild case, but for an enhancement in the case of spin 2 particles: in the massless case, however, the projected experimental sensitivity would be reached only for extremal black holes. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040101

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Abstract: The flows of phase trajectories in cosmological models based on the asymmetric scalar Higgs doublet are investigated. It is shown that phase flows tend to split into several diverging flows. In this case, initially close phase trajectories can diverge significantly over time, which indicates the instability of the model with respect to finite perturbations. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040071

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Abstract: We present exact solutions to the Friedmann equation in standard \(\Lambda\) CDM cosmology in Weierstrass and Jacobi functions. The right-hand side of the Friedmann equation, describing various contributions of matter sources, is considered in a generic form. It is proved that the problem of integration of the Friedmann equation for simple equations of state of a medium is reduced to solving Abel integrals for algebraic functions. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040113

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Abstract: Nonlinear quantization of the domain wall (relativistic membrane of codimension 1) is considered. The membrane dust equation is considered as an analogue of the Hamilton–Jacobi equation, which allows us to construct its quantum analogue. The resulting equation has the form of a nonlinear Klein–Fock–Gordon equation. It can be interpreted as the mean field approximation for a quantum domain wall. Dispersion relations are obtained for small perturbations (in a linear approximation). The group speed of perturbations does not exceed the speed of light. For perturbations propagating along the domain wall, in addition to the massless mode (as in the classical case), a massive one appears. The result may be interesting in condensed matter theory and in membrane quantization in superstring and supergravity theories. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040083

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Abstract: We discuss vacuum static, spherically symmetric asymptotically flat solutions of the generalized hybrid metric-Palatini theory of gravity (generalized HMPG) suggested by Böhmer and Tamanini, involving both a metric \(g_{\mu\nu}\) and an independent connection \(\hat{\Gamma}_{\mu\nu}^{\alpha}\) ; the gravitational field Lagrangian is an arbitrary function \(f(R,P)\) of two Ricci scalars, \(R\) obtained from \(g_{\mu\nu}\) and \(P\) obtained from \({\hat{\Gamma}}_{\mu\nu}^{\alpha}\) . The theory admits a scalar-tensor representation with two scalars \(\phi\) and \(\xi\) and a potential \(V(\phi,\xi)\) whose form depends on \(f(R,P)\) . Solutions are obtained in the Einstein frame and transferred back to the original Jordan frame for a proper interpretation. In the completely studied case \(V\equiv 0\) , generic solutions contain naked singularities or describe traversable wormholes, and only some special cases represent black holes with extremal horizons. For \(V(\phi,\xi)\neq 0\) , some examples of analytical solutions are obtained and shown to possess naked singularities. Even in the cases where the Einstein-frame metric \(g^{E}_{\mu\nu}\) is found analytically, the scalar field equations need a numerical study, and if \(g^{E}_{\mu\nu}\) contains a horizon, in the Jordan frame it turns to a singularity due to the corresponding conformal factor. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040046

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Abstract: Based on the mathematical model of a statistical system with scalar interaction of fermions, formulated earlier, a cosmological model based on a two-component statistical system of scalarly charged degenerate fermions interacting with an asymmetric scalar doublet of canonical and phantom scalar fields, is investigated. The asymptotic and limiting properties of the cosmological model are investigated, and it is shown that amon g all models there is a class of those with finite lifetime. The asymptotic behavior of the models near the corresponding singularities is investigated, and numerical implementations of such models are constructed. PubDate: 2021-10-01 DOI: 10.1134/S020228932104006X

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Abstract: We consider the metric \(f(R)\) gravity model for the four-dimensional metric tensor depending on two variables, time and one spacelike coordinate. We obtain exact analytical vacuum solutions for different forms of the function \(f(R)\) , which are solutions of cosmological type. These solutions are expressed in terms of arbitrary functions, which, under certain conditions, can be chosen as new variables. PubDate: 2021-10-01 DOI: 10.1134/S0202289321040125