arXiv subjects

Artyom V. Astashenok

Publications and source records attributed to Artyom V. Astashenok.

36 records · Page 2Linked to original sources

arXiv2017

The realistic models of relativistic stars in f(R) = R + alpha R^2 gravity

In the context of f(R)=R + alpha R^2 gravity, we study the existence of neutron and quark stars with no intermediate approximations in the generalised system of Tolman-Oppenheimer-Volkov equations. Analysis shows that for positive alpha's the scalar curvature does not drop to zero at the star surface (as in General Relativity) but exponentially decreases with distance. Also the stellar mass bounded by star surface decreases when the value alpha increases. Nonetheless distant observers would observe a gravitational mass due to appearance of a so-called gravitational sphere around the star. The non-zero curvature contribution to the gravitational mass eventually is shown to compensate the stellar mass decrease for growing alpha's. We perform our analysis for several equations of state including purely hadronic configurations as well as hyperons and quark stars. In all cases, we assess that the relation between the parameter $α$ and the gravitational mass weakly depend upon the chosen equation of state. Another interesting feature is the increase of the star radius in comparison to General Relativity for stars with masses close to maximal, whereas for intermediate masses around 1.4-1.6 solar masses, the radius of star depends upon alpha very weakly. Also the decrease in the mass bounded by star surface may cause the surface redshift to decrease in R^2-gravity when compared to Einsteinian predictions. This effect is shown to hardly depend upon the observed gravitational mass. Finally, for negative values of alpha our analysis shows that outside the star the scalar curvature has damped oscillations but the contribution of the gravitational sphere into the gravitational mass increases indefinitely with radial distance putting into question the very existence of such relativistic stars.

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arXiv2015

Modified Gauss-Bonnet gravity with Lagrange multiplier constraint as mimetic theory

In this paper we propose and extensively study mimetic $f({\cal G})$ modified gravity models, with various scenarios of cosmological evolution, with or without extra matter fluids. The easiest formulation is based on the use of Lagrange multiplier constraint. In certain versions of this theory, it is possible to realize accelerated expansion of the Universe or even unified evolution which includes inflation with dark energy, and at the same time in the same theoretical framework, dark matter is described by the theory. This is achieved by the re-parametrization of the metric tensor, which introduces a new degree of freedom in the cosmological equations and leads to appearance of the mimetic "dark matter" component. In the context of mimetic $f({\cal G})$ theory, we also provide some quite general reconstruction schemes, which enable us to find which $f({\cal G})$ gravity generates a specific cosmological evolution. In addition, we also provide the general reconstruction technique for Lagrange multiplier $f({\cal G})$ gravity. All our results are accompanied by illustrative examples, with special emphasis on bouncing cosmologies.

gr-qc↗

arXiv2014

Extreme neutron stars from Extended Theories of Gravity

We discuss neutron stars with strong magnetic mean fields in the framework of Extended Theories of Gravity. In particular, we take into account models derived from $f(R)$ and $f(\cal G)$ extensions of General Relativity where functions of the Ricci curvature invariant $R$ and the Gauss-Bonnet invariant ${\cal G}$ are respectively considered. Dense matter in magnetic mean field, generated by magnetic properties of particles, is described by assuming a model with three meson fields and baryons octet. As result, the considerable increasing of maximal mass of neutron stars can be achieved by cubic corrections in $f(R)$ gravity. In principle, massive stars with $M> 4 M_{\odot}$ can be obtained. On the other hand, stable stars with high strangeness fraction (with central densities $ρ_{c}\sim 1.5-2.0$ GeV/fm$^{3}$) are possible considering quadratic corrections of $f(\cal {G})$ gravity. The magnetic field strength in the star center is of order $6-8\times 10^{18}$ G. In general, we can say that other branches of massive neutron stars are possible considering the extra pressure contributions coming from gravity extensions. Such a feature can constitute both a probe for alternative theories and a way out to address anomalous self-gravitating compact systems.

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arXiv2014

The possible resolution of Boltzmann brains problem in phantom cosmology

We consider the well-known Boltzmann brains problem in frames of simple phantom energy models with little rip and big rip singularity. It is showed that these models (i) satisfy to observational data and (ii) may be free from Boltzmann brains problem. The human observers in phantom models can exist only in during for a certain period $t<t_{f}$ ($t_{f}$ is lifetime of universe) via Bekenstein bound. If fraction of unordered observers in this part of universe history is negligible in comparison with ordered observers than Boltzmann brains problem doesn't appear. The bounds on model parameters derived from such requirement don't contradict to allowable range from observational data.

astro-ph.CO↗

arXiv2014

Maximal neutron star mass and the resolution of hyperon puzzle in modified gravity

The so-called hyperon puzzle in the theory of neutron stars is considered in the framework of modified $f(R)$ gravity. We show that for simple hyperon equations of state, it is possible to obtain the maximal neutron star mass which satisfies the recent observational data for PSR J1614-2230, in higher-derivative models with power-law terms as $f(R) = R+αR^2+ βR^3$. The soft hyperon equation of state under consideration is usually treated as non-realistic in the standard General Relativity. The numerical analysis of Mass-Radius relation for massive neutron stars with hyperon equation of state in modified gravity turns out to be consistent with observations. Thus, we show that the same modified gravity can solve at once three problems: consistent description of the maximal mass of neutron star, realistic Mass-Radius relation and account for hyperons in equation of state.

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arXiv2014

Magnetic Neutron Stars in f(R) gravity

Neutron stars with strong magnetic fields are considered in the framework of f(R) gravity. In order to describe dense matter in magnetic field, the model with baryon octet interacting through $σ$$ρ$$ω$-fields is used. The hyperonization process results in softening the equation of state (EoS) and in decreasing the maximal mass. We investigate the effect of strong magnetic field in models involving quadratic and cubic corrections in the Ricci scalar $R$ to the Hilbert-Einstein action. For large fields, the Mass-Radius relation differs considerably from that of General Relativity only for stars with masses close to the maximal one. Another interesting feature is the possible existence of more compact stable stars with extremely large magnetic fields ($\sim 6\times 10^{18}$ G instead of $\sim 4\times 10^{18}$ G as in General Relativity) in the central regions of the stars. Due to cubic terms, a significant increasing of the maximal mass is possible.

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arXiv2013

Further stable neutron star models from f(R) gravity

Neutron star models in perturbative $f(R)$ gravity are considered with realistic equations of state. In particular, we consider the FPS, SLy and other equations of state and a case of piecewise equation of state for stars with quark cores. The mass-radius relations for $f(R)=R+R(e^{-R/R_{0}}-1)$ model and for $R^2$ models with logarithmic and cubic corrections are obtained. In the case of $R^2$ gravity with cubic corrections, we obtain that at high central densities ($ρ>10ρ_{ns}$, where $ρ_{ns}=2.7\times 10^{14}$ g/cm$^{3}$ is the nuclear saturation density), stable star configurations exist. The minimal radius of such stars is close to $9$ km with maximal mass $\sim 1.9 M_{\odot}$ (SLy equation). A similar situation takes place for AP4 and BSK20 EoS. Such an effect can give rise to more compact stars than in General Relativity. If observationally identified, such objects could constitute a formidable signature for modified gravity at astrophysical level. Another interesting result can be achieved in modified gravity with only a cubic correction. For some EoS, the upper limit of neutron star mass increases and therefore these EoS can describe realistic star configurations (although, in General Relativity, these EoS are excluded by observational constraints).

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arXiv2013

New exact cosmologies on the brane

We develop a method for constructing exact cosmological solutions in brane world cosmology. New classes of cosmological solutions on Randall-Sandrum brane are obtained. The superpotential and Hubble parameter are represented in quadratures. These solutions have inflationary phases under general assumptions and also describe an exit from the inflationary phase without a fine tuning of the parameters. Another class solutions can describe the current phase of accelerated expansion with or without possible exit from it.

astro-ph.CO↗

arXiv2013

Effective Dark Energy Models and Dark Energy Models with Bounce in frames of $F(T)$ Gravity

Various cosmological models in frames of $F(T)$ gravity are considered. The general scheme of constructing effective dark energy models with various evolution is presented. It is showed that these models in principle are compatible with $Λ$CDM model. The dynamics of universe governed by $F(T)$ gravity can mimics $Λ$CDM evolution in past but declines from it in a future. We also construct some dark energy models with the "real" (non-effective) equation-of-state parameter $w$ such that $w\leq-1$. It is showed that in $F(T)$ gravity the Universe filled phantom field not necessarily ends its existence in singularity. There are two possible mechanisms permitting the final singularity. Firstly due to the nonlinear dependence between energy density and $H^{2}$ ($H$ is the Hubble parameter) the universe can expands not so fast as in the general relativity and in fact Little Rip regime take place instead Big Rip. We also considered the models with possible bounce in future. In these models the universe expansion can mimics the dynamics with future singularity but due to bounce in future universe begin contracts.

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arXiv2012

Confronting dark energy models mimicking $Λ$CDM epoch with observational constraints: future cosmological perturbations decay or future Rip?

We confront dark energy models which are currently similar to $Λ$CDM theory with observational data which include the SNe data, matter density perturbations and baryon acoustic oscillations data. DE cosmology under consideration may evolve to Big Rip, type II or type III future singularity, or to Little Rip or Pseudo-Rip universe. It is shown that matter perturbations data define more precisely the possible deviation from $Λ$CDM model than consideration of SNe data only. The combined data analysis proves that DE models under consideration are as consistent as $Λ$CDM model. We demonstrate that growth of matter density perturbations may occur at sufficiently small background density but still before the possible disintegration of bound objects (like clusters of galaxies, galaxies, etc) in Big Rip, type III singularity, Little Rip or Pseudo-Rip universe. This new effect may bring the future universe to chaotic state well before disintegration or Rip.

gr-qc↗

arXiv2012

The cosmological constant as an eigenvalue of a Sturm-Liouville problem

It is observed that one of Einstein-Friedmann's equations has formally the aspect of a Sturm-Liouville problem, and that the cosmological constant, $Λ$, plays thereby the role of spectral parameter (what hints to its connection with the Casimir effect). The subsequent formulation of appropriate boundary conditions leads to a set of admissible values for $Λ$, considered as eigenvalues of the corresponding linear operator. Simplest boundary conditions are assumed, namely that the eigenfunctions belong to $L^2$ space, with the result that, when all energy conditions are satisfied, they yield a discrete spectrum for $Λ>0$ and a continuous one for $Λ<0$. A very interesting situation is seen to occur when the discrete spectrum contains only one point: then, there is the possibility to obtain appropriate cosmological conditions without invoking the anthropic principle. This possibility is shown to be realized in cyclic cosmological models, provided the potential of the matter field is similar to the potential of the scalar field. The dynamics of the universe in this case contains a sudden future singularity.

astro-ph.CO↗

arXiv2012

Equation-of-state formalism for dark energy models on the brane and the future of brane universes

Brane dark energy cosmologies, leading to various possible evolutions of our universe, are investigated. The discussion shows that while all these models can be made arbitrarily close to the standard $Λ$CDM cosmology at present, their future evolutions can be very different, even diverge with time in a number of ways. This includes asymptotic de-Sitter evolution, Little Rip with dissolution of bound structures, and various possible singularities, as the Big Rip, a sudden future singularity (Type II), and Type III and Type IV cases. Specifically, some interesting effects coming from the brane tension are investigated. It is shown, in particular, that the Little Rip occurs faster on the brane model than in usual FRW cosmology. And in the asymptotic de-Sitter regime the influence of the brane tension leads to a deviation of the value of the effective cosmological constant from that corresponding to ordinary dark energy. As a consequence, the value of the inertial force from the accelerating expansion can greatly exceed the corresponding inertial force in ordinary cosmological models.

gr-qc↗

arXiv2012

The Big Trip and Wheeler-DeWitt equation

Of all the possible ways to describe the behavior of the universe that has undergone a big trip the Wheeler-DeWitt equation should be the most accurate -- provided, of course, that we employ the correct formulation. In this article we start by discussing the standard formulation introduced by González-Díaz and Jimenez-Madrid, and show that it allows for a simple yet efficient method of the solution's generation, which is based on the Moutard transformation. Next, by shedding the unnecessary restrictions, imposed on aforementioned standard formulation we introduce a more general form of the Wheeler-DeWitt equation. One immediate prediction of this new formula is that for the universe the probability to emerge right after the big trip in a state with $w=w_0$ will be maximal if and only if $w_0=-1/3$.

gr-qc↗

arXiv2012

Scalar dark energy models mimicking $Λ$CDM with arbitrary future evolution

Dark energy models with various scenarios of evolution are considered from the viewpoint of the formalism for the equation of state. It is shown that these models are compatible with current astronomical data. Some of the models presented here evolve arbitrarily close to $Λ$CDM up to the present, but diverge in the future into a number of different possible asymptotic states, including asymptotic de-Sitter (pseudo-rip) evolution, little rips with disintegration of bound structures, and various forms of finite-time future singularities. Therefore it is impossible from observational data to determine whether the universe will end in a future singularity or not. We demonstrate that the models under consideration are stable for a long period of time (billions of years) before entering a Little Rip/Pseudo-Rip induced dissolution of bound structures or before entering a soft finite-time future singularity. Finally, the physical consequences of Little Rip, Type II, III and Big Crush singularities are briefly compared.

gr-qc↗

arXiv2012

Cosmological measure with volume averaging and the vacuum energy problem

In this paper, we give a possible solution to the cosmological constant problem. It is shown hat the traditional approach, based on volume weighting of probabilities, leads to an incoherent onclusion: the probability that a randomly chosen observer measures $Λ=0$ is exactly equal to 1. Using an alternative, volume averaging measure, instead of volume weighting can explain why the osmological constant is non-zero.

gr-qc↗

arXiv2012

An Infrared Divergence Problem in the cosmological measure theory and the anthropic reasoning

An anthropic principle has made it possible to answer the difficult question of why the observable value of cosmological constant ($Λ\sim 10^{-47}$ GeV${}^4$) is so disconcertingly tiny compared to predicted value of vacuum energy density $ρ_{SUSY}\sim 10^{12}$ GeV${}^4$. Unfortunately, there is a darker side to this argument, as it consequently leads to another absurd prediction: that the probability to observe the value $Λ=0$ for randomly selected observer exactly equals to 1. We'll call this controversy an infrared divergence problem. It is shown that the IRD prediction can be avoided with the help of a Linde-Vanchurin {\em singular runaway measure} coupled with the calculation of relative Bayesian probabilities by the means of the {\em doomsday argument}. Moreover, it is shown that while the IRD problem occurs for the {\em prediction stage} of value of $Λ$, it disappears at the {\em explanatory stage} when $Λ$ has already been measured by the observer.

astro-ph.CO↗

arXiv2012

Phantom Cosmology without Big Rip Singularity

We construct phantom energy models with the equation-of-state parameter $w$ such that $w<-1$, but finite-time future singularity does not occur. Such models can be divided into two classes: (i) energy density increases with time ("phantom energy" without "Big Rip" singularity) and (ii) energy density tends to constant value with time ("cosmological constant" with asymptotically de Sitter evolution). The disintegration of bound structure is confirmed in Little Rip cosmology. Surprisingly, we find that such disintegration (on example of Sun-Earth system) may occur even in asymptotically de Sitter phantom universe consistent with observational data. We also demonstrate that non-singular phantom models admit wormhole solutions as well as possibility of big trip via wormholes.

gr-qc↗

arXiv2007

Astronomical bounds on future big freeze singularity

Recently it was found that dark energy in the form of phantom generalized Chaplygin gas may lead to a new form of the cosmic doomsday, the big freeze singularity. Like the big rip singularity, the big freeze singularity would also take place at a finite future cosmic time, but unlike the big rip singularity it happens for a finite scale factor.Our goal is to test if a universe filled with phantom generalized Chaplygin gas can conform to the data of astronomical observations. We shall see that if the universe is only filled with generalized phantom Chaplygin gas with equation of state $p=-c^2s^2/ρ^α$ with $α<-1$, then such a model cannot be matched to the data of astronomical observations. To construct matched models one actually need to add dark matter. This procedure results in cosmological scenarios which do not contradict the data of astronomical observations and allows one to estimate how long we are now from the future big freeze doomsday.

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