International Journal of Modern Physics D

In this paper, we explore the wormhole solutions (which are initially proposed by R. Garattini [Eur. Phys. J. C 79 (2019) 951] who take generalized uncertainty principle (GUP) with Casimir energy) in the framework of two well-known dark energy (DE) models, generalized Chaplygin gas (GCG) and polytropic gas. We consider the GUP corrected pressure (force per unit surface area) in the equation of state (EoS) of these DE models to construct the GCG corrected energy density and polytropic gas corrected energy density. Three models of redshift function are taken into account to construct shape function as well as wormhole geometry for both cases of corrected energy densities. We find the behavior of EoS parameters, redshift function for isotropic and anisotropic fluid, equilibrium conditions, null energy condition and exotic volume at the throat of the wormhole with radius $r_0$.

The problem of cosmic acceleration and dark energy is one of the mysteries presently posed in the scientific society that general relativity has not been able to solve. In this work, we have considered alternative models to explain this late-time acceleration in a flat Friedmann–Lemaitre–Robertson–Walker (FLRW) Universe within the framework of the f (Q,T) modified gravity theory (where Q is the nonmetricity and T is the trace of the energy–momentum tensor) recently proposed by Y. Xu et al. [Eur. Phys. J. C 79 (2019) 708], which is an extension of f (Q) gravity with the addition of the T term. Here, we presume a specific form of $f(Q,T)=\alpha Q+\beta Q^2+\gamma T$ where $\alpha$, $\beta$ and $\gamma$ are free model parameters, and obtained the exact solutions by assuming the cosmic time-redshift relation as $t(z)=\frac{n{t}_0}{m}g(z)$ which produces the Hubble parameter of the form $H(z)=\frac{m{H}_0}{m+n}[\frac{1}{g(z)}+1]$, where m and n are the nonnegative constants, we find the best values for them using 57 data points of the Hubble parameter H(z). Also, we find the behavior of different cosmological parameters as the deceleration parameter (q), energy density $(\rho )$, pressure (p) and equation of state (EoS) parameter $(\omega )$ and compare them with the observational results. To ensure the validity of the results, we studied the energy conditions along with jerk parameter. Finally, we found that our model behaves similarly to the quintessence Universe.

This work is related to the stability of a nonstatic, very restricted class of axially symmetric cosmic matter configuration under the influence of modified gravitational theory (AGT) with a realistic model in anisotropic conditions. We construct dynamically modified equations, continuity equations, mass functions and collapse equations in the scenario of considered AGT. We continue our analysis under the influence of f(R,T) gravity along with a minimally coupled gravitational function of the form $f(R,T)=R+\beta R_c(1-{(1+\frac{R^2}{R_c^2})}^{-n})+\lambda T$, where $R_c$ indicates constant entity. In this gravitational model, f is a function of Ricci scalar invariant R and the trace of the stress–energy tensor T. This gravitational function gives significant findings only for $\beta >0,n>0$, and both belong to the set of positive real numbers and also $\lambda \ll 1$. We establish expressions of an adiabatic index, i.e. stiffness parameter of fluid $(\mathrm{\Gamma })$ utilizing relevant perturbation technique with the help of collapse equation in Newtonian $(\mathbb{N})$ and post-Newtonian $(\mathbb{P}\mathbb{N})$ domains. We consider the particular form of metric as well as material functions in $(\mathbb{N})$ and $(\mathbb{P}\mathbb{N})$ eras, and impose some constraints on physical quantities (i.e. pressure, density, mass, etc.) to carry out the stable behavior of a considered restricted geometry.

In this work, we explore the wormhole physics in a modified gravitational theory, called the generalized Brans–Dicke theory (GBD). In order to solve the equations in this theory, the parameterized forms for the shape function and the scalar-field function are taken. With the help of selecting a specific form of state equation, i.e. considering a relation between the transverse pressure (not the total pressure) and the energy density for matter that threads wormhole: $p_t=\alpha \rho =-\rho$, we provide some special solutions of the traversable wormhole in GBD. Given that the violation of energy conditions (ECs) of matter often induces problems on the gravitational theory, it is important to inspect the ECs of matter in modified theories. In this GBD theory, we find that the weak energy condition, the null energy condition, the dominated energy condition and the strong energy condition could be satisfied for the matter near the throat of wormhole, depending on the parameterized models and the model-parameters values. Finally, using the classical reconstruction technique in the traversable wormhole physics, we derive a Lagrangian function of gravitational field for the GBD theory.

It has recently been shown that any observed potential can in principle be generated via quantum mechanics using a suitable wave function. In this work, we consider the concrete example of the gravitational potential experienced by a test particle at length scales spanning from the planetary to the cosmological, and determine the wave function that would produce it as its quantum potential. In other words, the observed gravitational interaction at all length scales can be generated by an underlying wave function. We discuss implications of our result.

Traversable wormhole solutions in General Relativity require exotic matter sources that violate the null energy condition (NEC), and such behavior may be avoided in modified gravity. In this study, we analyze the energy conditions for static, spherically symmetric traversable Morris–Thorne wormholes in a recently proposed viable $f(R)$ gravity model. We numerically analyze solutions considering both constant and variable redshift functions, and present wormhole spacetimes respecting the NEC, supported by a phantom energy-like equation of state for the source. Moreover, we analyze the stability of the spacetimes using the generalized Tolman–Oppenheimer–Volkov equation. We demonstrate the effects of certain parameters in the $f(R)$ model in determining energy condition violations, and establish that stable wormholes can be formulated only at the expense of violating the NEC.

In this work, we study the inflationary cosmology in modified gravity theory $f(R,T)=R+2\lambda T$ ($\lambda$ is the modified gravity parameter) with three distinct classes of inflation potentials (i) ${\varphi }^p{e}^{-\alpha \varphi }$, (ii) $(1-{\varphi }^p)e^{-\alpha \varphi }$ and (iii) $\frac{\alpha {\varphi }^2}{1+\alpha {\varphi }^2}$ where $\alpha$, $p$ are the potential parameters. We have derived the Einstein equation, potential slow-roll parameters, the scalar spectral index $n_s$, tensor-to-scalar ratio r and tensor spectral index $n_T$ in modified gravity theory. We obtain the range of $\lambda$ using the spectral index constraints in the parameter space of the potentials. Comparing our results with PLANCK 2018 data and WMAP data, we found out the modified gravity parameter $\lambda$ lies between $-0.37<\lambda <1.483$.

In this paper, we discuss cosmic age problem in the case of ghost dark energy (GDE) model. To this end, we use two observational data sets to constrain the free parameter space of the models. Next, we try to set new limits on the mentioned free parameter space using the age of the universe. In this work, both of these tasks are done for the GDE model in the presence of the sign-changeable interaction terms. To this end, three classes of interaction terms are considered that provide a sign-changeability possibility. For one of these cases, the age problem passes completely and there is no new bound of free parameters. For two others the age test sets some constrains on the models. In addition to cosmic age problem, we investigate some other properties of these three models including ability to cross the phantom line, accelerating phase of cosmic expansion and the stability against cosmic perturbations.

The holographic Ricci dark energy (HRDE) can be treated as a running vacuum due to its analogy in the energy density, which is a combination of H and $Ḣ$, the model can predict either eternal acceleration or eternal deceleration. In the earlier works, we have shown that the presence of additive constant in the energy density or by considering possible interaction between dark sectors through a phenomenological term, the model can predict a transition from a prior decelerated to a late accelerated epoch. This paper analyzes the cosmic evolution of HRDE as running vacuum with a nonlinear interaction (NLI) between dark sectors in a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) universe. We consider three possible NLI forms which give analytically feasible solutions. We have constrained the model using the Type1a Supernova(Pantheon) + CMB(Planck 2018) + BAO(SDSS) data and evaluated the best estimated values of all the model parameters. We have analyzed the evolution of the Hubble parameter and deceleration parameter of all three cases. We perform state finder analysis of the model, which implies the quintessence nature of the model and found that it is distinguishably different from the standard $\mathrm{\Lambda }$CDM model. The dynamical system analysis of all three cases confirms the evolution of the universe from an unstable prior matter-dominated era to a stable end de Sitter phase.

In this paper, we present a detailed study of both nonrotating and rotating black hole solutions in $f(R)=R-2\alpha \sqrt{R}$ model. The complicated expression of Ricci scalar, which appears in the vacuum field equations, brings the main difficulty of searching analytic solutions. In the case of searching nonrotating black hole solution, we use the 00 component minus 11 component of field equations to constrain the Ricci scalar. In order to obtain analytic solutions of rotating black hole, two strategies are used. One is the simplification of the line element using Carter’s consideration that the Klein–Gordon equation is separable. The other is that we give the general solutions of the simultaneous equations of 00 minus 11 component equation and 01 component of the field equations. These strategies greatly simplify the process and difficulty of solving the axisymmetric solutions. The calculation process is more friendly to generalize to other modified gravities.

A regular Vaidya-type line-element is proposed in this work. The mass function depends both on the temporal and the spatial coordinates. The curvature invariants and the source stress tensor $T_b^a$ are finite in the whole space. The energy conditions for $T_b^a$ are satisfied if $k^2<2vr$, where k is a positive constant and v, r are coordinates. It is found that the radial pressure has a maximum very close to $r=2m(r>2m),v=2m$. The energy crossing a sphere of constant radius is akin to Lundgren–Schmekel–York quasilocal energy. The Newtonian acceleration of the timelike geodesics has an extra term (compared to the result of Piesnack and Kassner) which leads to rejecting effects.

In this work, we study corrections to black hole (BH) temperature and entropy in the context of the generalized uncertainty principle. In particular, we obtain corrected asymptotically Anti-de Sitter BH solutions following the extended phase-space scheme in which the cosmological constant is considered as a thermodynamic pressure satisfying certain equation of state (EoS). Among all the possibilities, we consider that the cosmological pressure satisfies either a polytropic or a Chaplygin EoS. The physical plausibility of the solutions is studied based on the energy conditions and the associated heat capacity.

In this paper, the effective potential approach in quantum field theory is used in order to investigate self-interaction loop correction to the Casimir energy density and generation of topological mass for both massless and massive real scalar fields. It is assumed that the scalar field obeys a helix boundary condition. In addition, it is also considered a CPT-even aether-type violation of the Lorentz symmetry. In the absence of the Lorentz violation, we obtain analytical expressions for the loop correction to the Casimir energy density and to the mass of the scalar field. The same expressions are also obtained assuming the Lorentz violation in each of the spacetime directions. We also show some graphs that exhibit how the loop correction and the Lorentz violation affect the Casimir energy density and the mass of the scalar field.

In this paper, we propose a generalization to the Schwarzschild metric and define noncommutative SUSY black holes. We introduce the noncommutative deformation to the minisuperspace variables and derive the noncommutative supersymmetric (SUSY) Wheeler–DeWitt (WDW) equation for the Schwarzschild black hole. We calculate the metric and find that the singularities are not removed.

In this paper, we present a quantum description of black holes given by coherent states of gravitons sourced by a matter core. The expected behavior in the weak-field region outside the horizon is recovered, with arbitrarily good approximation, but the classical central singularity is not resolved because the coherent states may not contain modes of arbitrarily short wavelength and the matter core must therefore have finite size. Ensuing quantum corrections both in the interior and exterior is also estimated by assuming that the mean-field approximation holds everywhere. These deviations from the classical black hole geometry can be viewed as quantum hair and will lead to a quantum corrected horizon radius and thermodynamics.