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The primary objective of this paper is to examine singularity-free solutions within the framework of anisotropic solutions for the Chaplygin equation of state in f (R , ϕ , X) modified gravity theory. Herein, R signifies the Ricci scalar, ϕ denotes the scalar field, and X represents the kinetic term associated with ϕ. The investigation employs the Krori–Barua metric to explore the characteristics of an anisotropic compact star. In the course of our analysis, we specifically consider observational data about three distinct stellar structures: Her X-I, LMC X-4, and 4U 1538-52. Our inquiry extends to the graphical examination of various physical parameters, encompassing energy density, pressure components, anisotropic components, equation of state parameters, equilibrium conditions, stability analysis, adiabatic index, and energy conditions. Furthermore, we delve into the intricacies of the mass-radius function, compactness factor, and surface redshift–integral features of charged compact stars. The culmination of our findings suggests that the f (R , ϕ , X) theory of gravity when subjected to observational scrutiny, manifests as realistic, stable, and devoid of singularities. [ABSTRACT FROM AUTHOR]

This paper is devoted to observe the physical attributes of isotropic relativistic spherical objects in the context of Rastall theory of gravity. In order to inspect the structural composition of spherical objects, the Tolman IV solution is taken into account. The unknown parameters associated with Tolman IV solution are evaluated through matching conditions with derived values of radii and masses of some specific star models. The consistency of the adopted ansatz is analyzed via graphical interpretation of matter variables, equation of state parameter, mass components, energy constraints, causality condition and stellar equation for several choices of Rastall parameter. It is observed that the considered solution does not admit singularities in the matter variables. Moreover, in accordance with the graphical analysis, the stellar models satisfy all physical requirements anticipated in a realistic star. It is also shown that for vanishing Rastall coupling parameter, the standard outcomes of general relativity can be retrieved. [ABSTRACT FROM AUTHOR]

This paper is devoted to study FRW universe in the context of Chern–Simons corrected f (R) gravity. In particular, the work provides the investigation of cosmological dynamics within modified gravity by assuming a suitable Hubble parameter that might result in a realistic bouncing universe. For this purpose, we consider a suitable Hubble parameter and examine the bouncing cosmology by assuming a well-known f (R) gravity model. Further, we explore the behavior of energy density and pressure profiles by employing the chosen Hubble parameter. We obtain positive and increasing trends of energy density during the bouncing phase while the negative behavior of pressure in our work could potentially be associated with cosmic expansion. We have also studied the energy conditions and in our case NEC is not violated perhaps due to the reason that the equation of state parameter is not crossing the phantom divide. However, the SEC is violated justifying a successful bounce. Moreover, the validation of DEC and TEC suggests that the chosen model is well behaved in nature. The Hubble parameter passing through the bouncing point shows an increasing trend, indicating an expansion phase after the bounce. The equation of state parameter is negative after the bouncing spot, particularly around ω ≈ − 1. This behavior justifies the current cosmic expansion. The deceleration parameter exhibits a negative behavior implies that the cosmological model demonstrates cosmic expansion. Thus all the results validate the existence of a bouncing universe with the chosen model in Chern–Simons corrected f (R) gravity. [ABSTRACT FROM AUTHOR]

The aim of this paper is to examine the singularity-free solutions for anisotropic stellar structures in f (R , ϕ) theory of gravity, where R denotes the Ricci scalar and ϕ represents the scalar field. For this purpose, we consider the spherically symmetric spacetime to discuss the stellar structures with anisotropic fluid. Moreover, the Einstein field equations are solved by using Chaplygin equation of state, and the Karori–Barua metric. To analyze the charged stellar structure, we use the matching conditions of spherically symmetric spacetime with the Reissner–Nordstrom exterior metric and develop some expressions for finding the unknown parameters. We further investigate the graphical behavior of energy density, pressure components, anisotropic component, equation of state parameters, equilibrium condition, stability analysis, adiabatic index and energy conditions. Conclusively, our considered stellar structures under observations are realistic, stable, and are free from any singularities. [ABSTRACT FROM AUTHOR]

In this paper, the basic Einstein–Hilbert cosmological model is extended by adding a new functional F (G , T μ ν T μ ν) in the fundamental action, encoding specific geometrical effects due to a nontrivial coupling with the Gauss–Bonnet invariant (G), and the energy–momentum squared term ( T μ ν T μ ν ). After obtaining the corresponding gravitational field equations for the specific decomposition where F (G , T μ ν T μ ν) = f (G) + g (T μ ν T μ ν) , we have explored the physical features of the cosmological model by considering the linear stability theory, an important analytical tool in the cosmological theory which can reveal the dynamical characteristics of the phase space. The analytical exploration of the corresponding phase space structure revealed that the present model can represent a viable dark energy model, with various stationary points where the effective equation of state corresponds to a de-Sitter epoch, possible explaining the early and late time acceleration of the Universe. [ABSTRACT FROM AUTHOR]

In this paper, we consider the framework of the newly proposed modified theory of gravity named f (R , A) gravity, wherein R and A signify the Ricci and anticurvature scalars, respectively. For our cosmic discussions, we evaluate a renowned f (R , A) model, incorporating various forms of the scale factor within the context of Friedmann–Robertson–Walker (FRW) spacetime. Our exploration delves into energy conditions, the effective equation of state parameter and the stability of models within the f (R , A) theory. These models are further elucidated through graphical representations plotted against time and their respective model constants. Notably, it is observed that scale factors with exponential terms yield more consistent results in comparison to those of power-law forms, given specific values of model constants. [ABSTRACT FROM AUTHOR]

In this paper, we study an isotropic and homogeneous Friedmann–Lemaitre–Robertson–Walker cosmological model in Weyl type f (Q , T) gravity with a dynamic deceleration parameter. The solution of the field equations is found using a variable deceleration parameter, viz., q = n + m H , with q the deceleration parameter, n and m constants, and H the Hubble parameter [Tiwari et al., Eur. Phys. J. Plus 132, 1 (2017)]. Then we calculate the values of the model parameters that are most likely to accord with the most recent observational data sets. We make use of the 57 CC points, SN and BAO datasets. We have contracted a graphical representation of the dynamical behavior of the cosmological parameters (the Hubble parameter, the deceleration parameter, the energy density and the equation of state) in terms of the redshift. To investigate the various eras of the universe, the evolution of the statefinder trajectory pairs (s , r) and (q , r) are studied. It is shown that the main energy component of the model is ordinary matter, which behaves initially as the SCDM model and then later as a fluid resembling quintessence, which behaves as the well-known Λ CDM model. A suitable cosmological model is constructed after analyzing the physical parameters of the model, the O m (z) diagnostic analysis. [ABSTRACT FROM AUTHOR]

In this paper, we study the Chaplygin gas equation in curvature-matter coupling gravity to describe the observed expansion of the universe. We consider the Chaplygin gas equation of state in terms of the energy density ρ and pressure p , by the relationship p = − A ρ and the curvature-matter coupling gravity as f (R , T) , where A is a positive constant, R and T denote, respectively, the Ricci scalar and the trace of energy–momentum tensor. Considering the gravitational lagrangian in the form f (R , T) = R + h (T) , where we used two forms of h (T) and by assuming a perfect fluid as matter source, we determined through the modified Friedmann equations, the corresponding energy and pressure for each model. By using the free parameters of our models, we performed the numerical results of the energy conditions to study its viability. Furthermore, we have derived some cosmological parameters like deceleration parameter and statefinder parameters and graphically investigated the nature of these parameters in Chaplygin gas model. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to study the feasibility and the appearance of charged thin-shell wormholes using generalized Chaplygin gas under the influence of minimally coupled f (R , T) gravitational theory. Here, f is a generic function of the scalar curvature R and the trace of stress-energy tensor T. We explore different components of Lanczos equations in the context of a specific f (R , T) functional form, i.e. f (R , T) = R + 2 λ T , by applying junction conditions on field equations. We study static stable solutions using radial perturbation and the generalized Chaplygin gas equation of state (EoS) with the isotropic environment. We investigate the stable (unstable) static wormhole solutions for various physical parametric values and illustrate them graphically. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated an anisotropic cosmological model in f (Q) gravity with string fluid in LRS Bianchi type-I universe. We have considered the arbitrary function f (Q) = Q + α Q + 2 Λ , where α is model free parameter and Λ is the cosmological constant. We have established a relationship between matter energy density parameter Ω m and dark energy density parameter Ω Λ through Hubble function using constant equation of state parameter ω. We have made observational constraint on the model using χ 2 -test with observed Hubble datasets H (z) and SNe Ia datasets, and obtained the best fit values of cosmological parameters. We have used these best fit values in the result and discussion. We have discussed our result with cosmographic coefficients and found a transit phase dark energy model. Also, we analyzed the Om diagnostic function for anisotropic universe and found that our model is quintessence dark energy model. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the late-time accelerated universe evolution in a flat, homogeneous and isotropic model in the context of f (Q , T) gravity, where Q and T are non-metricity scalar and trace of energy–momentum tensor, respectively. The scale factor, by construction, yields ekpyrotic contraction era followed by a non-singular bounce. The expanding era of the universe yields late-time dark energy era preceded by matter-dominating decelerating era. The model unifies an ekpyrotic, non-singular bounce with the present dark energy-dominated epoch. The model parameters in the functional form of f (Q , T) gravity affect the dynamical evolution of the equation of state (EoS) parameter. The theoretical value of EoS parameter is found to be ω 0 = − 1. 0 1 3 6 , − 1. 0 2 7 3 for n = 0 , 1 , respectively, and it lies in range of the estimated value of EoS parameter from the Planck+SNe+BAO observational data. Different aspects of this bouncing model including behavior of geometrical and physical quantities along with energy conditions have been discussed in detail. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the dynamic evolution of universe in the models of holographic dark energy with f (Q , T) gravity framework where, Q is the non-metricity scalar and T is the energy–momentum tensor trace. We have considered f (Q , T) = α Q + β T framework and investigated the evolution of cosmological quantities like energy density, equation of state (EoS) parameter and classical stability parameter with redshift. We parameterize the deceleration parameter and confront the Hubble parameter with the observational data. We investigate for the late-time accelerated expansion of universe and discuss the stability of the model by using adiabatic sound speed squared parameter. A comparison among these derived models suggest that the Renyi holographic dark energy model with GO cutoff satisfies the observational constraint of Planck+SNe+BAO on EoS parameter at present time as compared to other models. Diagnostic tools such as Statefinders and Om diagnostic have been used to classify the dark energy evolution regions. [ABSTRACT FROM AUTHOR]

In this paper, we have explored the field equations of f (T , B) gravity as an extension of teleparallel gravity in an isotropic and homogeneous space-time. In the basic formalism developed, the dynamical parameters are derived by incorporating the power law and exponential scale factor function. The models show accelerating behavior and approach Λ CDM at late time. The present value of the equation of state parameter for both the cases is obtained to be in accordance with the range provided by cosmological observations. The geometrical parameters and the scalar field reconstruction are performed to assess the viability of a late-time accelerating universe. Further, the stability of both the models is presented. It has been observed that both the models are parameter-dependent. Since most of the geometrically modified theories of gravity favor the violation of strong energy condition (SEC), we have derived the energy conditions both for the power law and exponential model. In both the models, the violation of SEC is established. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to examine the anisotropic matter distribution concerning one of the well-known f (R , T) theories of gravity, where R and T represent the Ricci scalar and trace of the energy–momentum tensor, respectively. We consider anisotropic matter distribution in spherically symmetric spacetime for our desired work. We investigate the physical behavior of pressure components, energy density, anisotropic function, equation of state parameters, and energy conditions. Furthermore, we analyze the stability of compact stars by investigating the causality condition, adiabatic index, and generalized Tolman–Oppenheimer–Volkoff equation. Some fundamental features of compact stars have been investigated, like compactness factor, mass function, and surface redshift. Our calculated solutions for Bardeen stellar structures are consistent and stable in the background of the f (R , T) theory of gravity. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the cosmic analysis of Kaniadakis holographic dark energy model in the frameworks of f (G) and f ̃ (T) modified theories of gravity. We reconstruct functional forms of f (G) and f ̃ (T) models by using a well-known power-law form of scale factor and flat FRW metric. The behavior of these functional forms with respect to their arguments is discussed. Using these models, various cosmic parameters like energy conditions, effective equation of state parameter, deceleration parameter and squared speed of sound parameters are explored to discuss the accelerated expansion of the universe. We obtain some consistent results for accelerated expansion of the universe related to specific values of model parameters. [ABSTRACT FROM AUTHOR]

The main emphasis of this paper is to find the viable solutions of Einstein Maxwell field equations of compact star in context of modified f (R) theory of gravity. Two different models of modified f (R) gravity are considered. In particular, we choose isotropic matter distribution and Bardeen's model for compact star to find the boundary conditions as an exterior space-time geometry. We use the conformal Killing geometry to compute the metric potentials. We discuss the behavior of energy density and pressure distribution for both models. Moreover, we analyze different physical properties such as behavior of energy density and pressure, equilibrium conditions, equation of state parameters, causality conditions and adiabatic index. It is noticed that both f (R) gravity models are suitable and provide viable results with Bardeen geometry. [ABSTRACT FROM AUTHOR]

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 ρ γ satisfies the barotropic equation of state p γ = (γ − 1) ρ γ , where γ = 4 / 3 and 1 , and the equation of state w = γ − 1. We derive the modified Friedmann equations, and the effective equation of state w ̃ in this universe model. For the dust-dominated case, the effective equation of state w ̃ is described by λ − 1 3 λ − 1 , where λ is the Rastall parameter. As 1 3 < λ < 2 3 , we have w ̃ < − 1 3 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 λ should be in the range of (1 2 , 2 3). In other words, the various w ̃ for 1 2 < λ < 2 3 correspond to those of quintessence with the general barotropic equation of state (γ = 1). For the general case, the Rastall universe full of radiation and dust, the expression of w ̃ 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 2 − 3 λ 2 (2 λ − 1) , w ̃ is less than − 1 3 . Moreover, to keep the density of dust positive, the Rastall parameter should be restricted in (1 2 , 2 3) too. By the numerical calculation, we find that w ̃ from 1 3 decreases as N (N ≡ l n a (t)) increases. And w ̃ for different λ ∈ (1 2 , 2 3) ultimately tend to λ − 1 3 λ − 1 , the value of which are in the range of (− 1 , − 1 3). Obviously, the expansion of the cosmology filled with radiation and dust or only dust in Rastall gravity (1 2 < λ < 2 3) accelerates without any "exotic" energy whose equation of state w < − 1 3 . [ABSTRACT FROM AUTHOR]

In this paper, in an extended theory of gravity, we have presented bouncing cosmological model at the backdrop of an isotropic, homogeneous space-time, in the presence of general relativistic hydrodynamics (GRH). The scale factor has been chosen in such a manner that with appropriate normalization, the quintom bouncing scenario can be assessed. Accordingly, the bounce occurs at t = 0 and the corresponding Hubble parameter vanishes at the bounce epoch. The equation of state (EoS) parameter and the energy conditions of the model have been analyzed. The violation of strong energy condition further supports the behavior of extended gravity. As the bouncing cosmology suffers with instability, this model also shows the similar behavior. [ABSTRACT FROM AUTHOR]

This paper explores the aspects of compact stars in f (T ,) gravity, where T and express the torsion and trace of the energy–momentum tensor. To achieve this goal, we consider a spherically symmetric spacetime with an anisotropic source of fluid. In particular, the Krori–Barua (KB) spacetime is considered to explore the solution of compact stars. In addition, we select three specific types of compact stars models, namely J1416-2230, 4U 1608-52 and Cen X-3. We develop the field equations using the KB spacetime with some specific form of f (T ,) gravity. In addition, we consider the Schwarzschild geometry for the matching conditions at the boundary. Then, we calculate the values of all the relevant parameters by imposing matching conditions. We provide detailed graphical analysis to discuss the physical acceptability of different parameters, namely energy density, pressure, anisotropy and gradient. We also examine the stability of compact stars by exploring energy conditions, equations of state, conditions of causality, redshift functions, mass functions and compactness functions. Finally, we find that the obtained solutions are physically viable and also having good properties for the compact star models. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the gravitational lensing for power-Maxwell charged quintessence black hole (BH) without cosmological constant in Rastall's theory of gravity in both the strong-field limit and weak-field limit. Furthermore, we discuss the gravitational lensing by the linear electrodynamic (LE) and nonlinear electrodynamic (NLE) Maxwell field with the equation of state (EoS) parameter of the quintessence (ω) and compared to the case with Einstein's gravity. We discuss the behaviors of the photon sphere radius r c , impact parameter u c , strong lensing coefficients b 1 , b 2 and the deflection angle α (u) on the equatorial plane (= π 2) with the different values of Rastall parameter λ for the case of LE and NLE Maxwell field in the strong field limit. We also investigate the angular separation X , angular position ∞ and the magnification of the relativistic images. Further, we also study about the weak lensing BHs in Rastall gravity. We investigate the behaviors of weak deflection angle with the different values of Rastall parameter λ for the case of LE and NLE Maxwell field in the weak-field limit. [ABSTRACT FROM AUTHOR]

In this paper, we derive the exact solution of traversable wormholes illustrating spherically symmetric geometry with anisotropic matter distribution entering the throat in the formalism of f (ℛ ,) gravity, where ℛ is Ricci scalar and is trace of energy–momentum tensor. For this purpose, we assume a power law-type generic function f (ℛ ,) = ℛ + γ ℛ 2 + α , where γ and α are being constants, with two different choices of shape functions a (r) = r 0 ( b r b r 0 ) , 0 < a (r) < 1 and a (r) = r 0 (cosh (r 0) cosh (r)) μ , 0 < μ < 1. For each approach, we find the exact solution and studied the existence of wormhole solutions in the presence of exotic and non-exotic matter. The graphical behavior of equation of state (EoS) parameter and energy condition bounds is also investigated for each shape function. The realistic wormhole solutions are obtained for the shape functions which satisfy necessary conditions at the throat radius r = r 0 = 1. Finally, we observe that there is small deviation in the results obtained by General Relativity and f (ℛ 2 ,) gravity. [ABSTRACT FROM AUTHOR]

There are many ways to probe alternative theories of gravity, namely, via experimental tests at solar system scale, cosmological data and models, gravitational waves and compact objects. In this paper, we consider a model of gravity with torsion f (T) applied to compact objects such as neutron stars (NSs) for a couple of realistic equations of state (EOS). To do so, we follow our previous articles, in which we show how to model compact stars in f (T) gravity by obtaining its corresponding Tolman–Oppenheimer–Volkof equations. In this modeling of NS in f (T) gravity presented here, we calculate, among other things, the maximum mass allowed for a given realistic EOS, which would also allow us to evaluate which models are in accordance with the observations. The results already known to General Relativity must be reproduced to some extent and, eventually, we can find models that allow higher maximum masses for NSs than Relativity itself, which could explain, for example, the secondary component of the event GW190814, if this star is a massive NS. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to study the physical characteristics and stability of anisotropic compact stars in energy–momentum squared gravity. We use Tolman solution and consider specific models of this theory to study the viable features of SAX J1808.4-3658, 4U 1538-52 and Her X-1 stars. We explore the behavior of energy density, pressure components, anisotropic factor, energy conditions and equation of state parameter in the interior of considered stars. To check the stability of these stars, we examine the behavior of causality conditions, Herrera cracking approach and adiabatic index. It is found that all the required conditions are satisfied for the proposed models, thus the considered stars are viable as well as stable in this modified theory. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the physical behavior of cosmological models in modified teleparallel gravity with a linear function f (T) = α T + β where α and β are model parameters and T 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 a (τ) and found a (τ) = c 1 [ sinh (n 2 τ) ] n 3 where n 2 = β (1 + ω) [ κ 2 ξ 0 − 3 (1 + ω) (1 + α) ] 8 and n 3 = 2 [ κ 2 ξ 0 − 3 (1 + ω) (1 + α) ] and c 1 is an integrating constant, ω is the equation of state (EoS) for normal matter and ξ 0 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 H (z) and Union 2. 1 compilation of SNe Ia data sets. We have observed that the β factor reveals the presence of cosmological constant and for β = 0 , 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 ω eff and found − 0. 2 8 0 7 ≤ ω eff ≤ − 0. 1 7 6 0. We have evaluated the age of the present universe as 1 3. 8 7 Gyrs. Also, we have studied the nature of deceleration parameter with the signature-flipping point at z t = 0. 5 , 1. 1 4 and the present value of deceleration parameter q 0 is obtained as − 0. 3 3 0 3 , − 0. 4 1 5 2 , respectively, for both observational datasets. [ABSTRACT FROM AUTHOR]

This paper examines the structure of gravastar (gravitationally vacuum star) in the background of f (ℜ , 2) theory. The gravastar can be regarded as a substitute for a black hole comprising of three regions (i) the interior domain, (ii) the intrinsic shell and (iii) the exterior domain. We investigate these domains by employing Karmarkar's condition by assuming a specific f (ℜ , 2) model. The field equations are solved to evaluate nonsingular solutions of both the interior and intermediate regions. Inner as well as outer spacetimes are matched together using Israel formalism to obtain density and pressure at the joining surface. Different attributes of gravastar like the equation of state parameter, proper length, entropy and energy are analyzed through graphical behavior. It is found that the gravastar model is a viable alternative to the black hole in the context of this gravity. [ABSTRACT FROM AUTHOR]

This paper is devoted to studying the structure of gravitationally vacuum condensate stars under the influence of matter and geometry coupled gravity. This alternative to black hole comprises three domains, i.e. interior, exterior and thin-shell. We investigate such regions by considering particular equations of state for a specific model of energy–momentum squared gravity. The nonsingular solutions of field equations are obtained to describe the interior region of gravastar and the Schwarzschild spacetime represents the exterior region. The junction conditions are used to match interior and exterior domains while Lanczos equations provide the surface density and pressure at thin-shell. We investigate various attributes of gravastar such as the equation of state parameter, entropy, proper length as well as energy. The physical features of a gravitationally vacuum star indicate a linear relationship with the thickness of the shell and provide a viable structure similar to general relativity. [ABSTRACT FROM AUTHOR]

The main objective of this paper is to examine the physical features and stability of anisotropic compact stellar objects in energy–momentum squared gravity. For this purpose, we apply Krori–Barua metric solutions and consider three different models of this modified theory to examine the physical characteristics of Her X-1, 4U 1538-52 and SAX J1808.4-3658 compact stars. We analyze the behavior of various physical quantities such as energy density, pressure components, energy conditions, equation of state parameter and anisotropic factor in the interior of these stars. We then use the estimated values of the sound speed and of the adiabatic index to explore the stability of these compact stars. It is found that all the required conditions are satisfied corresponding to all the selected models. We are therefore allowed to conclude that this modified theory provides viable and stable anisotropic compact stars. [ABSTRACT FROM AUTHOR]

In this paper, we study a cosmological model governed by a generalized scalar field nonminimally coupled to scalar torsion in the framework of teleparallel gravity. By applying the Noether symmetry approach, we attempt to explore the explicit forms of the coupling function and the scalar field potential in the Friedmann–Lemaitre–Robertson–Walker (FLRW) flat spacetime. The existence of such symmetry permits the equations of motion to take a simpler form and provides an exponential form cosmological solution for both the scale factor and scalar field. It is shown that the tachyon field behaves as the origin of inflation and contributes significantly to the accelerating expansion of the universe in the early times. While, in the late times, gravitational sources are a pressureless matter field together with the tachyon field, which plays the essential role of dark energy and is the origin of deceleration-acceleration phase transition. Later on, it is also shown that the present model contains a cosmological degeneracy in the sense that different coupling parameters and tachyonic potentials can contribute to the same cosmological behaviors such as the cosmic acceleration, age, equation of state and mean Hubble of FLRW universe. [ABSTRACT FROM AUTHOR]

This paper studies the influence of charge on a compact stellar structure also regarded as vacuum condensate star in the background of f (ℜ , 2) gravity. This object is considered the alternate of black hole whose structure involves three distinct regions, i.e. interior, exterior and thin-shell. We analyze these domains of a gravastar for a particular model of this modified theory. In the inner region of a gravastar, the considered equation of state defines that energy density is equal to negative pressure which is the cause of repulsive force on the spherical shell. In the intermediate shell, pressure and energy density are equal and contain ultra-relativistic fluid. The inward-directed gravitational pull of thin-shell counterbalances the force exerted by the inner region of a gravastar allowing the formation of a singularity-free object. The Reissner–Nordström metric presents the outer vacuum spherical domain. Moreover, we discuss the impact of the charge on physical attributes of a gravastar such as the equation of state parameter, entropy, proper length and energy. We conclude that singularity-free solutions of charged gravastar are physically consistent in this modified theory. [ABSTRACT FROM AUTHOR]

In this paper, the charged thin-shell wormholes have been constructed by using cut-and-paste approach in the framework of f (R) theory of gravity. The stability analysis is performed in f (R) = R + α R n gravity formalism, where α and n are nonzero constants, with a linear equation of state. The stable and unstable regions have been examined for different values of the parameters involved in the model. The effect of charge and mass on the throat radius is analyzed and stability of thin shell is obtained. [ABSTRACT FROM AUTHOR]

In this paper, we present a generalization of Rastall's gravity in which the conservation law of the energy–momentum tensor is altered, and as a result, the trace of the energy–momentum tensor is taken into account together with the Ricci scalar in the expression for the covariant derivative. Afterwards, we obtain the field equations in this theory and solve them by considering a spherically symmetric spacetime. We show that the external solution has two possible classes of solutions with spherical symmetry in the vacuum in generalized Rastall's gravity, and we analyze one of them explicitly. The generalization, in contrast to constant value k = 8 π G in general relativity, has a gravitational parameter k that depends on the Rastall constant α. As an application, we perform a careful analysis of the effects of the theory on neutron stars using realistic equations of state (EoS) as input. Our results show that important differences on the profile of neutron stars are obtained within two representatives EoS. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the behavior of anisotropic compact stars in generalized modified gravity, namely f (R , ϕ , X) gravity, where R represents the Ricci scalar, ϕ is the scalar potential function and X is a kinetic term of ϕ. We consider the spherically symmetric spacetime to analyze the feasible exposure of compact stars. We observe the behavior of anisotropic compact stars which includes Her X1, SAX J 1808.4-3658 and 4U 1820-30. From the graphical evaluation of energy density, tangential pressure, radial pressure, equilibrium conditions, energy conditions, mass–radius relationship, compactness and stability analysis of compact stars, it is concluded that the behavior of candidates of compact stars is regular in f (R , ϕ , X) gravity for the considered parameter. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the cosmic acceleration and the behavior of dark energy (DE) in the structure of the recently proposed κ (R , T) gravity theory [G. R. P. Teruel, κ (R , T) gravity, Eur. Phys. J. C 78 (2018) 660]. In this study, we obtained some fascinating cosmological features that are coherent with observational evidences and the touchstone Λ CDM model. To find the deterministic solution, we consider a periodic deceleration parameter q = − 1 + m cos (k t) , where m , k > 0 [M. Shen and L. Zhao, Oscillating quintom model with time periodic varying deceleration parameter, Chin. Phys. Lett. 31 (2014) 010401], which predicts the decelerating and accelerating phases of the universe. The Equation of State (EoS) parameter also supports the idea of DE, which is the dominant component and it is responsible for the universe's accelerated expansion. Here, we also construct cosmographic parameters, like, H , j , l , S , and studied their evolution in spatially flat κ (R , T) gravity. We find that these observations are sufficient in comparison with the universe's physical and kinematic properties and also consistent with ongoing (OHD + JLA) observation. Next, we apply the geometric diagnostics, the state-finder (r , s) in κ (R , T) gravity to discriminate from the Λ CDM model. We found that our model lies in quintessence and the Chaplygin Gas region. Finally, the model approaches Λ CDM at the present epoch of the universe. [ABSTRACT FROM AUTHOR]

This paper is dedicated to inspecting the appearance of electromagnetic field, which is caused by the presence of charge in the highly compact object named gravastar in the background of f (R , G) theory. According to conjecture introduced by Mazur and Mottola gravastar must be consisted of three sectors. We have interpreted these sectors by using specific equations of states. Spherically static metric, perfect fluid and the isotropic pressure have been used in order to calculate a singularity free solution. Gravitational mass present in the inner sector of gravastar is changed into electromagnetic mass in the existence of charge. The electromagnetic mass model of inner sector is responsible for stability by providing balance to the repulsive pressure, which is caused by the presence of charge. It is observed that the presence of electrical charge has a significant effect on energy content, entropy and proper length of the shell that is demonstrated with the help of graphs. [ABSTRACT FROM AUTHOR]

In this paper, evolution of a Friedmann–Robertson–Walker universe is studied in a higher derivative theory of gravity. The relativistic solutions admitting hybrid expansion law of the universe are explored here. Hybrid expansion law is a general form of scale factor from which one can recover both the power-law expansion and exponential expansion as a special case. The hybrid expansion law is interesting as it addresses the early deceleration phase and presents accelerating phase satisfactorily. It is found that an inflationary scenario with hybrid expansion law is permitted in the R 2 gravity fairly well. We consider universe filled with cosmic fluid that describes by an equation of state (EoS) parameter which varies with time. Consequently, we analyze the time variation of energy density parameter, cosmic pressure, equation of state parameter, deceleration parameter and jerk parameter in the cosmological model. The constraints of the model parameters imposed by the cosmological observational data set are determined. The present value of the deceleration parameter (q) , EoS parameter and the epoch at which the transition of decelerated phase to accelerated phase are estimated. In the higher derivative theory, we obtain some new and interesting cosmological solutions relevant for building cosmological models. [ABSTRACT FROM AUTHOR]

This paper investigates the new interior solution of stellar compact spheres in the framework of metric f (R) gravity. In this connection, we derived the Einstein field equations for static anisotropic spherically symmetric spacetime in the mechanism of Karmakar condition. The obtained results of the field equations have been studied with well-known Starobinskian model f (R) = R + ζ R 2 by using three different compact stars like RXJ1856-37, Her X - 1 , Vela X - 1 2. Moreover, the constants involved in the solution of metric potentials have been determined through smooth matching conditions between the interior geometry and exterior spacetime. Thereafter, the physical significance of the obtained results is examined in the form of fluid variables, equation of state (EoS) parameters, energy conditions, anisotropic stress and stability analysis by using the graphical plot. The approximated values of the constants and the mass-radius relation have been calculated through different stellar star objects (RXJ 1 8 5 6 - 3 7 ( r b = 6 Km), Her X - 1 ( r b = 6. 7 Km) and Vela X - 1 2 ( r b = 9. 9 9 Km)) shown in Table 1. Finally, we have concluded that our considered compact stellar objects with particular choice of f (R) model in the mechanism of Karmakar condition satisfies all the necessary bounds for potentially stable formation of the stars. [ABSTRACT FROM AUTHOR]

In this paper, we study a particular modified gravity Equation of State (EoS), the so-called Jaime–Jaber–Escamilla, that emerges from the first gravity modified action principle and can reproduce three cosmological viable f (R) theories: the Starobinsky, Hu–Sawicki and Exponential models. This EoS is a suitable candidate to reproduce the dynamical dark energy behaviour already reconstructed by the current data sets. Based on the joint statistical analysis, we found that our results are still in good agreement (within 1 σ) with the Λ CDM, while at perturbative level, we note that the matter power spectrum normalisation factor σ 8 shows an agreement with SDSS and SNeIa+IRAS at 1- σ for the Starobinsky model and with SDSS-vec for the Hu–Sawicki and Exponential models. Furthermore, we found that for the H 0 values, Starobinsky and Hu–Sawicki show the least tension in comparison with PL18 TT. All these aspects cannot be observed directly from other alternatives theories, where an equation of state is difficult to compute analytically. [ABSTRACT FROM AUTHOR]