arXiv subjects

Artyom V. Astashenok

Publications and source records attributed to Artyom V. Astashenok.

At least 19 recordsLinked to original sources

arXiv2026

Tsallis Holographic Dark Energy model with varying gravitational constant vs $Λ$CDM model: statistical analysis of observational data and tensions problem

We investigate the cosmological evolution of a flat Friedmann--Robertson--Walker Universe filled with Tsallis holographic dark energy (THDE) under the assumption that the gravitational constant $G$ varies with time according to the power-law parametrisation $d\ln G / d\ln a = α_G = \mathrm{const}$. In this framework, the holographic dark energy density is given by $ρ_{\mathrm{DE}} \propto L^{2γ- 4}$, where $L$ is the future event horizon and $γ$ is the Tsallis non-additivity parameter. We study both the past and future evolution of the Universe for various values of the model parameters $C$, $γ$, and $α_G$. The viability of the model is tested against a combination of observational data, including the Pantheon+ Type Ia supernova sample, direct Hubble parameter measurements $H(z)$, and baryon acoustic oscillation (BAO) data. Using Markov Chain Monte Carlo (MCMC) methods and Bayesian analysis tools such as the Deviance Information Criterion (DIC), the Bayes factor, and the suspiciousness metric, we perform a rigorous statistical comparison with the standard $Λ$CDM model. Our results show that models with a varying gravitational constant are preferred over $Λ$CDM. The best fit to the combined data is obtained for the THDE model with $C < 1$. Furthermore, we analyse the internal consistency of the data sets using the Index of Inconsistency, the $Q_{\mathrm{DMAP}}$ statistic, and Bayesian suspiciousness. We find that the THDE model with varying $G$ and $C < 1$ partially alleviates the tensions between different probes, reducing the significance from $\sim 7.4σ$ to $\sim 5.3σ$.

physics.gen-ph↗

arXiv2025

Non-flat Universe with Tsallis holographic dark energy

We investigated evolution of metric and density perturbations for Tsallis model of holographic dark energy with energy density $\sim L^{2γ- 4}$, where $L$ is length of event horizon or inverse Hubble parameter and $γ$ is parameter of non-additivity close to $1$. Because holographic dark energy is not an ordinary cosmological fluid but a phenomena caused by boundaries of the universe, the ordinary analysis for perturbations is not suitable. One needs to consider perturbations of the future event horizon. For realistic values of parameters it was discovered that perturbations of dark energy don't grow infinitely but vanish or freeze. We also considered the case of realistic interaction between holographic dark energy and matter and showed that in this case perturbations also can asymptotically freeze with time.

gr-qc↗

arXiv2024

General constraints on Tsallis holographic dark energy from observational data

We investigated Tsallis holographic dark energy (THDE) model in light of modern observations of supernovae, Hubble parameter measurements, data for baryon acoustic oscillations and fluctuations of matter density. The dark energy density for THDE model is written as $ρ_d = 3C^2 / L^{4-2γ}$ where $C$ and $γ$ are some constants. Scale $L$ is infrared cut-off lenght for which we use the event horizon. For analysis of type Ia supernovae (SNeIa) data Pantheon+ samples are involved. Dark Energy Spectroscopic Instrument (DESI) 2024 measurements serves as source of data about ratios between sound horizon $r_d$ and Hubble ($d_H$) or volume averaged ($d_V$) distances. The updated dataset of Hubble parameter for various redshift is also used in our analysis. Finally we considered the dependence of matter density fluctuations in past from redshift. The standard stratefy of $χ^2$ minimizing allows to estimate the optimal values of parameters ($Ω_{de}$ and $H_0$) for some fixed values of $C$ and $γ$. One note that best-fit values for parameters $H_0$ from Hubble parameter and SNeIa data are more close than in standard $Λ$CDM model for some $C$ and $γ$ although problem of Hubble tension remains unsolved. The combined data analysis also gives slightly better results in comparison with standard cosmology. We included in our consideration the possible interaction between matter and holographic component and estimated the acceptable interval of model parameters in this case.

astro-ph.CO↗

arXiv2024

Evolution of perturbations in the model of Tsallis holographic dark energy

gr-qc↗

arXiv2023

Compact Stars with Dark Energy in General Relativity and Modified Gravity

We investigate realistic models of compact objects, focusing on neutron and strange stars, composed by dense matter and dark energy in the form of a simple fluid or scalar field interacting with matter. For the dark energy component, we use equations of state compatible with cosmological observations. This requirement strongly constrains possible deviations from the simple $Λ$-Cold Dark-Matter model with EoS $p_{d}=-ρ_{d}$ at least for small densities of the dark component. But we can propose that the density of dark energy interacting with matter can reach large values in relativistic stars and affects the star parameters such as the mass and radius. Simple models of dark energy are considered. Then we investigated possible effects from modified gravity choosing to study the $R^2$ model combined with dark energy. Finally, the case of dark energy as scalar field non-minimally interacting with gravity is considered.

gr-qc↗

arXiv2022

Maximal Masses of White Dwarfs for Polytropes in $R^2$ Gravity and Theoretical Constraints

We examine the Chandrasekhar limit for white dwarfs in $f(R)$ gravity, with a simple polytropic equation of state describing stellar matter. We use the most popular $f(R)$ gravity model, namely the $f(R)=R+αR^2$ gravity, and calculate the parameters of the stellar configurations with polytropic equation of state of the form $p=Kρ^{1+1/n}$ for various values of the parameter $n$. In order to simplify our analysis we use the equivalent Einstein frame form of $R^2$-gravity which is basically a scalar-tensor theory with well-known potential for the scalar field. In this description one can use simple approximations for the scalar field $ϕ$ leaving only the potential term for it. Our analysis indicates that for the non-relativistic case with $n=3/2$, discrepancies between the $R^2$-gravity and General Relativity can appear only when the parameter $α$ of the $R^2$ term, takes values close to maximal limit derived from the binary pulsar data namely $α_{max}=5\times 10^{15}$ cm$^2$. Thus, the study of low-mass white dwarfs can hardly give restrictions on the parameter $α$. For relativistic polytropes with $n=3$ we found that Chandrasekhar limit can in principle change for smaller $α$ values. The main conclusion from our calculations is the existence of white dwarfs with large masses $\sim 1.33 M_{\odot}$, which can impose more strict limits on the parameter $α$ for the $R^2$ gravity model. Specifically, our estimations on the parameter $α$ of the $R^2$ model is $α\sim 10^{13}$ cm$^2$.

gr-qc↗

arXiv2022

Chandrasekhar Mass Limit of White Dwarfs in Modified Gravity

We investigate the Chandrasekhar mass limit for white dwarfs in various models of $f(R)$ gravity. Two equations of state for stellar matter are used: simple relativistic polytropic equation with polytropic index $n=3$ and the realistic Chandrasekhar equation of state. For calculations it is convenient to use the equivalent scalar-tensor theory in the Einstein frame and then to return in the Jordan frame picture. For white dwarfs we can neglect terms containing relativistic effects from General Relativity and we consider the reduced system of equations. Its solution for any model of $f(R)=R+βR^{m}$ ($m\geq 2$, $β>0$) gravity leads to the conclusion that the stellar mass decreases in comparison with standard General Relativity. For realistic equations of state we find that there is a value of the central density for which the mass of white dwarf peaks. Therefore, in frames of modified gravity there is lower limit on the radius of stable white dwarfs and this minimal radius is greater than in General Relativity.

gr-qc↗

arXiv2022

Tsallis holographic dark energy model with event horizon cutoff in modified gravity

We considered the Tsallis holographic dark energy model in frames of Nojiri-Odintsov gravity with $f(R)=R+λR^2-σμ/{R}$. For IR cutoff event horizon is taken. The cosmological evolution of such universe is investigated for various initial conditions and values of parameters. The dependence of the Hubble parameter $H$ from time in the future has an oscillations. It is shown that for $μ\neq 0$ appearance of singularities are typical and the time up to these singularities can be relatively small from cosmological viewpoint. The singularity is associated with the zero of second deribative of $f(R)$ on $R$. It is interesting to note that these models can describe observational data from Ia supernovae astrophysics and dependence of the Hubble parameter from redshift $z$ at least not worse than canonical $Λ$CDM model.

gr-qc↗

arXiv2022

Crossing of phantom divide line in model \\of interacting Tsallis holographic dark energy

We consider a Tsallis holographic dark energy model with interaction between dark energy and matter. The density of dark energy is taken as $ρ_d \sim 3C^2/L^{4-2γ}$, where $C$, $γ$ are constants. The event horizon is chosen as the characteristic scale $L$. The cosmological dynamics of the universe are analyzed, with special attention paid to the possibility of crossing the phantom line $w_{eff}=-1$. It is shown that for certain values of parameters this may occur not only once, but also twice.

gr-qc↗

arXiv2021

Dark Energy and Cosmological Horizon Thermal Effects

We investigate various dark energy models by taking into account the thermal effects induced from Hawking radiation on the apparent horizon of the Universe, for example near a finite-time future singularity. If the dark energy density increases as the Universe expands, the Universe's evolution reaches a singularity of II type (or sudden future singularity). The second derivative of scale factor diverges but the first remains finite. Quasi-de Sitter evolution can change on sudden future singularity in the case of having an effective cosmological constant larger than the maximum possible value of the energy density of the Universe. Another interesting feature of cosmological solution is the possibility of a transition between deceleration and acceleration for quintessence dark energy with a simple equation of state. Finally, we investigate which fluid component can remedy Big Rip singularities and other crushing type singularities.

gr-qc↗

arXiv2020

Extended Gravity Description for the GW190814 Supermassive Neutron Star

Very recently a compact object with a mass in the range $2.50÷2.67\, M_{\odot}$ has been discovered via gravitational waves detection of a compact binary coalescence. The mass of this object makes it among the heaviest neutron star never detected or the lightest black hole ever observed. Here we show that a neutron star with this observed mass, can be consistently explained with the mass-radius relation obtained by Extended Theories of Gravity. Furthermore, equations of state, consistent with LIGO observational constraints, are adopted. We consider also the influence of rotation and show that masses of rotating neutron stars can exceed $2.6 M_\odot$ for some equations of state compatible with LIGO data.

gr-qc↗

arXiv2020

Rotating Neutron Stars in F(R) Gravity with Axions

We investigate equilibrium configurations of uniformly rotating neutron stars in $R^2$ gravity with axion scalar field for GM1 equation of state (EoS) for nuclear matter. The mass-radius diagram, mass-central energy density are presented for some frequencies in comparison with static stars. We also compute equatorial and polar radii and moment of inertia for stars. For axion field $ϕ$ the coupling in the form $\sim R^2ϕ$ is assumed. Several interesting results follow from our consideration. Maximal possible star mass with given EoS increases due to the contribution of coupling term. We discovered the possibility to increase maximal frequency of the rotation in comparison with General Relativity. As a consequence the lower bound on mass of the fast rotating stars decreases. For frequency $f=700$ Hz neutron stars with masses $\sim M_\odot$ can exist for some choice of parameters (in General Relativity for same EoS this limit is around $1.2 M_{\odot}$). Another feature of our solutions is relatively small increase of stars radii for high frequencies in comparison with static case. Thus, eventually the new class of neutron stars in $R^2$ gravity with axions is discovered namely fast rotating compact stars with intermediate masses.

gr-qc↗

arXiv2020

Neutron Stars in f(R)-Gravity and Its Extension with a Scalar Axion Field

We present a brief review of general results about non-rotating neutron stars in simple R-square gravity and its extension with a scalar axion field. Modified Einstein equations are presented for metrics in isotropical coordinates. The mass-radius relation, mass profile and dependence of mass from central density on various equations of state are given in comparison to general relativity.

gr-qc↗

arXiv2020

Supermassive Neutron Stars in Axion $F(R)$ Gravity

We investigated realistic neutron stars in axion $R^{2}$ gravity. The coupling between curvature and axion field $ϕ$ is assumed in the simple form $\sim R^2ϕ$. For the axion mass in the range $m_{a}\sim 10^{-11}-10^{-10}$ eV the solitonic core within neutron star and corresponding halo with size $\sim 100$ km can exist. Therefore the effective contribution of $R^2$ term grows inside the star and it leads to change of star parameters (namely, mass and radius). We obtained the increase of star mass independent from central density for wide range of masses. Therefore, maximal possible mass for given equation of state grows. At the same time, the star radius increases not so considerably in comparison with GR. {Hence, our model may predict possible existence of supermassive compact stars with masses $M\sim 2.2-2.3M_\odot$ and radii $R_{s}\sim 11$ km for realistic equation of state (we considered APR equation of state). In General Relativity one can obtain neutron stars with such characteristics only for unrealistic, extremely stiff equations of state.} Note that this increase of mass occurs due to change of solution for scalar curvature outside the star. In GR curvature drops to zero on star surface where $ρ=p=0$. In the model under consideration the scalar curvature dumps more slowly in comparison with vacuum $R^2$ gravity due to axion "galo" around the star.

gr-qc↗

arXiv2019

Some models of holographic dark energy on the Randall-Sundrum brane and observational data

The some models of holographic dark energy for Randall-Sandrum brane are considered. For first class of dark energy models we take energy density in form $\sim L^{2γ-4}$ where $L$ is size of events horizon in Universe and $γ$ is parameter (Tsallis holographic energy). Analysis of observational data allows to define upper limit on value of $δ=ρ_{0}/2λ$ ($ρ_{0}$ is current energy density in the Universe and $λ$ is brane tension). Then we investigate models for which dark energy density has form $ρ_{de}=C^{2}L^{-2}-C_{1}^{2}H^{2}$ where $H$ is Hubble parameter.

gr-qc↗

arXiv2018

Neutron Stars in frames of $R^{2}$-gravity and Gravitational Waves

The realistic models of neutron stars are considered for simple $R+αR^2$ gravity and equivalent Brance-Dicke theory with dilaton field in Einsein frame. For negative values of $α$ we have no acceptable results from astrophysical viewpoint: the resulting solution for spherical stars doesn't coincide with Schwarzschild solution on spatial infinity. The mass of star from viewpoint of distant observer tends to very large values. For $α>0$ it is possible to obtain solutions with required asymptotics and well-defined star mass. The mass confined by stellar surface decreases with increasing of $α$ but we have some contribution to mass from gravitational sphere appearing outside the star. The resulting effect is increasing of gravitational mass from viewpoint of distant observer. But another interpretation take place in a case of equivalent Brance-Dicke theory with massless dilaton field in Einstein frame. The mass of star increases due to contribution of dilaton field inside the star. We also considered the possible constraints on $R^{2}$ gravity from GW 170817 data. According to results of Bauswein et al. the lower limit on threshold mass is $2.74^{+0.04}_{-0.01}$ $M_{\odot}$. This allows to exclude some equations of state for dense matter. But in $R^2$ gravity the threshold mass increases for given equation of state with increasing of $α$. In principle it can helps in future discriminate between General Relativity and square gravity (of course one need to know equation of state with more accuracy rather than now.

gr-qc↗

arXiv2018

Gravitational Collapse in General Relativity and in $R^2$-gravity: A Comparative Study

We compare the gravitational collapse of homogeneous perfect fluid with various equations of state in the framework of General Relativity and in $R^2$ gravity. We make our calculations using dimensionless time with characteristic timescale $t_{g}\sim (Gρ)^{-1/2}$ where $ρ$ is a density of collapsing matter. The cases of matter, radiation and stiff matter are considered. We also account the possible existence of vacuum energy and its influence on gravitational collapse. In a case of $R^2$ gravity we have additional degree of freedom for initial conditions of collapse. For barotropic equation of state $p=wρ$ the result depends from the value of parameter $w$: for $w>1/3$ the collapse occurs slowly in comparison with General Relativity while for $w<1/3$ we have opposite situation. Vacuum energy as expected slows down the rate of collapse and for some critical density gravitational contraction may change to expansion. It is interesting to note that for General Relativity such expansion is impossible. We also consider the collapse in the presence of so-called phantom energy. For description of phantom energy we use Lagrangian in the form $-X-V$ (where $X$ and $V$ are the kinetic and potential energy of the field respectively) and consider the corresponding Klein-Gordon equation for phantom scalar field.

gr-qc↗

arXiv2018

The Cosmological Models with Jump Discontinuities

The article is dedicated to one of the most undeservedly overlooked properties of the cosmological models: the behaviour at, near and due to a jump discontinuity. It is most interesting that while the usual considerations of the cosmological dynamics deals heavily in the singularities produced by the discontinuities of the second kind (a.k.a. the essential discontinuities) of one (or more) of the physical parameters, almost no research exists to date that would turn to their natural extension/counterpart: the singularities induced by the discontinuities of the first kind (a.k.a. the jump discontinuities). It is this oversight that this article aims to amend. In fact, it demonstrates that the inclusion of such singularities allows one to produce a number of very interesting scenarios of cosmological evolution. For example, it produces the cosmological models with a finite value of the equation of state parameter $w=p/ρ$ even when both the energy density and the pressure diverge, while at the same time keeping the scale factor finite. Such a dynamics is shown to be possible only when the scale factor experiences a finite jump at some moment of time. Furthermore, if it is the first derivative of the scale factor that experiences a jump, then a whole new and different type of a sudden future singularity appears. Finally, jump discontinuities suffered by either a second or third derivatives of a scale factor lead to cosmological models experiencing a sudden dephantomization -- or avoiding the phantomization altogether. This implies that theoretically there should not be any obstacles for extending the cosmological evolution beyond the corresponding singularities; therefore, such singularities can be considered a sort of a cosmological phase transition.

astro-ph.CO↗