Effective Equation Of State Research Articles

Structure, maximum mass, and stability of compact stars in f(Q,T)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {Q,T})$$\\end{document} gravity

We investigate the properties of compact objects in the f(Q, T) theory, where Q\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathcal {Q}$$\\end{document} is the non-metricity scalar and T\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${ \\mathcal {T}}$$\\end{document} is the trace of the energy–momentum tensor. We derive an interior analytical solution for anisotropic perfect-fluid spheres in hydrostatic equilibrium using the linear form of f(Q,T)=Q+ψT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {Q}, { \\mathcal {T}})=\\mathcal {Q}+\\psi { \\mathcal {T}}$$\\end{document}, where ψ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi $$\\end{document} represents a dimensional parameter. Based on the observational constraints related to the mass and radius of the pulsar SAX J1748.9-2021, ψ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi $$\\end{document} is set to a maximum negative value of ψ1=ψ/κ2=-0.04\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi _1=\\psi / \\kappa ^2=-0.04$$\\end{document}, where κ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\kappa ^2$$\\end{document} is the gravitational coupling constant. The solution results in a stable compact object, which does not violate the speed of sound condition cs2≤c23\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$c_s^2\\le \\frac{c^2}{3}$$\\end{document}. The effective equation of state is similar to the quark matter equation of state, and involves the presence of an effective bag constant. When ψ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi $$\\end{document} is negative, the star has a slightly larger size as compared to GR stars with the same mass. The difference in the predicted star size between the theory with a negative ψ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\psi $$\\end{document} and GR for the same mass is attributed to an additional force appearing in the hydrodynamic equilibrium equation. The maximum compactness allowed by the strong energy condition for f(Q,T)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {Q}, { \\mathcal {T}})$$\\end{document} theory and for GR is C=0.514\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C = 0.514$$\\end{document} and 0.419, respectively, with the f(Q,T)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {Q}, { \\mathcal {T}})$$\\end{document} prediction about 10%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10\\%$$\\end{document} higher than the GR one. Assuming a surface density at saturation nuclear density of ρnuc=4×1014g/cm3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rho _{\ ext {nuc}} = 4\ imes 10^{14}~\\hbox {g}/\\hbox {cm}^3$$\\end{document}, the maximum mass of the star is 4.66M⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$4.66 M_\\odot $$\\end{document}, with a radius of 14.9 km.

Reheating constraints on mutated hilltop inflation

Future research studies of cosmic microwave background (CMB) polarization seem likely to provide a more improved upper bound of [Formula: see text].03 on the tensor-to-scalar ratio (r). In this work, we present the reheating predictions of mutated hilltop inflation (MHI). We find that the model is well consistent with combined Planck+BK18+BAO observations over a range of effective equation of state (EoS) ([Formula: see text]) and model parameter ([Formula: see text]). For lower values of [Formula: see text], MHI generates considerably smaller r compared to normal hilltop potential by lying well within the observational bounds, making MHI a better choice as per recent and future observational studies.

Reconstruction of the singularity-free f(R)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f({\\mathcal {R}})$$\\end{document} gravity via Raychaudhuri equations

We study the bounce cosmology to construct a singularity-free f(R)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f({\\mathcal {R}})$$\\end{document} model using the reconstruction technique. The formulation of the f(R)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f({\\mathcal {R}})$$\\end{document} model is based on the Raychaudhari equation, a key element employed in reconstructed models to eliminate singularities. We explore the feasibility of obtaining stable gravitational Lagrangians, adhering to the conditions fR>0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{{\\mathcal {R}}}>0$$\\end{document} and fRR>0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{{\\mathcal {R}}{\\mathcal {R}}}>0$$\\end{document}. Consequently, both models demonstrate stability, effectively avoiding the Dolgov–Kawasaki instability. Our assessment extends to testing the reconstructed model using energy conditions and the effective equation-of-state (EoS). Our findings indicate that the reconstructed super-bounce model facilitates the examination of a singularity-free accelerating universe for both phantom and non-phantom phases. However, in the case of the reconstructed oscillatory bounce model, two scenarios are considered with ω=-1/3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega =-\\,1/3$$\\end{document} and ω=-2/3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega =-\\,2/3$$\\end{document}. While the model proves suitable for studying a singular-free accelerating universe in the ω=-1/3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega =-\\,1/3$$\\end{document} case, it fails to demonstrate such behavior under energy conditions for the ω=-2/3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega =-\\,2/3$$\\end{document} scenario. The reconstructed models accommodate early-time bouncing behavior and late-time cosmic acceleration within a unified framework.

Cosmological dynamics in f(R) gravity models through cosmographic energy conditions

A proposed f(R) functional form’s constraints caused by energy conditions are presented. Raychaudhuri’s equation and the premise that gravity is attractive are both used to derive null and strong energy conditions for the proposed f(R) gravity model. Furthermore, the weak and dominant energy conditions are established by analysing the energy conditions induced by an effective energy-momentum tensor for the f(R)-theory of gravity. The most recent estimates of the cosmographic parameters that mimic Chaplygin gas-type dark energy are used on the parameters of a pair of f(R) models as a tangible extension of the energy conditions to the Robertson-Walker universe. It is found that under the various values of the coupling parameters the proposed f(R) model satisfy null, weak and dominant energy conditions while strong energy conditions are breached. Further, constraining the coupling parameters as a result of energy conditions, our analysis shows that the proposed model accurately captures the observed cosmic acceleration, as indicated by key parameters such as the effective equation of state (ω eff ), and the deceleration parameter (q). Through a geometric analysis of cosmographic parameters, we observe deviations of f(R) gravity from the ΛCDM model. We identify distinct evolutionary pathways that resemble characteristics of either quintessence or Chaplygin gas dark energy across various epochs, indicating a dynamic rate of cosmic acceleration. Our findings highlight the significant influences of coupling parameters and initial conditions on the cosmic evolution.

Effective equation of state of a radiatively cooling gas

Context. The temperature of the interstellar medium (ISM) is governed by several physical processes, including radiative cooling, external UV/cosmic-ray heating, and mechanical work due to compression and expansion. In regimes where the dynamical effect is important, the temperature deviates from that derived by simply balancing the heating and cooling functions. This renders the expression of the gas energy evolution with a simple equation of state (EOS) less straightforward. Aims. Given a cooling function, the behavior of the gas is subject to the combined effect of dynamical compression and radiative cooling. The goal of the present work is to derive the effective EOS of a collapsing gas within a full fluid solution. Methods. We solved the Navier-Stokes equations with a parametric cooling term in spherical coordinates, and looked for a self-similar collapse solution. Results. We present a solution that describes a cloud that is contracting while losing energy through radiation. This yields an effective EOS that can be generally applied to various ISM contexts, where the cooling function is available from first principles and is expressed as a power-law product of the density and temperature. Conclusions. Our findings suggest that a radiatively cooling gas under self-gravitating collapse can easily manifest an effective polytropic EOS, even isothermal in many scenarios. The present model provides theoretical justification for the simplifying isothermal assumptions of simulations at various scales, and can also provide a more realistic thermal recipe without additional computation costs.

A space dependent Cosmological Constant

In a specific adiabatic perfect fluid, intrinsic entropy density perturbations are the source of a space-dependent cosmological constant responsible for local void inhomogeneity. Assuming an anisotropic Locally Rotationally Symmetric space time, using the 1+1+2 covariant approach and a Lemaître space time metric, we study the cosmological implication of such a scenario giving a proper solution to the Hubble constant tension and providing, locally, also an effective equation of state with w ≤ -1.

Cosmic dynamics of isotropic models with inhomogeneous EoS: A dynamical system perspective

We examine the homogeneous and isotropic models sourced by the fluid having an inhomogenous equation of state (EoS) using qualitative methods. The flat Friedmann–Lemaitre–Robertson–Walker (FLRW) model explains the accelerating universe expansion of the present era having past of decelerating era dominated by radiation and matter. The open FLRW model incorporates the radiation, matter and dark energy dominated era in order with the spatial curvature playing role just before the accelerated universe evolution. The dominating behaviors of radiation, matter and spatial curvature affect the evolution trajectories of the deceleration and effective EoS parameter, which are obtained by the numerical solutions of autonomous system. The model parameters affect the nature of dark energy and therefore the EoS parameter may also lie in the quintessence or phantom regions. The closed FLRW model having constant positive curvature may yield the bouncing and turnaround universe evolution with dark energy-dominated phase at late-times. The application of statefinder diagnostic in the cosmological dynamical systems highlights that the universe may approach the [Formula: see text] cold dark matter ([Formula: see text]CDM) universe as a particular case in the future.

A tilt instability in the cosmological principle

We show that the Friedmann–Lemaître–Robertson–Walker (FLRW) framework has an instability towards the growth of fluid flow anisotropies, even if the Universe is accelerating. This flow (tilt) instability in the matter sector is invisible to Cosmic No-Hair Theorem-like arguments, which typically only flag shear anisotropies in the metric. We illustrate our claims in the setting of “dipole cosmology”, the maximally Copernican generalization of FLRW that can accommodate a flow. Simple models are sufficient to show that the cosmic flow need not track the shear, even in the presence of a positive cosmological constant. We also emphasize that the growth of the tilt hair is fairly generic if the effective equation of state w rightarrow -1 at late times (as it does in standard cosmology), irrespective of the precise model of dark energy. The generality of our theoretical result puts various recent observational claims about late time anisotropies and cosmic dipoles in a new light.

Impact of dark energy on the equation of state in light of the latest cosmological data

Abstract We reconstruct the effective equation of state (EoS) within the framework of the general theory of relativity in a homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker universe, which is assumed to be composed of matter and dark energy (DE). Our analysis employs a dataset consisting of 31 cosmic chronometer data points, six data points of baryon acoustic oscillations, and 1048 type Ia supernovae from the Pantheon sample, and we determine the best-fitting values of the model parameters through Markov chain Monte Carlo simulation. We then use these parameter values to calculate various cosmological parameters, such as the DE EoS parameter, the energy density, the deceleration parameter, the state-finder parameters, and the Om(z) diagnostic. All the analyzed cosmological parameters show behavior consistent with the accelerated universe scenario.

Constraining the cosmological model using recent observational data* *Supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) ( IMSIU-RG23008)

In this study, we conduct a comprehensive investigation of the cosmological model described by (where λ represents a free parameter) in light of the most recent observational data. By constraining the model using the and datasets, we determine its compatibility with the observed behavior of the Universe. For this purpose, we adopt a parametric form for the effective equation of state (EoS) parameter. This parametric form allows us to describe the evolution of the EoS parameter with respect to redshift and investigate its behavior during different cosmic epochs. The analysis of the deceleration parameter reveals an accelerating Universe with a present value of , indicating the current phase of accelerated expansion. The transition redshift is found to be , marking the epoch of transition from deceleration to acceleration. We also analyze the evolution of important cosmological parameters, including the density parameter, pressure, effective EoS, and stability. These findings collectively demonstrate the viability of the cosmological model as a robust candidate capable of engendering the requisite negative pressure, thereby efficiently propelling cosmic expansion. Moreover, the undertaken stability analysis underscores the model's stability within the broader cosmic landscape. By providing the best-fit values for the coupling parameter λ, this approach motivates and encourages further exploration into the extensive landscape of this model and its potential applications across diverse realms of cosmology and astronomy.

Cosmological constraints on dark energy in [formula omitted] gravity: A parametrized perspective

In this paper, we focus on the parametrization of the effective equation of state (EoS) parameter within the framework of f(Q) symmetric teleparallel gravity. Here, the gravitational action is represented by an arbitrary function of the non-metricity scalar Q. By utilizing a specific parametrization of the effective EoS parameter and a power-law model of f(Q) theory, namely f(Q)=βQm+1 (where β and m are arbitrary constants), we derive the cosmological solution of the Hubble parameter H(z). To constrain model parameters, we employ recent observational data, including the Observational Hubble parameter Data (OHD), Baryon Acoustic Oscillations data (BAO), and Type Ia supernovae data (SNe Ia). The current constrained value of the deceleration parameter is found to be q0=−0.50−0.01+0.01, indicating that the current Universe is accelerating. Furthermore, we examine the evolution of the density, EoS, and Om(z) diagnostic parameters to deduce the accelerating nature of the Universe. Finally, we perform a stability analysis with linear perturbations to confirm the model’s stability.

Viscous Fluid Cosmology in Symmetric Teleparallel Gravity

AbstractIn this article, the viscous fluid cosmological model is analyzed in the framework of recently proposed gravity by assuming three different forms of bulk viscosity coefficients, specifically, , , and and a linear model, particularly, where is free model parameter. The bulk viscosity coefficients and the model parameter values using the combined H(z)+Pantheon+BAO data set are estimated. The asymptotic behavior of our cosmological bulk viscous model is studied by utilizing the phase space method. It is found that corresponding to all three cases, our model depicts the evolution of the universe from matter dominated decelerated epoch (a past attractor) to a stable de‐sitter accelerated epoch (a future attractor). Furthermore, the physical behavior of effective pressure, effective equation of state (EoS), and the statefinder parameters are studied. It is also found that the pressure component in the presence of bulk viscosity shows negative behavior and the effective EoS parameter predicts the accelerated expansion phase of the universe for all three cases. Moreover, the trajectories of the model lie in the quintessence region and it converges to the ΛCDM fixed point in the far future. It is found that the accelerated de‐Sitter like phase comes purely from the case without any geometrical modification to GR. Moreover, the late‐time behavior of all three cases of viscosity coefficients are identical. Furthermore, a non‐linear model is considered, specifically, , and then analyzed the behavior of model using dynamical approach. It is also found that the late‐time behavior of the considered non‐linear model with is similar to the linear case, whereas for the case results are quite different.

Constraining the Viscous Dark Energy Equation of State in f (R, Lm) Gravity

In this article, we attempt to describe the cosmic late-time acceleration of the universe in the framework of f(R,Lm) gravity, by using an effective equation of state, when bulk viscosity is taken into account. We presume a non-linear f(R,Lm) functional form, specifically, f(R,Lm)=R2+Lmα, where α is a free model parameter. We obtain the exact solution of our bulk viscous matter dominated f(R,Lm) model, and then we utilize the combined H(z) + Pantheon + Analysis datasets to estimate the best fit values of the free parameters of our model. Then, we characterize the behavior of the matter–energy density, effective pressure, and the equation of state (EoS) parameter, incorporating the viscous type fluid. The evolution profile of the effective EoS parameter depicts an acceleration phase of the cosmic expansion, whereas the pressure, with the effect of viscosity, exhibits negative behavior, that can lead to the accelerating expansion of the universe. Moreover, the cosmic matter–energy density shows the expected positive behavior. Further, we investigate the behavior of the statefinder parameters for the assumed f(R,Lm) model. We find that the evolutionary trajectory of the given model lies in the quintessence region. In addition, we employ the Om diagnostic test, that indicates that our model exhibits quintessence behavior. Lastly, we check the energy condition criteria and find that the violation of SEC occurs in the past, whereas NEC and DEC satisfy the positivity criteria. We find that our f(R,Lm) cosmological model, with the effect of bulk viscosity, provides a good fit of the recent observational data and can efficiently describe the cosmic expansion scenario.

Thermodynamics of the Acceleration of the Universe in the κ(R, T) Gravity Model

In this article, we examined the behavior of dark energy (DE) and the cosmic acceleration in the framework of κ(R,T) gravity in the standard spherically symmetric coordinates (xi) = t,r,θ,ϕ, a spatially homogeneous and isotropic FLRW space–time. We discovered some remarkable cosmic characteristics in this investigation that are in line with both observations and the accepted ΛCDM model. We made two assumptions in order to determine a deterministic solution of the modified field equations (MFEs): (i) p=γρ, where γ(1≥γ≥0) is a constant, (ii) Λ = βH2, where β is an arbitrary constant. We solved the MFEs and obtained the expression for the Hubble parameter. The depicted model of κ(R,T) gravity was taken into consideration when discussing the behavior of the accelerating Universe. In κ(R,T) gravity, the statefinder analysis was utilized to distinguish our model from the ΛCDM model. The evolution of the cosmos was studied using an effective equation of state (EoS). We investigated the thermodynamic quantities and the generalized energy conditions in order to test the viability of our model. When dominant and weak energy conditions are satisfied, this validates the model; when the strong energy condition is not satisfied, this accelerates the expansion of the Universe.

Squared Torsion f(T,T)$f(T,\mathcal {T})$ Gravity and Its Cosmological Implications

AbstractWe present the coupling of the torsion scalar T and the trace of energy‐momentum tensor , which produces new modified gravity. Moreover, we consider the functional form where α and β are free parameters. As an alternative to a cosmological constant, the theory may offer a theoretical explanation of the late‐time acceleration. The recent observational data to the considered model especially the bounds on model parameters is applied in detail. Furthermore, we analyze the cosmological behavior of the deceleration, effective equation of state and total equation of state parameters. However, it is seen that the deceleration parameter depicts the transition from deceleration to acceleration and the effective dark sector shows a quintessence‐like evolution.

Observational constraints on Tsallis holographic dark energy with Ricci horizon cutoff

In this research, we are interested in constraining the nonlinear interacting and noninteracting Tsallis holographic dark energy (THDE) with Ricci horizon cutoff by employing three observational datasets. To this aim, the THDE with Ricci horizon considering the noninteraction and nonlinear interaction terms will be fitted by the SNe Ia, SNe Ia+H(z), and SNe Ia+H(z)+GRB samples to investigate the Hubble ($H(z)$), dark energy equation of state ($\omega_{DE}$), effective equation of state ($\omega_{eff}$), and deceleration ($q$) parameters. Investigating the $H(z)$ parameter illustrates that our models are in good consistent with respect to observation. Also, it can reveal the turning point for both noninteracting and nonlinear interacting THDE with Ricci cutoff in the late time era. Next, the analysis of the $\omega_{DE}$ for our models displays that the dark energy can experience the phantom state at the current time. However, it lies in the quintessence regime in the early era and approaches the cosmological constant in the late-time epoch. Similar results will be for the $\omega_{eff}$ parameter with this difference that the $\omega_{eff}$ will experience the quintessence region at the current redshift. Next to the mentioned parameters, the study of the $q$ parameter indicates that the models satisfy an acceptable transition phase from the matter to the dark energy-dominated era. After that, the classical stability ($v_{s}^{2}$) will be analyzed for our models. The $v_{s}^{2}$ shows that the noninteracting and nonlinear interacting THDE with Ricci cutoff will be stable in the past era, unstable in the present, and progressive epochs. Then, we will employ the $Jerk$ ($J$) and $OM$ parameters to distinguish between our models and the $\Lambda CDM$ model. Finally, we will calculate the age of the Universe for the THDE and nonlinear interacting THDE with Ricci as the IR cutoff.

The effective equation of state in Rastall gravity

In this paper, we study a homogeneous and isotropic flat Friedmann–Robertson–Walker (FRW) universe in Rastall gravity, in which there exist the components of radiation and dust. That is to say, the density of fluid [Formula: see text] satisfies the barotropic equation of state [Formula: see text], where [Formula: see text] and [Formula: see text], and the equation of state [Formula: see text]. We derive the modified Friedmann equations, and the effective equation of state [Formula: see text] in this universe model. For the dust-dominated case, the effective equation of state [Formula: see text] is described by [Formula: see text], where [Formula: see text] is the Rastall parameter. As [Formula: see text], we have [Formula: see text] which means an accelerated expansion of the universe occurs. Along with the conditions for the convergent modified Friedmann equations and the positive density of dust, the Rastall parameter [Formula: see text] should be in the range of [Formula: see text]. In other words, the various [Formula: see text] for [Formula: see text] correspond to those of quintessence with the general barotropic equation of state [Formula: see text]. For the general case, the Rastall universe full of radiation and dust, the expression of [Formula: see text] is complicated involving the Rastall parameter, the density of radiation and the density of dust. As the ratio of the densities of dust and radiation is smaller than [Formula: see text], [Formula: see text] is less than [Formula: see text]. Moreover, to keep the density of dust positive, the Rastall parameter should be restricted in [Formula: see text] too. By the numerical calculation, we find that [Formula: see text] from [Formula: see text] decreases as [Formula: see text] increases. And [Formula: see text] for different [Formula: see text] ultimately tend to [Formula: see text], the value of which are in the range of [Formula: see text]. Obviously, the expansion of the cosmology filled with radiation and dust or only dust in Rastall gravity [Formula: see text] accelerates without any “exotic” energy whose equation of state [Formula: see text].

Aspects of non-interacting and interacting universe: a complete scenario

The evolution of the dark energy within the scope of a spatially homogeneous and isotropic Friedmann–Robertson–Walker model filled with barotropic fluid and dark energy is examined. We have considered the two cases of an interacting and non-interacting two-fluid (barotropic and dark energy) scenario and obtained the exact solutions to the corresponding field equations. In an interacting model, we have considered linear as well as quadratic interaction between dark sectors of the universe. We have also given the quadrature form for general interaction coupling parameter via Q = αρ m n, n ⩾ 1 and discussed the evolution of energy densities and effective equation of state with cosmic time for flat, closed, and open universes. It is also examined that the matter energy density is dominated in early phase of the universe, whereas the dark energy density is dominated in later phase of the universe for both scenarios, which is in concurrence with current observations.

Rapidly rotating Dirac stars

Within general relativity, we construct sequences of rapidly rotating Dirac stars consisting of a spinor fluid described by an effective equation of state. We find the physically relevant domain of stable configurations and calculate their principal characteristics which are completely determined by the central density of the spinor fluid, the mass of the nonlinear spinor field, and the velocity of rotation. It is demonstrated that for a certain choice of the spinor field mass, the main physical characteristics of the Dirac stars are close to those that are typical of rotating neutron stars.

Accelerating scenarios of viscous fluid universe in modified f(T) gravity

In this paper, we have investigated the physical behavior of cosmological models in modified teleparallel gravity with a linear function [Formula: see text] where [Formula: see text] and [Formula: see text] are model parameters and [Formula: see text] is the torsion scalar. We have considered a homogeneous and isotropic Friedman universe filled with bulk viscosity fluid. We have solved the field equations for the scale factor [Formula: see text] and found [Formula: see text] where [Formula: see text] and [Formula: see text] and [Formula: see text] is an integrating constant, [Formula: see text] is the equation of state (EoS) for normal matter and [Formula: see text] is generated from bulk viscosity fluid. We have calculated the several cosmological parameters for this scale factor and studied their physical and geometrical behavior along with the observational data sets [Formula: see text] and Union [Formula: see text] compilation of SNe Ia data sets. We have observed that the [Formula: see text] factor reveals the presence of cosmological constant and for [Formula: see text], the acceleration drives by the bulk viscosity of the fluid and it behaves just like dark energy model without cosmological constant. We have studied the effective EoS [Formula: see text] and found [Formula: see text]. We have evaluated the age of the present universe as [Formula: see text] Gyrs. Also, we have studied the nature of deceleration parameter with the signature-flipping point at [Formula: see text] and the present value of deceleration parameter [Formula: see text] is obtained as [Formula: see text], respectively, for both observational datasets.