FLRW Cosmological Models Research Articles

Cosmological FLRW phase transitions and micro-structure under Kaniadakis statistics

This article studies the thermodynamics phase transitions and critical phenomena of an FLRW cosmological model under truncated Kaniadakis's statistics. The EoS is derived from the corrected Friedmann field equations and the thermodynamics unified first law. To approach thermodynamics, we use an approximation for KSAH≪1, on the cosmological equation, and also check the validity of our approximation. Then for different values from Kaniadakis parameter K, order O(10−37) for the relic-abundance, O(10−84) for 7Li-abundance and O(10−125) in the recombination era, the EoS reveals non-trivial critical points where a first-order phase transition occurs a sort of a van der Waals fluid. Interestingly, the numerical values of the critical exponents are the same as those of the van der Waals system. Besides, to obtain more insights into the thermodynamics description, the so-called Ruppeiner's geometry is studied through the normalized scalar curvature, disclosing this invariant zone where the system undergoes repulsive/attractive interactions. Near the critical point, this curvature provides again the same critical exponent and universal constant value as for van der Waals fluid. Despite the similarity, both systems are quite different because the present one considers a relativistic entropy (Kaniadakis entropy) while the Van der Waals gas responds to a classical entropy (Maxwell-Boltzmann).

Dynamical system method of viscous fluid in f(T) gravity theory

In this paper, using the recently introduced f(T) gravity framework, we have analyzed the viscous fluid cosmological model in FLRW cosmological model by assuming a specific form of the bulk viscosity coefficient as, ζ=ζ0+ζ1H+ζ2ḢH+H and a non-linear f(T) model particularly, f(T)=T−α−T where α is the model parameter. The bulk viscosity coefficients are estimated by using Hubble 31 data set. Using the phase space technique, we examine the asymptotic behaviour of our cosmological bulk viscous model. We found one stable critical point. Phase space analysis and the geometrical interpretations are given. We discover that according to our model, the Universe evolved from a matter-dominated decelerated phase (a past attractor) to a stable de-Sitter accelerated epoch (a future attractor). The evolution of EoS parameter shows the acceleration phase of the cosmic expansion whereas the negative behavior of viscosity-induced pressure indicates the accelerated expansion of the Universe. Additionally, the cosmic matter-energy density exhibits positive behavior. We look into how statefinder parameters behave for our model . We discover that the trajectory of our model starts at the quintessence and ends at the ΛCDM region. We use the Om diagnostic test, which reveals that our model displays quintessence behaviour. The energy condition criteria are analyzed, and we discover that SEC has been violated, whereas WEC, NEC and DEC satisfy the positivity criteria. Our f(T) cosmological model, with the influence of bulk viscosity successfully describes the expansion history of the Universe and provides a good fit to recent observational data.

Energy Conditions with Interacting Field in f(R) Gravity

In the context of current scenario, it is crucial to look beyond Einstein’s theory, which opens the door to certain modified theories of gravity. So, present study is devoted to investigate the various energy conditions, particularly, strong energy condition (SEC), weak energy condition (WEC), null energy condition (NEC) and dominant energy condition (DEC) corresponding to different functional forms of f(R) gravity. We have studied for flat, isotropic and homogeneous FLRW cosmological model filled with interacting field i.e., perfect fluid is coupled with mass less scalar field for different models of modified f(R) gravity in which R is the Ricci scalar. We have observed, the accelerated expansion of the Universe which exact match with recent observational evidences.

Bulk viscous fluid in extended symmetric teleparallel gravity

In this paper, we investigate the existence of bulk viscous FLRW cosmological models in a recently proposed extended symmetric teleparallel gravity or fQ,T gravity in which Q is the non-metricity and T is the trace of the energy–momentum tensor. We consider a simple coupling between matter and non-metricity, specifically, fQ,T=αQm+1+λT and fQ,T=αQ+λT where α, λ and m are free model parameters. The exact cosmological solutions are found by assuming the scale factor in the form of the hybrid expansion law. This type of relation generates a time-varying deceleration parameter with the transition of the Universe from the early decelerating phase to the present accelerating phase. In the presence of viscous fluid, we analyze some cosmological parameters of our cosmological model such as the energy density, bulk viscous pressure, bulk viscous coefficient, equation of state parameter, and energy conditions. Finally, we conclude our fQ,T cosmological models agree with the recent astronomical observations.

Cosmic acceleration and energy conditions in symmetric teleparallel f(Q) gravity

In this paper, we study a spatially homogeneous and isotropic FLRW cosmological model in framework of the symmetric teleparallel f(Q) gravity proposed by Jimenez et al. (Physical Review D 98.4 (2018): 044048), where the non-metricity Q is responsible for the gravitational interaction. For the whole analysis we have considered a special form of f(Q) gravity function i.e. f(Q)=αQn+β (where α, β, and n are the dynamical model constant parameters) in the direction of t(z)=kt0bf(z), where, f(z)=W[bkeb−ln⁡(1+z)k] and W denotes the Lambert function. We have discussed two cases of the function: linear f(Q) model towards n=1 and quadratic for n=2. In both the model we had verified the validity of the models with the help of energy conditions along with some physical parameters like equation of state parameter, jerk parameter, statefinder parameters are also discussed in details and compared it with the observational data.

Cosmology in f(R, T) gravity with quadratic deceleration parameter

Abstract In this work, we have developed FLRW cosmological models in f(R, T) gravity. The solution of the modified field equations are obtained under the newly proposed Bakry and Shafeek, “The periodic universe with varying deceleration parameter of the second degree,” Astrophys. Space Sci., vol. 364, p. 135, 2019, quadratic form of the deceleration parameter. Further, we have discussed the state-finder parameter, om-diagnostic analysis and energy conditions of the proposed model. The variation of deceleration parameter with respect to cosmic time and red-shift is consistent with observational data.

Models of holographic dark energy in f(Q) gravity

Present analysis dedicated to the dynamical investigation of models of holographic dark energy using FLRW cosmological model in the context of modified gravity in which Lagrangian be the arbitrary function of the trace of the non-metricity tensor Q say f ( Q ) gravity where the nonmetricity Q is responsible for the gravitational interaction. We govern the features of the derived cosmological model in view of the relation between cosmic time and redshift as t = 1 α k l o g 1 + 1 ( 1 + z ) α and discuss the evolution trajectories of the equation of state parameter and stability parameters in the evolving universe. The classification of the field equations toward f ( Q ) = Q + m Q n makes available that the model is purely accelerating corresponds to 0 ≤ q ≤ − 1 .

FLRW Cosmological Models with Dynamic Cosmological Term in Modified Gravity

Although the standard lambda cold dark matter cosmological model is quite successful in describing the universe, there are still several issues that are still not resolved. Some of these are the cosmological constant problem, certain anomalies in the cosmic microwave background radiation and whether general relativity is valid on large scales. Therefore, it is interesting to examine modified theories in an attempt to solve these problems, and to examine the entire range of possibilities that are allowed. In this work, we examine one of these modified theories, viz., f(R,T) gravity. We study the homogeneous and isotropic models in this theory, which have some pleasing features, such as no initial singularity, a dynamic cosmological term, and a transition from early deceleration to late-time acceleration as intimated by observations. The physical parameters of the model, as well as the energy conditions, are discussed and a viable cosmological model can be constructed.

Semiconformal curvature tensor and perfect fluid spacetimes in general relativity

The semiconformal curvature tensor and its divergence have been studied for perfect fluid spacetimes. It is seen, apart from other results, that the perfect fluid spacetimes with divergence-free semiconformal curvature tensor either satisfy the vacuum-like equation of state or represent FLRW cosmological model.

Minisuperspace FLRW oscillator cosmological model and generalized uncertainty principle

This paper deals with the effects of the generalized uncertainty principle on the one-dimensional minisuperspace FLRW cosmological model with a mixture of noninteracting dust and radiation. The model is presented in a classical and quantum case. In the classical case, the Lagrangian of the model is reduced by a suitable coordinate transformation to the Lagrangian of a linear harmonic oscillator. Within the standard quantum approach Wheeler–DeWitt equation and its general solution, i.e. a wave function of the model is written. The effects of the modified standard commutation relations on the dynamics of the model are considered in both of the approaches, classical and quantum.

FLRW cosmological models with quark and strange quark matters in f ( R , T ) $f(R,T)$ gravity

In this paper, we have studied the magnetized quark matter (QM) and strange quark matter (SQM) distributions in the presence of $ f(R,T)$ gravity in the background of Friedmann--Lema\^itre--Robertson--Walker (FLRW) metric. To get exact solutions of modified field equations we have used $f(R,T) = R + 2 f(T)$ model given by Harko et al. with two different parametrization of geometrical parameters \textit{i.e.} the parametrization of the deceleration parameter $ q $, and the scale factor $ a $ in hybrid expansion form. Also, we have obtained Einstein Static Universe (ESU) solutions for QM and SQM distributions in $f(R,T)$ gravity and General Relativity (GR). All models in $f(R,T)$ gravity and GR for FRW and ESU Universes with QM also SQM distributions, we get zero magnetic field. These results agree with the solutions of Akta{\c{s} and Ayg\"un in $f(R,T)$ gravity. However, we have also discussed the physical consequences of our obtained models.

Some Remarks on the $$C^0$$ C 0 -(In)Extendibility of Spacetimes

The existence, established over the past number of years and supporting earlier work of Ori [14], of physically relevant black hole spacetimes that admit $C^0$ metric extensions beyond the future Cauchy horizon, while being $C^2$-inextendible, has focused attention on fundamental issues concerning the strong cosmic censorship conjecture. These issues were recently discussed in the work of Jan Sbierski [17], in which he established the (nonobvious) fact that the Schwarschild solution in global Kruskal-Szekeres coordinates is $C^0$-inextendible. In this paper we review aspects of Sbierski's methodology in a general context, and use similar techniques, along with some new observations, to consider the $C^0$-inextendibility of open FLRW cosmological models. We find that a certain special class of open FLRW spacetimes, which we have dubbed `Milne-like,' actually admit $C^0$ extensions through the big bang. For spacetimes that are not Milne-like, we prove some inextendibility results within the class of spherically symmetric spacetimes.

Evolving Hořava cosmological horizons

Several sets of radially propagating null congruence generators are exploited in order to form 3-dimensional marginally trapped surfaces, referred to as black hole and cosmological apparent horizons in a Hořava universe. Based on this method, we deal with the characteristics of the 2-dimensional space-like spheres of symmetry and the peculiarities of having trapping horizons. Moreover, we apply this method in standard expanding and contracting FLRW cosmological models of a Hořava universe to investigate the conditions under which the extra parameters of the theory may lead to trapped/anti-trapped surfaces both in the future and in the past. We also include the cases of negative time, referred to as the finite past, and discuss the formation of anti-trapped surfaces inside the cosmological apparent horizons.

The equivalence principle and the relative velocity of local inertial frames

In this paper, we explicitly show that in general relativity, the relative velocity of two local inertial frames is always less than the velocity of light. This fact is a by-product of the equivalence principle. The general result is then illustrated within two examples, the FLRW cosmological model and the Schwarzschild metric.

Cosmology based on f(R) gravity with \U0001d4aa(1) eV sterile neutrino

We address the cosmological role of an additional \U0001d4aa(1) eV sterile neutrino in modified gravity models. We confront the present cosmological data with predictions of the FLRW cosmological model based on a variant of f(R) modified gravity proposed by one of the authors previously. This viable cosmological model which deviation from general relativity with a cosmological constant Λ decreases as R−2n for large, but not too large values of the Ricci scalar R (while no Λ is introduced by hand at small R) provides an alternative explanation of present dark energy and the accelerated expansion of the Universe (the case n=2 is considered in the paper). Various up-to-date cosmological data sets exploited include measurements of the cosmic microwave background (CMB) anisotropy, the CMB lensing potential, the baryon acoustic oscillations (BAO), the cluster mass function and the Hubble constant. We find that the CMB+BAO constraints strongly restrict the sum of neutrino masses from above. This excludes values of the model parameter λ∼ 1 for which distinctive cosmological features of the model are mostly pronounced as compared to the ΛCDM model, since then free streaming damping of perturbations due to neutrino rest masses is not sufficient to compensate their extra growth occurring in f(R) modified gravity. Thus, in the gravity sector we obtain λ>8.2 (2σ) with the account of systematic uncertainties in galaxy cluster mass function measurements and λ>9.4 (2σ) without them. At the same time in the latter case we find for the sterile neutrino mass 0.47 eV < mν, sterile < 1 eV (2σ) assuming that the sterile neutrinos are thermalized and the active neutrinos are massless, not significantly larger than in the standard ΛCDM with the same data set: 0.45 eV < mν, sterile < 0.92 eV (2σ). However, a possible discovery of a sterile neutrino with the mass mν, sterile ≈ 1.5 eV motivated by various anomalies in neutrino oscillation experiments would favor cosmology based on f(R) gravity rather than the ΛCDM model.

A Cyclic Cosmological Model Based on the &amp;lt;i&amp;gt;f(ρ)&amp;lt;/i&amp;gt; Modified Theory of Gravity

We consider FLRW cosmological models for perfect fluid (with ρ as the energy density) in the frame work of the f(ρ) modified theory of gravity [V. N. Tunyak, Russ. Phys. J. 21, 1221 (1978); J. R. Ray, L. L. Smalley, Phys. Rev. D. 26, 2615 (1982)]. This theory, with total Lagrangian R-f(ρ), can be considered as a cousin of the F(R) theory of gravity with total Lagrangian F(R)-ρ. We can pick proper function forms f(ρ) to achieve, as the F(R) theory does, the following 4 specific goals, 1) producing a non-singular cosmological model (Ricci scalar and Ricci tensor curvature are bounded); 2) explaining the cosmic early inflation and late acceleration in a unified fashion; 3) passing the solar system tests; 4) unifying the dark matter with dark energy. In addition we also achieve goal number 5) unify the regular matter/energy with dark matter/energy in a seamless fashion. The mathematics is simplified because in the f(ρ) theory the leading terms in Einstein’s equations are linear in second order derivative of metric wrt coordinates but in the F(R) theory the leading terms are linear in fourth order derivative of metric wrt coordinates.

A square‐torsion modification of Einstein‐Cartan theory

AbstractIn the present paper a theory of gravity is considered in which not only curvature but also torsion is explicitly present in the Lagrangian, both with their own coupling constant. In particular, the couplings to Dirac fields and spin fluids are discussed: in the case of Dirac fields, it is discusses how the Dirac self‐interactions depend on the coupling constant as a parameter that may even make these non‐linearitiesmanifest at subatomic scales, showing different applications according to the value of the parameter assigned; in the case of spin fluids, FLRW cosmological models arising from the proposed theory are discussed.

RECONSTRUCTING THE HISTORY OF ENERGY CONDITION VIOLATION FROM OBSERVATIONAL DATA

We study the likelihood of energy condition violations in the history of the Universe. Our method is based on a set of functions that characterize energy condition violation. FLRW cosmological models are built around these "indication functions". By computing the Fisher matrix of model parameters using type Ia supernova and Hubble parameter data, we extract the principal modes of these functions' redshift evolution. These modes allow us to obtain general reconstructions of energy condition violation history independent of the dark energy model. We find that the data suggest a history of strong energy condition violation, but the null and dominant energy conditions are likely to be fulfilled. Implications for dark energy models are discussed.

Equation of state and singularities in FLRW cosmological models

We consider FLRW cosmological models with standard Friedmann equations, but leaving free the equation of state. We assume that the dark energy content of the universe is encoded in an equation of state p = f(ρ), which is expressed with most generality in the form of a power expansion. The inclusion of this expansion in Friedmann equations allows us to construct a perturbative solution and to relate the coefficients of the equation of state with the formation of singularities of different types.

Inhomogeneous generalizations of the FLRW cosmological models

Inhomogeneous generalizations of the FLRW cosmological models