Narrow Results

In the framework of $f(R,T)=R+2\beta T$ gravity, wherein R denotes the Ricci scalar and T represents the trace of the energy–momentum tensor, the equilibrium structure of charged anisotropic white dwarfs is investigated. The stellar equations for the general scenario are derived, and numerical solutions are obtained for the Chandrasekhar equation of state alongside the charge density distribution proportional to the energy density, ${\rho }_{\mathrm{\text{ch}}}=\alpha \rho$. By scrutinizing the properties of the solutions under various parameter combinations, we conducted an exhaustive analysis of the influences of electric charge, modified gravity, and anisotropy on the structural characteristics of white dwarfs. Crucially, white dwarfs can transcend the trivial instances of General Relativity in various ways, thereby demonstrating super-Chandrasekhar phenomena. As an inquiry into the structure of white dwarfs, the findings of this research may contribute to elucidating astronomical observations such as superluminous type Ia supernovae.

Recently, the Rastall–Rainbow gravity emerged as an intriguing modified gravity theory. This theory is a combination of two theories, namely, the Rastall theory and the Rainbow description. This study aims to investigate the potential existence of static, spherically symmetric wormholes in this gravity theory. Specifically, we provide precise solutions to asymptotically flat wormhole geometries for the specific choice of the redshift and shape functions. With the specific examples, we investigate some issues concerning energy conditions and adiabatic sound velocity. We also consider the “volume integral quantifier” to assess the total amount of energy-condition violation matter. Finally, we conclude that reducing Rastall’s parameters and adjusting the Rainbow functions may alleviate the violation of energy conditions.

This work examines whether evolving wormhole solution is possible or not in $f(R,T)$ modified gravity theory. In the background of inhomogeneous FLRW-type wormhole configuration, the field equations are investigated for different choices of scale factors and shape functions. For the power law and exponential choice of the scale factor from cosmological context and decoupled power law of $f(R,T)$ in each variable, wormhole configuration has been examined for two viable choices of shape function. Energy conditions are examined graphically for a range of values of the parameters involved. Finally, the possibility of emergent scenario at early cosmic evolution has been examined.

In this paper, we investigate the quintessential inflation in the logarithmic Cartan $F(R)$ gravity associated with local Lorenz symmetry. A small logarithmic modification of general relativity has the potential to introduce both inflation and dark energy. We evaluate the time evolution of the Universe such as inflation, reheating, and dark energy. The parameters in the model are fixed to introduce the inflation and the dark energy scales. We show that the CMB fluctuations induced by the inflation are consistent with the current observations. In the reheating process, it is possible to achieve the reheating temperature required for nucleosynthesis in the Big Bang scenario. It can be seen that by choosing an appropriate value for the scalaron field after reheating, the scalaron field again dominates the energy of the Universe and causes the current accelerating expansion as dark energy.

This paper explores a cosmological reconstruction scheme in the background of $f(Q)$ gravity theory from a Holographic perspective. The basic motivation for this work is that the reconstruction is performed from a holographic origin, which has its roots in the black hole thermodynamics and quantum gravity. Dark energy models inspired by holographic prescription are used to reconstruct the $f(Q)$ gravity models. Two such models, namely, the Granda–Oliveros holographic dark energy model and its generalization, the Chen-Jing model, are considered for the study. Different scale factors are used and a thorough reconstruction scheme is set up using the dark energy models. The observationally constrained values of the free model parameters have been used to form the reconstructed models. Finally, a thorough investigation of the energy conditions has been performed to check the cosmological viability of the reconstructed $f(Q)$ models. As an outcome, we get some very promising and cosmologically viable $f(Q)$ models that present some interesting properties and demand further investigation. Finally, a method is discussed how the constructed $f(Q)$ models can be reconciled with a generalized holographic dark energy.

The objective of this study is to examine the possible existence of traversable wormhole geometries within the context of $f(R,L_m,T)$ gravity. To meet this objective, we employ the Karmarkar condition to construct the shape function that aids in identifying the wormhole configurations. This developed function is found to satisfy the essential conditions and provides a link between two asymptotically flat spacetime regions. We then assume the Morris–Thorne line element that expresses the wormhole configuration and formulate the anisotropic gravitational equations for a particular minimal matter-spacetime coupled model of the modified theory. Afterward, we develop three solutions and determine their viability by analyzing whether they violate the null energy conditions. Different stability methods are applied to the resulting geometries to explore the acceptance of the considered modified model. We conclude that the developed wormhole structures potentially fulfill the required criteria and thus exist in this modified gravity under all choices of the matter Lagrangian density.

Using modified $f(T)$ gravity and the diagonal tetrad, we propose a new kind of analytical solution to describe anisotropic charged stellar structures in this study. We obtain precise solutions by applying the linear form of $f(T)$ as $f(T)=\gamma T+\delta$ in the framework of Tolman–Kuchowicz physically sound metric potentials. The stellar structures of the compact star EXO 1785-248 are then investigated using the model by incorporating a linear equation of state relating the radial pressure $p_r$ and the matter density $\rho$. We computed the closed-form expressions for the model parameters and illustrated their characteristics. The comprehensive graphical analysis demonstrates the scientific plausibility, causality, and dynamical stability of the model describing charged anisotropic stars. Finally, using the M-R curve, we estimate the numerical values of mass for different values of $\gamma$. The observational data of the neutron star candidates 4U 1820-30, PSR J1614-2230, and PSR J2215 + 5135 are covered by the maximum allowable masses for different values of $\gamma$ additionally, when $\gamma =2.5$, the charged compact star’s maximum permissible mass is calculated to be $2.56M_{\odot }$, which could indicate the secondary component of the GW190814 event discovered by LIGO/VIRGO experiments.

This paper explores cosmic evolution in $f(R,T)$ gravity considering two distinct models with minimal coupling that are linear and quadratic in $T$; $f(R,T)=R+\alpha T$ and $f(R,T)=R+\alpha T^2$. The flat FRW space-time under the matter and radiation distribution has been considered to complete the analysis. In this background, the dynamical equations for dust in terms of dimensionless variables are developed. Via solutions of these dynamical equations, we develop significant cosmological parameters to compare the obtained results with the $\mathrm{\Lambda }$CDM model and analyze them graphically. It is found that these parameters efficiently explain the expanding phenomenon of the universe for constrained values of the involved model parameters using some recent observational data. Moreover, the results are found to be stable via the square speed of sound. The matter and radiation fixed points are calculated by an autonomous set of dynamical equations, which are the stable and unstable saddle points. Also, the growth of matter perturbation in the $f(R,T)$ theory has been discussed. Under this scenario, it is also checked that exact solutions of the dynamical equations are not possible for non-minimal coupling between geometry and matter.

In this study, we delve into the propagation characteristics of the even modes of the primordial gravity waves (GWs) represented by the Regge–Wheeler perturbation scheme within a locally rotationally symmetric Bianchi type-I universe from the perspective of the $f(R,T^{\psi })$ gravity to incorporate the effects of a scalar field. Initially, we consider the cosmological background without wave fluctuations and derive the corresponding field equations. Subsequently, by utilizing the even Regge–Wheeler perturbation scheme, we explore polar wave dynamics by deriving the pertinent field equations. Analytical solutions of the perturbed metric potentials are obtained under certain assumptions and the study observes the impact of the scalar field, examining its effects through the model parameter. The graphical analysis sheds light on the implications of the scalar field on the propagation of the polar modes of GWs. It is concluded that the presence of a scalar field influences the temporal perturbations however, the spatial part remains unaffected and is characterized by a combination of exponential functions.

In this research, we have reconstructed the extended $f({𝒫})$ cubic gravity and symmetric $f({𝒬})$ teleparallel gravity from the $(m,n)$-type Barrow Holographic Dark Energy (BHDE) model. We have derived the unknown functions $f({𝒫})$ and $f({𝒬})$ in terms of ${𝒫}$ and ${𝒬}$, assuming a flat, homogeneous, and isotropic universe. To constrain our model parameters, we employed cosmic chronometer datasets and Baryon Acoustic Oscillation (BAO) datasets, utilizing Markov Chain Monte Carlo (MCMC) method. We analyzed the behavior and stability of each model throughout the universe’s evolution by studying crucial parameters such as the deceleration parameter, equation of state (EoS) parameter ${\omega }_{\mathrm{\text{DE}}}$, density parameter $\mathrm{\Omega }(z)$ and the square of the speed of sound $v_s^2$. Additionally, we explored the cosmographic behavior by plotting the jerk parameter, snap parameter, and lerk parameter against the redshift. Furthermore, we examined the ${\omega }_{\mathrm{\text{DE}}}^{\prime }-{\omega }_{\mathrm{\text{DE}}}$ phase plane, the $(r,s^{\ast })$, $(r,q)$ statefinder parameters, and the $Om(z)$ parameter offers profound revelations about the dynamics of the universe and the distinctive features of dark energy. Our analyses indicated that our model could produce a universe undergoing accelerated expansion with quintessence-type dark energy. These findings contribute to our understanding of the nature of dark energy and the evolution of the cosmos.

We investigate the cosmic evolution of the universe in context of $f(R,G)$ gravity with homogeneous and isotropic background, where $R$ represents the Ricci scalar, and $G$ denotes the Gauss–Bonnet invariant. With the field equations of the $f(R,G)$ gravity in the non-flat spacetime, we examine the characteristics of model with two distinct forms: $f_1(R,G)=\mu R^n{G}^m$ and $f_2(R,G)={\beta }_{1}R+{\beta }_2\mu R^n{G}^m$, where $n$, $m$, ${\beta }_1$ and ${\beta }_2$ are the model parameters. The power-law solutions for the field equations are obtained by constraining the model parameters by employing Bayesian analysis tools in conjunction with the Markov Chain Monte Carlo (MCMC) method. We constraint the parameters by utilizing the combined datasets (OHD$+$BAO$+$Pantheon) for open Friedmann–Lemaitre–Robertson–Walker (FLRW) model. The parameters of power-law solutions are determined as $H_0=70.76732_{-0.00088}^{+0.00010}$ km/(s ⋅ Mpc) and $q=-0.6478_{-0.000075}^{+0.000091}$. The present dark energy model estimates $H_0$ value higher than the $\mathrm{\Lambda }$CDM model. This is one of the motivations to consider the $f(R,G)$ gravity to describe dark energy era. The best-fit values of model parameters in an open model yield the power index of power-law solution as $\alpha =2.394949_{-0.00060}^{+.000089}$. In the present model, the joint analysis by incorporating data from OHD, BAO, and Pantheon datasets favors the open FLRW model with power-law solutions at the late times. The behavior of cosmological parameters has been studied to explore the evolution of dark energy. The equation of state parameter ($\omega \simeq -1$) supports the accelerated expansion of the Universe at late times. The energy conditions are also studied to test the cosmological viability of the model.

In this study, we construct a theoretical framework based on the generalized Hubble parameter form which may arise within the particle creation, viscous and $f(R)$ gravity theory. The Hubble parameter is scrutinized for its compatibility with the observational data relevant to the late-time universe. By using Bayesian statistical techniques based on ${\chi }^2$ minimization method, we determine model parameters’ best fit values for the cosmic chronometer and supernovae Pantheon datasets. For the best fit values, the cosmographic and physical parameters are analyzed to understand the cosmic dynamics in model. We also analyze the model section criterion in comparison to the $\mathrm{\Lambda }$ cold dark matter model.

In this paper, I propose a static wormhole model within modified $f(R,T)$ gravity where $f(R,T)=R+2\lambda T$. The wormhole solutions have been evolved in four cases — three different shape function along with redshift $\varphi =\frac{{\varphi }_0}{r}$ and a variable EoS parameter $\omega (r)$ with constant redshift function. I also have explored the energy conditions and the behavior of exotic matter within the wormhole in all scenarios. The presence of exotic matter violates necessary energy conditions near wormhole throat which gives constraint on modified gravity parameter $\lambda$ in all different cases.

To explain the cosmic speed up, brought to light by the recent SNIa and CMB observations, we propose the following: (a) In a spacetime endowed with a FRW metric, we choose an empirical scale factor that best explains the observations. (b) We assume a modified gravity, generated by an unspecified field Lagrangian, f(R). (c) We use the adopted empirical scale factor to work back retroactively to obtain f(R), hence the term "Inverse f(R)". (d) Next we consider the classic GR and a conventional FRW universe that, in addition to its known baryonic content, possesses a hypothetical "Dark Energy" component. We compare the two scenarios and find the density, the pressure, and the equation of the state of the Dark Energy required to make up for the differences between the conventional and the modified GR models.

In the general picture of high order theories of gravity, recently, the R^-1 theory has been analyzed in two different frameworks. In this letter a third context is added, considering an explicit coupling between the R^-1 function of the Ricci scalar and the matter Lagrangian. The result is a non-geodesic motion of test particles which, in principle, could be connected with Dark Matter and Pioneer anomaly problems.

We review recent work toward constructing, via five-dimensional holographic duals, four-dimensional theories in which spin-2 states (gravitons) are emergent. The basic idea is to extend to gravity model-building the applications of holographic duality to phenomenology construction.

We investigate the modified gravity theories in terms of the effective dark energy models. We compare the cosmic expansion history and the linear growth in different models. We also study the evolution of linear cosmological perturbations in modified theories of gravity assuming the Palatini formalism. We find the stability of the superhorizon metric evolution depends on models. We also study the matter density fluctuation in the general gauge and show the differential equations in super and sub-horizon scales.

In this work we study the spherical symmetric solutions of f(R) gravity in the metric formalism. We show that for a generic f(R) gravity, the spherical symmetric solution is consistent with the modified gravity equations except in the case of imposing an extra condition for the metric.

In this paper we consider the Kaluza–Klein field equations in the presence of a generic 5D matter tensor which is governed by a conservation equation due to 5D Bianchi identities. Following a previous work, we provide a consistent approach to matter where the problem of huge massive modes is removed, without relaxing the compactification hypotheses; therefore we perform the dimensional reduction either for metric fields and for matter, thus identifying a pure 4D tensor term, a 4D vector term and a scalar one. Hence we are able to write down a consistent set of equations for the complete dynamics of matter and fields; with respect to the pure Einstein–Maxwell system we now have two additional scalar fields: the usual dilaton one plus a scalar source term. Some significant scenarios involving these terms are discussed and perspectives for cosmological applications are suggested.

We review a number of approaches that have been adopted to explain the origin of dark energy responsible for the late-time cosmic acceleration. This includes the cosmological constant and dynamical dark energy models such as quintessence, k-essence, Chaplygin gas, f(R) gravity, scalar-tensor theories, and braneworld models. We also discuss observational and local gravity constraints on those models and clarify which models are favored or ruled out in current observations.