SearcharxivSearch

arXiv subjects

M. Khurshudyan

Publications and source records attributed to M. Khurshudyan.

At least 19 recordsLinked to original sources

Late time attractors of some varying Chaplygin gas cosmological models

The goal of this paper is to study new cosmological models where the dark energy is a varying Chaplygin gas. This specific dark energy model with non-linear EoS had been often discussed in modern cosmology. Contrary to previous studies, we consider new forms of non-linear non-gravitational interaction between dark matter and assumed dark energy models. We applied the phase space analysis allowing understanding the late time behavior of the models. It allows demonstrating that considered non-gravitational interactions can solve the cosmological coincidence problem. On the other hand, we applied Bayesian Machine Learning technique to learn the constraints on the free parameters. In this way, we gained a better understanding of the models providing a hint which of them can be ruled out. Moreover, the learning based on the simulated expansion rate data shows that the models cannot solve the $H_{0}$ tension problem.

gr-qc

Cosmological model involving an interacting van der Waals fluid

A model for the late-time accelerated expansion of the universe is considered where a van der Waals fluid interacting with matter plays the role of dark energy. The transition towards this phase in the cosmic evolution history is discussed in detail and, moreover, a complete classification of the future finite-time singularities is obtained for six different possible forms of the non-gravitational interaction between dark energy (the van der Waals fluid) and dark matter. This study shows, in particular, that a universe with a non-interacting three-parameter van der Waals fluid can evolve into a universe characterized by a Type IV~(Generalized Sudden) Singularity. On the other hand, for certain values of the parameters, exit from the accelerated expanding phase is possible in the near future, what means that the expansion of the universe in the future could become decelerated. On the other hand, our study shows that space can be divided into different regions. For some of them, in particular, the non-gravitational interactions $Q = 3 H b ρ_{de}$, $Q = 3 H b ρ_{dm}$ and $Q = 3 H b (ρ_{de} + ρ_{de})$ may completely suppress future finite-time singularity formation, for sufficiently high values of $b$. On the other hand, for some other regions of the parameter space, the mentioned interactions would not affect the singularity type, namely the Type IV singularity generated in the case of the non-interacting model would be preserved. A similar conclusion has been archived for the cases of $Q = 3 b H ρ_{de}ρ_{dm}/(ρ_{de}+ρ_{dm})$, $Q = 3 b H ρ_{dm}^{2}/(ρ_{de}+ρ_{dm})$ and $Q = 3 b H ρ_{de}^{2}/(ρ_{de}+ρ_{dm})$ non-gravitational interactions, with only one difference: the $Q = 3 b H ρ_{dm}^{2}/(ρ_{de}+ρ_{dm})$ interaction will change the Type IV singularity of the non-interacting model into a Type II~(The Sudden) singularity.

gr-qc

Phenomenological modification of horizon temperature

In this paper a study of the accelerated expansion problem of the large scale universe is presented. To derive Friedmann like equations, describing the background dynamics of the recent universe we take into account, that it is possibile to interpret the spacetime dynamics as an emergent phenomenon. It is a consequence of the deep study of connection between gravitation and thermodynamics. Considered models are based on phenomenological modifications of the horizon temperature. In general, there are various reasons to modify the horizon temperature and one of them can be related to the feedback from the spacetime on the horizon, which will generate additional heat. In order to constraint the parameters of the models we use $Om$ analysis and the constraints on this parameter at $z=0.0$, $z=0,57$ and $z=2.34$.

gr-qc

Non-linear logarithmic interactions and a varying polytropic gas

In this paper we will demonstrate possible existence of more exotic forms of interaction taking into account, that dark energy can be parametrized as a varying polytropic uid suggested recently by the first author. On going research in both directions, i.e. dark energy and non-gravitational interaction, opens very interesting perspectives for solving the problems which are related to the accelerated expansion of the Universe. The fact that available observational data allows to parametrize the dark side of our Universe in form of interacting dark uids is not less surprising, than the accelerated expansion itself. Therefore, the consideration of new forms of non-gravitational interactions in modern cosmology still is one of the actual topics and requires deep and systematic research. This fact motivates us to consider various new forms of non-gravitational logarithmic interactions and study their cosmological consequences. Appropriate classification of the models is presented in the light of actively discussed Om analysis and the result for the Hubble parameter measured at z = 2:34 in BOSS experiment.

gr-qc

On a holographic dark energy model with a Nojiri-Odintsov cut-off in general relativity

In this paper we consider the models of the accelerated expanding large scale universe~(according to general relativity) containing a generalized holographic dark energy with a Nojiri - Odintsov cut - off. The second component of the darkness is assumed to be the pressureless cold dark matter according to observed symmetries of the large scale universe. Moreover, we assume specific forms of the interaction between these two components and besides the cosmographic analysis, we discuss appropriate results from $Om$ and $Om3$ analysis and organize a closer look to the models via the statefinder hierarchy analysis, too. In this way we study mainly impact of the interaction on the dynamics of the background of our universe~(within specific forms of interaction). To complete the cosmographic analysis, the present day values of the statefinder parameters $(r,s)$ and $(ω^{\prime}_{de}, ω_{de})$ has been estimated for all cases and the validity of the generalized second law of thermodynamics is demonstrated. Our study showed that theoretical results from considered phenomenological models are consistent with the available observational data and symmetries.

gr-qc

Interacting Ghost Dark Energy Models in the Higher Dimensional Cosmology

We investigate interacting ghost dark energy models in higher dimensional cosmology. We attempt to model dark matter within a barotropic fluid with $P_{b}=ω(t)_{b}ρ$. In this work we consider four different models based on choosing equation of state parameter and interaction term. We confirm that our models agree with observational data.

gr-qc

Phase space analysis of some interacting Chaplygin gas models

The goal of this paper is to discuss phase space analysis of some interacting Chaplygin gas models in General Relativity. Chaplygin gas is one of the fluids actively considered in modern cosmology due to the fact that it is a joint model of dark energy and dark matter. In this paper we have considered various forms of interaction term Q including linear and non linear sign changeable interactions. For each case late time attractors for the field equations are found.

gr-qc

A Universe with a generalized ghost dark energy and Van der Waals fluid interacting with a fluid

In this paper, we consider an unusual connection between different fluids. Having established a research goal we would like to consider a toy model of the Universe and investigate its behavior, especially for later time evolution for well known facts. The main goal of the article is to consider a toy model of the Universe with generalized ghost dark energy, Van der Waals gas and a phenomenologically modified fluid. The origin of the last component can be understood as a result of interaction between some original fluid and some source of energy or matter in Universe. By unusual connection we mean an assumption that generalized ghost dark energy has its contribution to the model by an interaction term $Q$ and we suppose an interaction of the form $Q=3Hb(ρ_{\small{tot}}-ρ_{GDe})$. Graphical analysis is performed and the questions of validity of the generalized second law of thermodynamics and stability of the model also approached in this paper.

physics.gen-ph

Higher order corrections of the extended Chaplygin gas cosmology with varying $G$ and $Λ$

In this paper, we study two different models of dark energy based on Chaplygin gas equation of state. The first model is the variable modified Chaplygin gas while the second one is the extended Chaplygin gas. Both models are considered in the framework of higher order $f(R)$ modified gravity. We also consider the case of time varying gravitational constant $G$ and $Λ$ for both models. We investigate some cosmological parameters such as the Hubble, the deceleration and the equation of state parameters. Then we showed that the model that we considered, extended Chaplygin gas with time-dependent $G$ and $Λ$, is consistent with the observational data. Finally we conclude with the discussion of cosmological perturbations of our model.

gr-qc

An effective quintessence field with a power-law potential

In this paper, we consider an effective quintessence scalar field with a power-law potential interacting with a $P_{b}=ξqρ_{b}$ barotropic fluid as a first model, where $q$ is a deceleration parameter. For the second model we assume viscous polytropic gas interacting with the scalar field. We investigate problem numerically and analyze behavior of different cosmological parameter concerning to components and behavior of Universe. We also compare our results with observational data to fix parameters of the models. We find some instabilities in the first model which may disappear in the second model for the appropriate parameters. Therefore, we can propose interacting quintessence dark energy with viscous polytropic gas as a successful model to describe Universe.

gr-qc

Higher derivative corrections of $f(R)$ gravity with varying equation of state in the case of variable $G$ and $Λ$

In this paper, we study three models of f(R) modified gravity including higher order terms based on different equation of state parameter. We also assume variable G and $Λ$. By using numerical analysis, we obtain the behavior of some important cosmological parameters, like the Hubble expansion H parameter and the deceleration parameter q. The statefinder diagnostics is also performed for all models.

gr-qc

Interacting quintessence dark energy models in Lyra manifold

In this paper, we consider two-component dark energy models in Lyra manifold. The first component is assumed as a quintessence field while the second model may be a viscous polytropic gas, a viscous Van der Waals gas or a viscous modified Chaplygin gas. We also consider the possibility of interaction between components. By using numerical analysis, we study some cosmological parameters of the models and compare them with observational data.

gr-qc

Observational constraints on models of the Universe with time variable Gravitational and Cosmological constants along MOG

The subject of this paper is to investigate the weak regime covariant scalar-tensor-vector gravity (STVG) theory, known as the MOdified gravity (MOG) theory of gravity. First, we show that the MOG in the absence of scalar fields is converted into $Λ(t),G(t)$ models. Time evolution of the cosmological parameters for a family of viable models have been investigated. Numerical results with the cosmological data have been adjusted. We've introduced a model for dark energy (DE) density and cosmological constant which involves first order derivatives of Hubble parameter. To extend this model, correction terms including the gravitational constant are added. In our scenario, the cosmological constant is a function of time. To complete the model,interaction terms between dark energy and dark matter (DM) manually entered in phenomenological form. Instead of using the dust model for DM, we have proposed DM equivalent to a barotropic fluid. Time evolution of DM is a function of other cosmological parameters. Using sophisticated algorithms, the behavior of various quantities including the densities, Hubble parameter, etc. have been investigated graphically. The statefinder parameters have been used for the classification of DE models. Consistency of the numerical results with experimental data of $SneIa+BAO+CMB$ are studied by numerical analysis with high accuracy.

gr-qc

Mutually interacting Tachyon dark energy with variable $G$ and $Λ$

In this paper we consider Tachyonic scalar field as dark energy with interaction between components in the case of variable $G$ and $Λ$. We assume a flat Universe with specific form of scale factor and study cosmological parameters numerically and graphically. Statefinder analysis also performed as well. In the special choice of interaction parameters we succeed to obtain analytical expression of densities. We find that our model will be stable in the late stage but there is an instability at the early Universe.

gr-qc

Hubble parameter corrected interactions in cosmology

In this paper we make steps in a new direction by considering fluids with EoS of more general form $F(ρ,P)=0$. It is thought that there should be interaction between cosmic fluids, but this assumption for this stage carries only phenomenological character opening a room for different kind of manipulations. In this article we will consider a modification of an interaction $Q$, where we accept that interaction parameter $b_{1}$ (order of unity) in $Q=3Hb_{1}ρ$ is time dependent and presented as a linear function of Hubble parameter $H$ of the form $b_{0}+btH$, where $b$ and $b_{0}$ are constants. We consider two different models include modified Chaplygin gas and palotropic gas which have bulk viscosity. Then, we investigate problem numerically and analyze behavior of different cosmological parameters concerning to fluids and behavior of Universe.

gr-qc

FRW cosmology of the generalized model of LQG

In this paper, we study the main cosmological properties of the classical Friedmann equations in the case of homogeneous and isotropic Friedmann-Robertson-Walker Universe and we also generalized the expression of the Friedmann equation in the case of Loop Quantum Cosmology (LQC). Considering the $M_{35}$-model, we found the solutions of the equations considered for two particular cases, i.e. $Q=0$ (i.e., the de Sitter solution) and $Q>0$. Moreover, we considered and studied two exact cosmological solutions of the $M_{35}$-model, in particular the power-law and the exponential ones. Futhermore, we also considered a third more complicated case and we derived the solution for an arbitrary function of the time $f\left(t\right)$. A scalar field description of the model is presented by constructing its self-interacting potential.

gr-qc

Phenomenologically varying $Λ$ and a toy model for the Universe

We consider a model of the Universe with variable G and Λ. Subject of our interest is a phenomenological model for Λ proposed and considered in this article first time (up to our knowledge). Modification based on an assumption that ghost dark energy exists and Universe will feel it through Λ. In that case we would like to consider possibility that there exist some unusual connections between different components of the fluids existing in Universe. We would like to stress, that this is just an assumption and could be very far from the reality. We are interested by this model as a phenomenological and mathematical and unfortunately, we will not discuss about physical conditions and possibilities of having such modifications. To test our assumption and to observe behavior of the Universe, we will consider toy models filled by a barotropic fluid and modified Chaplyagin gas. To complete the logic of the research we will consider interaction between barotropic fluid or Chaplygin gas with ghost dark energy as a separate scenario. Statefinder diagnostic also provided with stability analysis of the models. All free parameters of the model fixed to satisfy generalized second law of thermodynamics.

gr-qc

Phenomenological models of Universe with varying $G$ and $Λ$

In this article we will consider several phenomenological models for the Universe with varying $G$ and $Λ(t)$, where $G$ is the gravitational "constant" and $Λ(t)$ is a varying cosmological "constant". Two-component fluid model taken into account. An interaction of the phenomenological form between a barotropic fluid and a quintessence DE is supposed. Three different forms of $Λ(t)$ will be considered. The problem is analysed numerically and behavior of different cosmological parameters investigated graphically. Conclusion and discussions are given at the end of the work. In an Appendix an information concerning to the other cosmological parameters is presented.

gr-qc