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Using Tsallis statistics and its relation with Boltzmann entropy, the Tsallis entropy content of black holes is achieved, a result in full agreement with a recent study (Mejrhit and Ennadifi in Phys Lett B 794:24, 2019). In addition, employing Kaniadakis statistics and its relation with that of Tsallis, the Kaniadakis entropy of black holes is obtained. The Sharma-Mittal and Rényi entropy contents of black holes are also addressed by employing their relations with Tsallis entropy. Thereinafter, relying on the holographic dark energy hypothesis and the obtained entropies, two new holographic dark energy models are introduced and their implications on the dynamics of a flat FRW universe are studied when there is also a pressureless fluid in background. In our setup, the apparent horizon is considered as the IR cutoff, and there is not any mutual interaction between the cosmic fluids. The results indicate that the obtained cosmological models have (i) notable powers to describe the cosmic evolution from the matter-dominated era to the current accelerating universe, and (ii) suitable predictions for the universe age.

In this study we present evolving wormhole configurations in third-order {Lovelock} gravity and investigate the possibility that these solutions satisfy the energy conditions. Using a generalized Friedmann-Robertson-Walker spacetime, we derive evolving wormhole geometries by considering a constraint on the Ricci scalar. In standard cosmological models, the Ricci scalar is independent of the radial coordinate $r$ and is only a function of time. We use this property to introduce dynamic wormhole solutions expanding in an inflationary cosmological background and explore the effects of higher-order {Lovelock} terms on the dynamics of such wormholes. Our analysis shows that for suitable third-order {Lovelock} coefficients, there are wormhole solutions that respect the weak energy condition {(WEC)}. In addition to this, we also present other wormhole solutions that satisfy the WEC throughout their respective evolution., 23 pages & 8 figures

In the present work we study spherically symmetric gravitational collapse of a homogeneous perfect fluid in the context of Generalized Rastall Theory (GRT). In this modified version of the original {Rastall Gravity (RG)}, the coupling parameter which is a representative of matter-curvature interaction is no longer a constant parameter. Such a dynamic coupling may play the role of dark energy which is responsible for the present accelerating expansion of the Universe. Assuming then a linear equation of state (EoS) for the fluid profiles, we seek for physically reasonable collapse scenarios in which the spacetime singularity that occurs in general relativity (GR) is replaced by a non-singular bounce. We therefore find that depending on model parameters, the collapse process which starts from regular initial data, will halt at a minimum value for the scale function and then turns into an expansion at a finite time. We further find that there exists a minimum value for the initial radius of collapsing object so that for radii smaller than this minimum radius, formation of apparent horizon can be avoided and hence the bounce can be visible to the observers within the Universe. We also compare our results to quantum corrected collapse scenarios and find that the mutual interaction between matter and geometry can play the role of quantum corrections to energy density., Comment: 10 Pages & 5 Figures

In the framework of the generalized Rastall theory (GRT), we study the ability of a non-minimal coupling between geometry and matter fields in order to provide a setting which allows for a variable G during the cosmic evolution. In this regard, the compatibility of this theory with Dirac hypothesis on the variations of G is investigated, and additionally, the possibility of obtaining the current accelerated universe is also addressed. In summary, our study indicates that, in GRT, having in hand the G profile, one may find the corresponding non-minimal coupling between the energy source and geometry and vise versa, in a compatible way with the current accelerated universe., 8 pages, 3 figures

In present research, we construct Kaniadakis holographic dark energy (KHDE) model within a non-flat Universe by considering the Friedmann-Robertson-Walker (FRW) metric with open and closed spatial geometries. We therefore investigate cosmic evolution by employing the density parameter of the dark energy (DE), the equation of state (EoS) parameter and the deceleration parameter (DP). The transition from decelerated to accelerated expanding phase for the KHDE Universe is explained through dynamical behavior of DP. With the classification of matter and DE dominated epochs, we find that the Universe thermal history can be defined through the KHDE scenario, and moreover, a phantom regime is experienceable. The model parameters are constrained by applying the newest $30$ data cases of $H(z)$ measurements, over the redshift span $0.07 \leq z \leq 2.36$, and the distance modulus measurement of the recent Union $2.1$ data set of type Ia supernovae. The classical stability of KHDE model has also been addressed., Comment: 14 pages, 6 figures

Hartree-Fock approximation suffers from two inabilities including i) the divergence of electron Fermi velocity , and ii) existence of bandwidth not confirmed experimentally. Here, we study the effects of minimal length on the ground state energy of the electron gas in the Hartree-Fock approximation. Our results indicate that considering some mathematical terms, similar to those of used for the minimal length correction to the Hamiltonian of system, can eliminate the weaknesses of Hartree-Fock approximation. These corrections, on the other hand, can be considered as relativistic corrections of electron in solids. Physically, it is obtained that electrons in metals can be employed to test the quantum gravity scenario, if the value of its parameter ($\beta$) lies within the range of 2 to 10, depending on the used metal. Indeed, the latter addresses an upper bound on $\beta$ which is comparable with previous works meaning that these types of systems may be employed in testing quantum gravity scenarios. To overcome the infinite Fermi velocity in Hartree-Fock method, the screening potential is used based on the Lindhard theory. We also found that considering the generalized Heisenberg uncertainly leads to some additional oscillating terms in the Friedel oscillations., Comment: 10 pases, 5 figs

In recent years, the implications of the generalized (GUP) and extended (EUP) uncertainty principles on Maxwell–Boltzmann distribution have been widely investigated. However, at high energy regimes, the validity of Maxwell–Boltzmann statistics is under debate and instead, the Jüttner distribution is proposed as the distribution function in relativistic limit. Motivated by these considerations, in the present work, our aim is to study the effects of GUP and EUP on a system that obeys the Jüttner distribution. To achieve this goal, we address a method to get the distribution function by starting from the partition function and its relation with thermal energy which finally helps us in finding the corresponding energy density states.

It has been recently shown that the Bekenstein entropy bound is not respected by the systems satisfying modified forms of Heisenberg uncertainty principle (HUP) including the generalized and extended uncertainty principles, or even their combinations. On the other, the use of generalized entropies, which differ from Bekenstein entropy, in describing gravity and related topics signals us to different equipartition expressions compared to the usual one. In that way, The mathematical form of an equipartition theorem can be related to the algebraic expression of a particular entropy, different from the standard Bekenstein entropy, initially chosen to describe the black hole event horizon, see E. M. C. Abreu et al., MPLA 32, 2050266 (2020). Motivated by these works, we address three new uncertainty principles leading to recently introduced generalized entropies. In addition, the corresponding energy-time uncertainty relations and Unruh temperatures are also calculated. As a result, it seems that systems described by generalized entropies, such as those of Tsallis, do not necessarily meet HUP and may satisfy modified forms of HUP., Accepted version by EPL

The paper deals with a theoretical model for interacting Tsallis holographic dark energy (THDE) whose infrared (IR) cut-off scale is set by the Hubble length. The interaction $Q$ between the dark sectors (dark energy and pressureless dark matter) of the universe has been assumed to be non-gravitational in nature. The functional form of $Q$ is chosen in such a way that it reproduces well known and most used interactions as special cases. We then study the nature of the THDE density parameter, the equation of state parameter, the deceleration parameter and the jerk parameter for this interacting THDE model. Our study shows that the universe exhibits the usual thermal history, namely the successive sequence of radiation, dark matter and dark energy epochs, before resulting in a complete dark energy domination in the far future. It is shown the evolution of the Hubble parameter for our model and compared that with the latest Hubble parameter data. Finally, we also investigate both the stability and thermodynamic nature of this model in the present context., v3: minor corrections, published in EPJC

Recently a modified version of Rastall theory of gravity has been introduced in which a varying coupling parameter could act as dark energy (DE) and thus, it can be held responsible for the current accelerated expansion of the Universe. Motivated by this modification, we study here the evolution of linear and non-linear perturbations in the matter content of the Universe, utilizing spherically symmetric top-hat collapse scenario. The exact solutions we obtain in linear regime show that as the Universe evolves, matter density perturbations grow and reach a maximum value at a certain redshift after which these perturbations start decreasing towards a finite positive value at the present time. Depending on model parameters, exact oscillatory solutions can be also found representing that matter perturbations could experience either overdense and underdense regions during the dynamical evolution of the Universe. Numerical solutions in non-linear regime show that the amplitude of perturbations grow much faster than the linear one and diverges at a critical redshift. However, the formation of collapsed structures is delayed as compared to $\Lambda${\rm CDM} model. It is found that the running mutual interaction between matter and geometry, encoded in the variable Rastall coupling parameter, could drastically affect the dynamics of matter perturbations and their growth rate during the evolution of the Universe., Comment: 8 pages and 5 figures

Bearing holographic dark energy hypothesis in mind, the ability of vacuum energy in describing the current accelerated universe is studied in the framework of Rastall theory. Here, in addition to the ordinary approach in which it is expected that this energy plays the role of dark energy, we also address a new approach where the sum of this energy and Rastall term is responsible for the current accelerated universe. We also investigate the cosmological outcomes of using Tsallis entropy in quantifying the energy of fields in vacuum for both mentioned approaches. The implications of considering an interaction between the various segments of cosmic fluid have been addressed in each studied cases. The normalized Hubble parameter for the models have also been plotted and compared that with the H(z) data consisting of 41 data points in the redshift range of 0.07< z, 25 pages, 24 figures

Very recently, Josset and Perez (Phys. Rev. Lett. 118:021102, 2017) have shown that a violation of the energy-momentum tensor (EMT) could result in an accelerated expansion state via the appearance of an effective cosmological constant, in the context of unimodular gravity. Inspired by this outcome, in this paper we investigate cosmological consequences of a violation of the EMT conservation in a particular class of $$f(\mathsf{R},\mathsf{T})$$ f ( R , T ) gravity when only the pressure-less fluid is present. In this respect, we focus on the late time solutions of models of the type $$f(\mathsf{R},\mathsf{T})=\mathsf{R}+\beta \Lambda (-\mathsf{T})$$ f ( R , T ) = R + β Λ ( - T ) . As the first task, we study the solutions when the conservation of EMT is respected, and then we proceed with those in which violation occurs. We have found, provided that the EMT conservation is violated, that there generally exist two accelerated expansion solutions of which the stability properties depend on the underlying model. More exactly, we obtain a dark energy solution for which the effective equation of state depends on the model parameters and a de Sitter solution. We present a method to parametrize the $$\Lambda (-\mathsf{T})$$ Λ ( - T ) function, which is useful in a dynamical system approach and has been employed in the model. Also, we discuss the cosmological solutions for models with $$\Lambda (-\mathsf{T})=8\pi G(-\mathsf{T})^{\alpha }$$ Λ ( - T ) = 8 π G ( - T ) α in the presence of ultra-relativistic matter.

We study the collapse process of a homogeneous perfect fluid (in FLRW background) with a barotropic equation of state in Brans-Dicke (BD) theory in the presence of phase space deformation effects. Such a deformation is introduced as a particular type of non-commutativity between phase space coordinates. For the commutative case, it has been shown in the literature [1], that the dust collapse in BD theory leads to the formation of a spacetime singularity which is covered by an event horizon. In comparison to general relativity (GR), the authors concluded that the final state of black holes in BD theory is identical to the GR case but differs from GR during the dynamical evolution of the collapse process. However, the presence of non-commutative effects influences the dynamics of the collapse scenario and consequently a non-singular evolution is developed in the sense that a bounce emerges at a minimum radius, after which an expanding phase begins. Such a behavior is observed for positive values of the BD coupling parameter. For large positive values of the BD coupling parameter, when non-commutative effects are present, the dynamics of collapse process differs from the GR case. Finally, we show that for negative values of the BD coupling parameter, the singularity is replaced by an oscillatory bounce occurring at a finite time, with the frequency of oscillation and amplitude being damped at late times., 23 pages, 10 figures

The generalized and extended uncertainty principles affect the Newtonian gravity and also the geometry of the thermodynamic phase space. Under the influence of the latter, the energy-temperature relation of ideal gas may change. Moreover, it seems that the Newtonian gravity is modified in the framework of the R\'{e}nyi entropy formalism motivated by both the long-range nature of gravity, and the extended uncertainty principle. Here, the consequences of employing the generalized and extended uncertainty principles, instead of the Heisenberg uncertainty principle, on the Jeans mass are studied. The results of working in the R\'{e}nyi entropy formalism are also addressed. It is shown that unlike the extended uncertainty principle and the R\'{e}nyi entropy formalism which lead to the same increase in the Jeans mass, the generalized uncertainty principle can decrease it. The latter means that a cloud with mass smaller than the standard Jeans mass, obtained in the framework of the Newtonian gravity, may also undergo the gravitational collapse process., Comment: 6 pages, 2 figures

Using the Sharma-Mittal entropy, we study some properties of the Schwarzschild and Schwarzschild-de Sitter black holes. The results are compared with those obtained by attributing the Bekenstein entropy bound to the mentioned black holes. Our main results show that while the Schwarzschild black hole is always stable in the micro-canonical ensemble, it can be stable in the canonical ensemble if its mass is bigger than the mass of the coldest Schwarzschild black hole. A semi-classical analysis has also been used to find an approximate relation between the entropy free parameters. Throughout the paper, we use units $c=G=\hbar=k_B=1$, where $k_B$ denotes the Boltzmann constant., 5 pages, 5 figures

A Vaidya spacetime is considered for gravitational collapse of a type II fluid in the context of Rastall theory of gravity. For a linear equation of state (EoS) for the fluid profiles, the conditions under which the dynamical evolution of the collapse can give rise to the formation of a naked singularity are examined. It is shown that depending on the model parameters, strong-curvature naked singularities would arise as exact solutions to the Rastall's field equations. The allowed values of these parameters satisfy certain conditions on the physical reliability, nakedness and the curvature strength of the singularity. It turns out that Rastall gravity, in comparison to general relativity (GR), provides a wider class of physically reasonable spacetimes that admit both locally and globally naked singularities., Comment: Version accepted in Annalen der Physik

In recent years, Rastall gravity is undergoing a considerable surge in popularity. This theory purports to be a modified gravity theory with a non-conserved energy-momentum tensor ({\rm EMT}) and an unusual non-minimal coupling between matter and geometry. The present work looks for the evolution of homogeneous spherical perturbations within the Universe in the context of Rastall gravity. Using the spherical Top-Hat collapse model we seek for exact solutions in linear regime for density contrast of dark matter (\rm DM) and dark energy ({\rm DE}). We find that the Rastall parameter affects crucially the dynamics of density contrasts for {\rm DM} and {\rm DE} and the fate of spherical collapse is different in comparison to the case of general relativity ({\rm GR}). Numerical solutions for perturbation equations in non-linear regime reveal that {\rm DE} perturbations could amplify the rate of growth of {\rm DM} perturbations depending on the values of Rastall parameter.

In the present work we consider the existence and stability of Einstein static {\sf ES} Universe in Brans-Dicke ({\sf BD}) theory with non-vanishing spacetime torsion. In this theory, torsion field can be generated by the {\sf BD} scalar field as well as the intrinsic angular momentum (spin) of matter. Assuming the matter content of the Universe to be a Weyssenhoff fluid, which is a generalization of perfect fluid in general relativity ({\sf GR}) in order to include the spin effects, we find that there exists a stable {\sf ES} state for a suitable choice of the model parameters. We analyze the stability of the solution by considering linear homogeneous perturbations and discuss the conditions under which the solution can be stable against these type of perturbations. Moreover, using dynamical system techniques and numerical analysis, the stability regions of the {\sf ES} Universe are parametrized by the {\sf BD} coupling parameter and first and second derivatives of the {\sf BD} scalar field potential, and it is explicitly shown that a large class of stable solutions exists within the respective parameter space. This allows for non-singular emergent cosmological scenarios where the Universe oscillates indefinitely about an initial {\sf ES} solution and is thus past eternal., Comment: 24 pages, 9 figures and 2 tables

We study the cosmological consequences of interacting Tsallis holographic dark energy model in the framework of the fractal universe, in which, the Hubble radius is considered as the IR cut-off. We drive the equation of state (EoS) parameter, deceleration parameter and the evolution equation for the Tsallis holographic dark energy density parameter. Our study shows that this model can describe the current accelerating Universe in both noninteracting and interacting scenarios, and also a transition occurs from the deceleration phase to the accelerated phase, at the late time. Finally, we check the compatibility of free parameters of the model with the latest observational results by using the Pantheon supernovae data, eBOSS, 6df, BOSS DR12, CMB Planck 2015, Gamma-Ray Burst., Comment: 9 pages, 6 figures

In the present work we investigate wormhole configurations described by a constant redshift function in Einstein-Cubic gravity ({{\sf ECG}}). We derive analytical wormhole geometries by assuming a particular equation of state ({{\sf EoS}}) and investigate the possibility that these solutions satisfy the standard energy conditions. We introduce exact asymptotically flat and anti-de Sitter (AdS) spacetimes that admit traversable wormholes. These solutions are obtained by imposing suitable values for the parameters of the theory so that the resulted geometries satisfy the weak energy condition ({\sf WEC}) in the vicinity of the throat, due to the presence of higher-order curvature terms. Moreover, we find that AdS solutions satisfy the {\sf WEC} throughout the spacetime. A description of the geodesic motion of timelike and null particles is presented for the obtained wormhole solutions. Also, using gravitational lensing effects, observational features of the wormhole structure are discussed., Comment: 20 pages and 7 figures

Recently a Lagrangian formulation for Rastall Gravity (RG) has been proposed in the framework of f(R, T) gravity. In the present work we obtain Lagrangian formulation for the standard Rastall theory assuming the matter content is a perfect fluid with linear Equation of State (EoS). We therefore find a relation between the coupling constant of the Lagrangian, the Rastall gravitational coupling constant and the EoS parameter. We also propose a Lagrangian for the Generalized Rastall Gravity (GRG). In this case the Rastall parameter which is a constant is replaced by a variable one. More exactly, it appears as a function of the Ricci scalar and the trace of the Energy Momentum Tensor (EMT). In both mentioned models, the Lagrangians are constructed by a linear function of R and T., 12 pages

Focusing on the special case of generalized Rastall theory, as a subclass of the non-minimal curvature-matter coupling theories in which the field equations are mathematically similar to the Einstein field equations in the presence of cosmological constant, we find two classes of black hole (BH) solutions including (i) conformally flat solutions and (ii) non-singular BHs. Accepting the mass function definition and by using the entropy contents of the solutions along with thermodynamic definitions of temperature and pressure, we study the validity of Euler equation on the corresponding horizons. Our results show that the thermodynamic pressure, meeting the Euler equation, is not always equal to the pressure components appeared in the gravitational field equations and satisfies the first law of thermodynamics, a result which in fact depends on the presumed energy definition. The requirements of having solutions with equal thermodynamic and Hawking temperatures are also studied. Additionally, we study the conformally flat BHs in the Rastall framework. The consequences of employing generalized Misner–Sharp mass in studying the validity of the Euler equation are also addressed.

In this work we study classical bouncing solutions in the context of $$f(\mathsf{R},\mathsf{T})=\mathsf{R}+h(\mathsf{T})$$ gravity in a flat FLRW background using a perfect fluid as the only matter content. Our investigation is based on introducing an effective fluid through defining effective energy density and pressure; we call this reformulation as the “effective picture”. These definitions have been already introduced to study the energy conditions in $$f(\mathsf{R},\mathsf{T})$$ gravity. We examine various models to which different effective equations of state, corresponding to different $$h(\mathsf{T})$$ functions, can be attributed. It is also discussed that one can link between an assumed $$f(\mathsf{R},\mathsf{T})$$ model in the effective picture and the theories with generalized equation of state (EoS). We obtain cosmological scenarios exhibiting a nonsingular bounce before and after which the Universe lives within a de-Sitter phase. We then proceed to find general solutions for matter bounce and investigate their properties. We show that the properties of bouncing solution in the effective picture of $$f(\mathsf{R},\mathsf{T})$$ gravity are as follows: for a specific form of the $$f(\mathsf{R,T})$$ function, these solutions are without any future singularities. Moreover, stability analysis of the nonsingular solutions through matter density perturbations revealed that except two of the models, the parameters of scalar-type perturbations for the other ones have a slight transient fluctuation around the bounce point and damp to zero or a finite value at late times. Hence these bouncing solutions are stable against scalar-type perturbations. It is possible that all energy conditions be respected by the real perfect fluid, however, the null and the strong energy conditions can be violated by the effective fluid near the bounce event. These solutions always correspond to a maximum in the real matter energy density and a vanishing minimum in the effective density. The effective pressure varies between negative values and may show either a minimum or a maximum.

Static solutions representing wormhole configurations in Einstein-Cartan theory ({{\sf ECT}}) in the presence of electric charge are obtained. The solutions are described by a constant redshift function with matter content consisting of a Weyssenhoff fluid along with an anisotropic matter and energy momentum tensor ({\sf EMT}) of the electric field which together generalize the anisotropic energy momentum tensor in Einstein-Maxwell theory in order to include the effects of the intrinsic angular momentum (spin) of the particles. Assuming the equation of state ({{\sf EoS}}) $p_r={\sf w}_1\rho$ and $p_t={\sf w}_2\rho$, we derive exact wormhole solutions satisfying weak and null energy conditions. Depending on the value of the spin square density at the wormhole throat these solutions can be asymptotically flat, de-Sitter or anti de-Sitter. Observational aspects of the wormhole solutions are also studied, using gravitational lensing effects. It is found that the throat can act as a photon sphere near which the light deflection angle has arbitrarily large values. Moreover, for a particular class of solutions, when ${\sf w}_1\rightarrow-{\sf w}_2$ the lensing features of the present model mimic those of the Ellis wormhole in the weak field limit., Comment: 18 pages, 6 figures

In the present work we investigate evolving wormhole configurations described by a constant redshift function in Einstein-Cartan theory ({{\sf ECT}}). The matter content consists of a Weyssenhoff fluid along with an anisotropic matter which together generalize the anisotropic energy momentum tensor in general relativity ({{\sf GR}}) in order to include the effects of intrinsic angular momentum (spin) of particles. Using a generalized Friedmann-Robertson-Walker ({\sf FRW}) spacetime, we derive analytical evolving wormhole geometries by assuming a particular equation of state ({{\sf EoS}}) for energy density and pressure profiles. We introduce exact asymptotically flat and anti-de Sitter spacetimes that admit traversable wormholes and respect energy conditions throughout the spacetime. The rate of expansion of these evolving wormholes is determined only by the Friedmann equation in the presence of spin effects., Comment: 15 pages, 3 figures. arXiv admin note: substantial text overlap with arXiv:1704.06923

In this work, we study the late-time cosmological solutions of $$f(\mathsf{R},\mathsf{T})=g(\mathsf{R})+h(-\mathsf{T})$$ f ( R , T ) = g ( R ) + h ( - T ) models assuming that the conservation of the energy-momentum tensor (EMT) is violated. We perform our analysis through constructing an autonomous dynamical system for the equations of motion. We study the stability properties of solutions via considering linear perturbations about the related equilibrium points. Moreover, we parameterize the Lagrangian by introducing the parameters m(r) and n(s). These parameters which are constructed out of the functions $$g(\mathsf{R})$$ g ( R ) and $$h(-\mathsf{T})$$ h ( - T ) play the main role in finding the late-time behavior of the solutions. We find that there exist, in general, three classes of solutions; all models with $$n>0$$ n > 0 include a proper transition from a prolonged matter era to a de Sitter solution. Models with $$-0.51$$ n ′ > 1 , for at least a root of equation $$n(s)=s-1$$ n ( s ) = s - 1 , include an unphysical dark energy solution preceding an improper matter era. Finally, for $$n

In the present work we introduce a novel approach to study $f({\sf R},{\sf T})$ gravity theory from a different perspective. Here, ${\sf T}$ denotes the trace of energy-momentum tensor ({\sf EMT}) of matter fluids. The usual method (as discussed in the literature) is to choose an $h({\sf T})$ function and then solve for the resulted Friedman equations. Nevertheless, our aim here is, without loss of generality, to reformulate a particular class of $f({\sf R},{\sf T})$ gravity models in which the Einstein-Hilbert action is promoted by an arbitrary function of the trace of {\sf EMT}. The strategy is the redefinition of the equation of motion in terms of the components of an effective fluid. We show that in this case the {\sf EMT} is automatically conserved. As we shall see, adopting such a point of view (at least) in $f({\sf R},{\sf T})$ gravity is accompanied by two significant points. On one hand, $h({\sf T})$ function is chosen based upon a physical concept and on the other, we clearly understand the overall or effective behavior of matter in terms of a conserved effective fluid. To illustrate the idea, we study some models in which different physical properties for the effective fluid is attributed to each model. Particularly, we discuss models with constant effective density, constant effective pressure and constant effective equation of state ({\sf EoS}) parameter. Moreover, two models with a relation between the effective density and the effective pressure will be considered. An elegant result is that in $f({\sf R},{\sf T})$ gravity, there is a possibility that a perfect fluid could effectively behave as a modified Chaplygin gas with four free parameters., 21 pages, 3 figures

The Einstein static (ES) universe has played a major role in various emergent scenarios recently proposed in order to cure the problem of initial singularity of the standard model of cosmology. In the herein model, we study the existence and stability of ES universe in the context of $f(R,T)$ modified theories of gravity. Considering specific forms of $f(R,T)$ function, we seek for the existence of solutions representing ES state. Using dynamical system techniques along with numerical analysis, we find two classes of solutions: the first one is always unstable of the saddle type while the second is always stable so that its dynamical behavior corresponds to a center equilibrium point. The importance of the second class of solutions is due to the significant duty they have in constructing non-singular emergent models in which the universe could have experienced past-eternally, a series of infinite oscillations about such an initial static state after which, it enters through a suitable physical mechanism, to an inflationary era. Considering specific forms for the functionality of $f(R,T)$, we show that this theory is capable of providing cosmological solutions which admit emergent universe (EU) scenarios. We also investigate homogeneous scalar perturbations for the mentioned models. The stability regions of the solutions are parametrized by a linear equation of state (EoS) parameter and other free parameters that will be introduced for the models. Our results suggest that modifications in $f(R,T)$ gravity would lead to stable solutions which are unstable in $f(R)$ gravity model., Comment: 27 pages, 6 figures, major revision

In the present work we investigate wormhole structures and the energy conditions supporting them, in Einstein-Cartan theory ({\sf ECT}). The matter content consists of a Weyssenhoff fluid along with an anisotropic matter which together generalize the anisotropic energy momentum tensor in general relativity ({\sf GR}) to include spin effects. Assuming that the radial pressure and energy density obey a linear equation of state, we introduce exact asymptotically flat and anti-de-Sitter spacetimes that admit traversable wormholes and respect energy conditions. Such wormhole solutions are studied in detail for two specific forms for the redshift function, namely a constant redshift function and the one with power law dependency., Comment: 16 pages, 2 figures

The Einstein-Cartan-Kibble-Sciama ({\sf ECKS}) theory of gravity naturally extends Einstein\rq{}s general relativity ({\sf GR}) to include intrinsic angular momentum (spin) of matter. The main feature of this theory consists of an algebraic relation between spacetime torsion and spin of matter which indeed deprives the torsion of its dynamical content. The Lagrangian of {\sf ECKS} gravity is proportional to the Ricci curvature scalar constructed out of a general affine connection so that owing to the influence of matter energy-momentum and spin, curvature and torsion are produced and interact only through the spacetime metric. In the absence of spin, the spacetime torsion vanishes and the theory reduces to {\sf GR}. It is however possible to have torsion propagation in vacuum by resorting to a model endowed with a non-minimal coupling between curvature and torsion. In the present work we try to investigate possible effects of the higher order terms that can be constructed from spacetime curvature and torsion, as the two basic constituents of Riemann-Cartan geometry. We consider Lagrangians that include fourth-order scalar invariants from curvature and torsion and then investigate the resulted field equations. The solutions that we find show that there could exist, even in vacuum, nontrivial static spacetimes that admit both black holes and naked singularities., Comment: 25 pages, 3 figures

We use the extended uncertainty principle (EUP) in order to obtain the R\'{e}nyi entropy for a black hole (BH). The result implies that the non-extensivity parameter, appeared in the R\'{e}nyi entropy formalism, may be evaluated from the considerations which lead to EUP. It is also shown that, for excited BHs, the R\'{e}nyi entropy is a function of the BH principal quantum number, i.e. the BH quantum excited state. Temperature and heat capacity of the excited BHs are also investigated addressing two phases while only one of them can be stable. At this situation, whereas entropy is vanished, temperature may take a non-zero positive minimum value, depending on the value of the non-extensivity parameter. The evaporation time of excited BH has also been studied.

The description of the accelerated phases for the current and primordial cosmos has been obtained in the framework of R\'{e}nyi cosmology. The results show that corrections to the Friedmann equations, originated from differences between R\'{e}nyi and Bekenstein entropies, can describe both the current accelerated and inflationary eras. The effects of these corrections can be appointed to a hypothetical fluid which is classically unstable. Although inflation can be obtained in R\'{e}nyi cosmology as the vacuum solution, the values found for the parameters of the hypothetical fluid which supports the current accelerated and primary inflationary cosmos differ from each other., Comment: 4 pages, 4 figures

We study the gravitational collapse of a star with barotropic equation of state $p=w\rho$ in the context of $f({\mathcal R})$ theories of gravity. Utilizing the metric formalism, we rewrite the field equations as those of Brans-Dicke theory with vanishing coupling parameter. By choosing the functionality of Ricci scalar as $f({\mathcal R})=\alpha{\mathcal R}^{m}$, we show that for an appropriate initial value of the energy density, if $\alpha$ and $m$ satisfy certain conditions, the resulting singularity would be naked, violating the cosmic censorship conjecture. These conditions are the ratio of the mass function to the area radius of the collapsing ball, negativity of the effective pressure, and the time behavior of the Kretschmann scalar. Also, as long as parameter $\alpha$ obeys certain conditions, the satisfaction of the weak energy condition is guaranteed by the collapsing configuration.

The evolution of inhomogeneous space-times composed of uncharged fermions is studied for Szekeres metrics which have no Killing vectors, in general. Using the Einstein-Cartan theory to include the effects of (intrinsic) matter spin in general relativity, the dynamics of a perfect fluid with nonnull spin degrees of freedom is considered. It is shown that, if the matter is composed by effective dust and certain constraints on the initial data are verified, a singularity will not form. Various special cases are discussed, such as Lemaitre-Tolman-Bondi and Bianchi I space-times, where the results are further extended or shown explicitly to be verified., AHZ is grateful to D Malafarina and R Goswami for useful discussions and to F W Hehl for helpful correspondence. PL thanks IDPASC and FCT-Portugal for financial support through Grant No. PD/BD/114074/2015. FCM was supported by Portuguese Funds through FCT-Fundacao para a Ciencia e Tecnologia, within the Projects UID/MAT/00013/2013 and PTDC/MAT-ANA/1275/2014 as well as grant SFRH/BSAB/130242/2017. This work has been supported financially by Research Institute for Astronomy & Astrophysics of Maragha (RIAAM) under research project No. 1/5750-61., info:eu-repo/semantics/publishedVersion

We consider the gravitational collapse of a spherically symmetric homogeneous matter distribution consisting of a Weyssenhoff fluid in the presence of a negative cosmological constant. Our aim is to investigate the effects of torsion and spin averaged terms on the final outcome of the collapse. For a specific interior spacetime setup, namely the homogeneous and isotropic FLRW metric, we obtain two classes of solutions to the field equations where depending on the relation between spin source parameters, $(i)$ the collapse procedure culminates in a spacetime singularity or $(ii)$ it is replaced by a non-singular bounce. We show that, under certain conditions, for a specific subset of the former solutions, the formation of trapped surfaces is prevented and thus the resulted singularity could be naked. The curvature singularity that forms could be gravitationally strong in the sense of Tipler. Our numerical analysis for the latter solutions shows that the collapsing dynamical process experiences four phases, so that two of which occur at the pre-bounce and the other two at post-bounce regimes. We further observe that there can be found a minimum radius for the apparent horizon curve, such that the main outcome of which is that there exists an upper bound for the size of the collapsing body, below which no horizon forms throughout the whole scenario., 23 pages, 11 figures

In the present work, we revisit the process of gravitational collapse of a spherically symmetric homogeneous dust fluid which is known as the Oppenheimer-Snyder (OS) model [1]. We show that such a scenario would not end in a spacetime singularity when the spin degrees of freedom of fermionic particles within the collapsing cloud are taken into account. To this purpose, we take the matter content of the stellar object as a homogeneous Weyssenhoff fluid which is a generalization of perfect fluid in general relativity (GR) to include the spin of matter. Employing the homogeneous and isotropic FLRW metric for the interior spacetime setup, it is shown that the spin of matter, in the context of a negative pressure, acts against the pull of gravity and decelerates the dynamical evolution of the collapse in its later stages. Our results bode a picture of gravitational collapse in which the collapse process halts at a finite radius whose value depends on the initial configuration. We thus show that the spacetime singularity that occurs in the OS model is replaced by a non-singular bounce beyond which the collapsing cloud re-expands to infinity. Depending on the model parameters, one can find a minimum value for the boundary of the collapsing cloud or correspondingly a threshold value for the mass content below which the horizon formation can be avoided. Our results are supported by a thorough numerical analysis., 16 pages, 5 figures, revised version

We study the gravitational collapse of a homogeneous scalar field, minimally coupled to gravity, in the presence of a particular type of dynamical deformation between the canonical momenta of the scale factor and of the scalar field. In the absence of such a deformation, a class of solutions can be found in the literature [R. Goswami and P. S. Joshi, arXiv:gr-qc/0410144], %\cite{JG04}, whereby a curvature singularity occurs at the collapse end state, which can be either hidden behind a horizon or be visible to external observers. However, when the phase-space is deformed, as implemented herein this paper, we find that the singularity may be either removed or instead, attained faster. More precisely, for negative values of the deformation parameter, we identify the emergence of a negative pressure term, which slows down the collapse so that the singularity is replaced with a bounce. In this respect, the formation of a dynamical horizon can be avoided depending on the suitable choice of the boundary surface of the star. Whereas for positive values, the pressure that originates from the deformation effects assists the collapse toward the singularity formation. In this case, since the collapse speed is unbounded, the condition on the horizon formation is always satisfied and furthermore the dynamical horizon develops earlier than when the phase-space deformations are absent. These results are obtained by means of a thoroughly numerical discussion., Comment: 17 pages, 17 figures

The purpose of this paper is to extend the analysis presented in Goswami et al. [Phys. Rev. Lett. 96, 031302 (2006)] by means of investigating how a specific type of loop (quantum) effect can alter the outcome of gravitational collapse. To be more concrete, a particular class of spherically symmetric spacetime is considered with a tachyon field $\ensuremath{\phi}$ and a barotropic fluid constituting the matter content; the tachyon potential $V(\ensuremath{\phi})$ is assumed to be of the form ${\ensuremath{\phi}}^{\ensuremath{-}2}$. Within inverse triad corrections, we then obtain, for a semiclassical description, several classes of analytical as well as numerical solutions. Moreover, we identify a subset whose behavior corresponds to an outward flux of energy, thus avoiding either a naked singularity or a black hole formation.

A particular class of space-time, with a tachyon field, \phi, and a barotropic fluid constituting the matter content, is considered herein as a model for gravitational collapse. For simplicity, the tachyon potential is assumed to be of inverse square form i.e., V(\phi) \sim \phi^{-2}. Our purpose, by making use of the specific kinematical features of the tachyon, which are rather different from a standard scalar field, is to establish the several types of asymptotic behavior that our matter content induces. Employing a dynamical system analysis, complemented by a thorough numerical study, we find classical solutions corresponding to a naked singularity or a black hole formation. In particular, there is a subset where the fluid and tachyon participate in an interesting tracking behaviour, depending sensitively on the initial conditions for the energy densities of the tachyon field and barotropic fluid. Two other classes of solutions are present, corresponding respectively, to either a tachyon or a barotropic fluid regime. Which of these emerges as dominant, will depend on the choice of the barotropic parameter, \gamma. Furthermore, these collapsing scenarios both have as final state the formation of a black hole., Comment: 18 pages, 7 figures. v3: minor changes. Final version to appear in GRG

Motivated by the geometrical interpretation of Brans-Dicke (BD) scalar field which may also act as a torsion potential in Lyra geometry, we study the effects of spacetime torsion on the dynamics of a collapsing massive star. Taking the interior spacetime as the FLRW metric and the matter content as spherically symmetric, homogeneous perfect fluid with the equation of state (EoS) $p=w\rho$, we show that the collapse ends in a spacetime singularity which is of the strong curvature type in the sense of Tipler. Whether the trapped surfaces form during the dynamical evolution of the collapse depends on the torsion parameter, related to the BD coupling parameter, and the EoS subject to the conditions on physical reasonableness of the collapse configuration. Hence, the space of torsion and EoS parameters is divided into two portions, one for which the collapse process leads to the formation of apparent horizon and the other for which the apparent horizon is failed to form in the interior region. The nature of the singularity is examined from the exterior perspective, by searching for the existence of radial null geodesics reaching the faraway observers. Moreover, it is found that the effects of a dynamical torsion can be transferred to the outside region of the collapsing star, making the exterior region dynamic.

We study the fate of gravitational collapse of a tachyon field matter. In presence of an inverse square potential a black hole forms. Loop quantum corrections lead to the avoidance of classical singularities, which is followed by an outward flux of energy., Contribution to the conference of Loops'11, Madrid

Gravitational collapse of the Brans-Dicke scalar field with non-zero potential in the presence of matter fluid obeying the barotropic equation of state, $p=w\rho$ is studied. Utilizing the concept of expansion parameter, it is seen that the cosmic censorship conjecture may be violated for $w=-\frac{1}{3}$ and $ w=-\frac{2}{3}$ which correspond to cosmic string and domain wall, respectively. We show that physically, it is the rate of collapse and the presence of Brans-Dicke scalar field that govern the formation of a black hole or a naked singularity as the final fate of dynamical evolution and only for these two cases the singularity can be naked as the collapse end state. Also the weak energy condition is satisfied by the collapsing configuration., Comment: 20 pages, 7 figures; some corrections added