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The observational evidence of f (R , , T) gravity in the FRW universe is the focus of the study. We provide a novel cosmological theory in this paper that was created within the f (R , , T) framework. We used the Markov Chain Monte Carlo (MCMC) method to limit the model parameters using the 31 Cosmic Chronometer points and 1048 Pantheon data points. We can efficiently explore the parameter space using this method. In order to compare our proposed f (R , , T) model to the well-known Λ CDM model, we take into account a parameterization of the Hubble parameter for solving the modified field equation. By studying these factors in the context of our new model, we get essential knowledge into the dynamics and evolution of the cosmos. State finder and Om Diagnostic, among other diagnostic tests, have been used to further analyze the behavior and consistency of our f (R , , T) model. These tests give us a greater understanding of our model's properties and their applicability to observational data. [ABSTRACT FROM AUTHOR]

In this paper, we refer to the early hypotheses on superconductivity in the models of the electron as an extended particle, and show how Nonlinear Electrodynamics minimally coupled to Gravity (NED-GR) predicts, without additional assumptions, the existence of primary superconductivity as the universal property of regular electrically charged rotating black holes and electromagnetic spinning solitons, replacing naked singularities and including that with the parameters of the electron. NED-GR dynamical equations describe basic generic properties of these objects in a self-consistent and the model-independent way, and establish the existence of the superconducting ring current in their deep interiors, which presents the non-dissipative source of the electromagnetic field of an object and provides its practically unlimited life time. [ABSTRACT FROM AUTHOR]

In this paper, we explore the physical properties and characteristics of static, spherically symmetric wormholes in the background of Rastall–Rainbow gravity. The Rastall–Rainbow gravity theory has recently been proposed as a combination of two theories, namely, the Rastall theory and the Rainbow description. We implemented noncommutativity by adopting two different distributions of energy density (Gaussian and Lorentzian) in the Morris and Thorne metric. We solve the field equations analytically and discuss all the properties of wormholes depending on the two model parameters. Notably, for specific parameter ranges, one can alleviate the violation of the WEC at the throat and its neighborhood. [ABSTRACT FROM AUTHOR]

General relativity has been the dominant paradigm theory of gravity for the last century, it manages to explain a plethora of astronomical and cosmological observations with high accuracy. A major challenge faced by general relativity is, that it predicts a singularity as the beginning of our universe. In this paper, we shall explore a well-known idea in the literature that of the emergent universe and its viability according to Planck 2018 data and BBN constraints. [ABSTRACT FROM AUTHOR]

In this paper, we consider a charged BTZ black hole in asymptotically AdS spacetime of massive gravity to study the effect of the thermal fluctuations on the black hole thermodynamics. We consider the Einstein–Born–Infeld solution and investigate critical points and stability. We also compare the results with the case of Einstein–Maxwell solutions. Besides, we find that thermal fluctuations, which appear as a logarithmic term in the entropy, affect the stability of the black hole and change the phase transition point. Moreover, we study the geometrical thermodynamics and find that the behavior of the linear Maxwell solution is the same as the nonlinear one. [ABSTRACT FROM AUTHOR]

Second-order-derivative plus fourth-order-derivative gravity is the ultraviolet completion of second-order-derivative quantum Einstein gravity. While it achieves renormalizability through states of negative Dirac norm, the unitarity violation that this would entail can be postponed to Planck energies. As we show in this paper, the theory has a different problem, one that occurs at all energy scales, namely that the Dirac norm of the vacuum of the theory is not finite. To establish this, we present a procedure for determining the norm of the vacuum in any quantum field theory. With the Dirac norm of the vacuum of the second-order-derivative plus fourth-order-derivative theory not being finite, the Feynman rules that are used to establish renormalizability are not valid, as is the assumption that the theory can be used as an effective theory at energies well below the Planck scale. This lack of finiteness is also manifested in the fact that the Lorentzian path integral for the theory is divergent. Because the vacuum Dirac norm is not finite, the Hamiltonian of the theory is not Hermitian. However, it turns out to be P T symmetric. When one continues the theory into the complex plane and uses the P T symmetry inner product, viz. the overlap of the left-eigenstate of the Hamiltonian with its right-eigenstate, one then finds that for the vacuum this norm is both finite and positive, the Feynman rules now are valid, the Lorentzian path integral now is well behaved, and the theory now can serve as a low energy effective theory, one that we show to have a real classical gravity limit. Consequently, the theory can now be offered as a fully consistent, unitary and renormalizable theory of quantum gravity. [ABSTRACT FROM AUTHOR]

Circa 1962 Hans A. Buchdahl pioneered a program — and made significant progress — seeking vacuo configurations in pure ℛ 2 gravity (H. A. Buchdahl, Nuovo Cimento 23 (1962) 141). Unfortunately, he deemed the final step in his calculations impassable and prematurely suspended his pursuit. Since then, his achievements on this front have faded into dormancy. Unbeknownst to Buchdahl himself, the ℛ 2 vacua that he sought were within his striking distance. In our recent three-paper body of work published in Nguyen (Phys. Rev. D 106 (2022) 104004; Phys. Rev. D 107 (2023) 104008; Phys. Rev. D 107 (2023) 104009), we broke this six-decades-old impasse and accomplished his goal: A novel class of metrics, describing non-Schwarzschild spacetimes in quadratic gravity and carrying a hallmark of higher-derivative characteristic. Intriguing properties of Buchdahl-inspired spacetimes and their associated Morris–Thorne–Buchdahl wormholes — summarized herein — embody a new branch of phenomenology that transcends the Einstein–Hilbert paradigm. May this unsung legacy of Hans A. Buchdahl long "forsaken" and "forgotten" finally live on Our essay is a passageway bridging the past and the future. The past is rooted in a research program originated but left unfinished by Buchdahl going back threescore years ago. The future is the new horizons his program brings forth. In between is our labor — summarized in a three-installment "Beyond Schwarzschild–de Sitter spacetimes" series[1–3] — to advance the Buchdahl program and achieve his ultimate goal. [ABSTRACT FROM AUTHOR]

In this paper, we would like to examine whether a novel Starobinsky–Bel–Robinson (SBR) gravity model admits stable exponential inflationary solutions with or without spatial anisotropies. As a result, we are able to derive an exact de Sitter inflationary to this SBR model. Furthermore, we observe that an exact Bianchi type I inflationary solution does not exist in the SBR model. However, we find that a modified SBR model, in which the sign of coefficient of R 2 term is flipped from positive to negative, can admit the corresponding Bianchi type I inflationary solution. Unfortunately, stability analysis using the dynamical system approach indicates that both of these inflationary solutions turn out to be unstable. Interestingly, we show that a stable de Sitter inflationary solution can be obtained in the modified SBR gravity. [ABSTRACT FROM AUTHOR]

This paper aims to conduct an extensive examination of hyperbolically symmetrical static fluid distributions, wherein a precise analysis of their physical characteristics is carried out in the background of modified f (G , T) gravity, where G and T stands for Gauss Bonnet invariant and energy-momentum trace, respectively. The outcomes reveal that the energy density exhibits negative value, thus implying that any utilization of such fluids necessitates extremely demanding circumstances where quantum effects would be significantly influential. Furthermore, it has been determined that these fluid distributions cannot take the vicinity surrounding their central point of symmetry and leave behind an empty vacuum cavity in its place. Additionally, an appropriate definition for mass function and the complexity factor is determined. Eventually, we exhibit a broad strategy for accomplishing particular solutions and showcase several instances of exact analytical solutions in the presence of f (G , T) correction terms. [ABSTRACT FROM AUTHOR]

In this paper, we examine the homogeneous and isotropic flat Universe in the frame of symmetric teleparallel gravity say f (Q) gravity (where Q is the nonmetricity scalar). In this work, we parametrized the field equations with the help of Hubble's parameter defined as H (z) = η [ 1 + (z + 1) − γ ] , where η and γ are model/free parameters which are constrained with updated 57 data points of the Hubble data set within the redshift range 0. 0 7 < z < 2. 3 6. For this, we have used a Markov Chain Monte Carlo Technique (MCMCT). Some physical parameters of the model are discussed. In addition, we analyze the jerk parameter and the statefinder parameters and we also study the energy conditions to assess the compatibility of our model with dark energy models; we determine that the Strong Energy Condition (SEC) is violated due to the fact that the Universe is currently accelerating. [ABSTRACT FROM AUTHOR]

The work reported in this paper explores holographic bounce. In the first phase of the study, we chose a non-singular bouncing scale factor. Then we reconstructed f (T) gravity and analytically derived constraints on the bouncing parameter σ. These constraints helped us understand the scale factor's quintessence or phantom behavior. Furthermore, we also explored the statefinder parameters for reconstructed f (T) and observed the attainment of Λ CDM fixed point. Next, we considered the multiplicative bouncing scale factor inspired by S. D. Odintsov and V. K. Oikonomou Phys. Rev. D 94, (2016) 064022. For this choice, we discussed the types of singularities realizable for different cases. Through the Talyor series expansion, we analytically presented cases and subcases for different ranges of α of the scale factor. In the last phase of the study, we demonstrated holographic bounce with the choice of the multiplicative scale factor. In this case, we considered holographic Ricci dark energy and Barrow holographic dark energy. We concluded that it is possible to generate constraints on the bouncing parameter for its feasibility for the EoS parameter. We concluded that the realization of holographic bounce is possible, and different suitable constraints can be derived for this multiplicative bouncing scale factor focusing on the realization of cosmic bounce. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to explore the anisotropic exact Bianchi type II solutions in f (T) gravity, where T denotes the torsion scalar. We utilize the solutions to discuss conformal vector fields (CVFs) and energy conditions. In the first slot of this study, we find the CVFs. The CVFs being a generalization of the Killing vector fields (KVFs) are affiliated with the conservation laws of physics. Corresponding to the obtained solutions, we observe law of conservation of (linear or generalized) momentum. In the second slot, we derive the constraints under which the solution classes can admit certain energy conditions. [ABSTRACT FROM AUTHOR]

Delving into the realm of scalar–tensor theory of gravity, this paper uncovers the intricate details of the conformal factor and its correspondence to quantum mechanical mass fluctuations. Shedding light on the theory's key findings, we explore the quantum mechanical nature of the wave equation associated with mass fluctuations, predicting the creation of scalar gravitational waves. The association of conformal fluctuations with the quantum potential presents a remarkable feature of this theory, which predicts a scalar component of gravitational waves. With the potential to generate such waves in a laboratory set-up, this theory invites exciting possibilities for empirical testing, highlighting the quantum mechanical origin of scalar gravitational waves. [ABSTRACT FROM AUTHOR]

In this paper, we worked in the framework of an inflationary f (R , T) theory in the presence of a canonical scalar field. More specifically, the f (R , T) = γ R + 2 κ α T gravity. The values of the dimensionless parameters α and γ are taken to be α ≥ 0 and 0 < γ ≤ 1. The motivation for this study was the striking similarities between the slow-roll parameters of the inflationary model used in this work and the ones obtained by the rescaled Einstein–Hilbert gravity inflation f (R) = α R. We examined a variety of potentials to determine if they agree with the current Planck Constraints. In addition, we checked whether these models satisfy the Swampland Criteria and we specified the exact region of the parameter space that produces viable results for each model. As we mention in Sec. 4, the inflationary f (R , T) theory used in this work cannot produce a positive n T which implies that the stochastic gravitational wave background will not be detectable. [ABSTRACT FROM AUTHOR]

In this paper, our interest lies in investigating the possibilities of reconstructing analytical solutions for some familiar bouncing models in the flat Friedmann–Lemaître–Robertson–Walker (FLRW) geometry. We inspect the various gravitational Lagrangians, which are efficient enough for reproducing analytical solutions for symmetric, oscillatory, power law and bounce from loop quantum cosmology settings. Equation of state parameter, energy conditions, stability analysis have been performed for each model to examine its validity. The outcomes determine that the f (T ,) modified theory is the competitive candidate of extended theories on the cosmological scale. [ABSTRACT FROM AUTHOR]

In this paper, the modified F (R) gravity theory with the function F (R) = − (1 / β) ln (1 − β R) is studied. The action at small coupling β becomes Einstein–Hilbert action. The bound on the parameter β from local tests is β ≤ 2 × 1 0 − 6 cm2. We find the constant curvature solutions and it was shown that the de Sitter space is unstable but a solution with zero Ricci scalar is stable. The potential and the mass of the scalar field (scalaron) are obtained in Einstein's frame. The slow-roll cosmological parameters are studied and e-folds number is evaluated. The critical points of autonomous equations are analyzed. The function m (r) that describes the deviation from the Λ CDM model is calculated. [ABSTRACT FROM AUTHOR]

Here, we discuss a topic that comes up more often than expected: A same theory or theoretical model arises in two different presentations which are assumed to be actually different theories so that these are independently developed. Sometimes this leads to an unwanted doubling of the results. In this paper, we illustrate this issue with the example of two apparently different gravitational theories: (i) the (minimally coupled) Einstein-massless-scalar (EMS) system and (ii) the Sáez–Ballester theory (SBT). We demonstrate that the latter is not a scalar–tensor theory of gravity, as widely acknowledged. Moreover, SBT is identified with the EMS theory. As illustrations of this identification we show that several known solutions of SBT are also solutions of the EMS system and vice versa. Cosmological arguments are also considered. In particular, a dynamical systems-based demonstration of the dynamical equivalence between these theories is given. The study of the asymptotic dynamics of the Sáez–Ballester-based cosmological model shows that there are no equilibrium points which could be associated with accelerated expansion, unless one includes a cosmological constant term or a self-interacting scalar field. This is a well-known result for cosmological models which are based on the Einstein-self-interacting-scalar theory, also known as quintessence. [ABSTRACT FROM AUTHOR]

In this paper, we study black hole entropy universality within the conformal Weyl gravity paradigm. We do this by first computing the entropy of specific vacuum and non-vacuum solutions, previously unexplored in conformal Weyl gravity via both the Nöther current method and Wald's entropy formula. For the vacuum case, we explore the near horizon near extremal Kerr metric, which is also a vacuum solution to conformal Weyl gravity and not previously studied in this setting. For the non-vacuum case, we couple the conformal Weyl gravity field equations to a near horizon (linear) U (1) gauge potential and analyze the respective found solutions. We highlight the non-universality of black hole entropy between our studied black hole solutions of varying symmetries. However, despite non-universality, the respective black hole entropies are in congruence with Wald's entropy formula for the specific gravity theory. Finally and despite non-universality, we comment on the construction of a near horizon CFT dual to one of our unique non-vacuum solutions. Due to the non-universality, we must introduce a parameter (similarly to entropy calculations in LQG) which we also call γ and relating to the Weyl anomaly coefficient. The construction follows an A d S 2 / C F T 1 correspondence in the near horizon, which enables the computation of the full asymptotic symmetry group of the chosen non-vacuum conformal Weyl black hole and its near horizon quantum CFT dual. We conclude with a discussion and outlook for future work. [ABSTRACT FROM AUTHOR]

In this paper, we consider an inflation scenario in which the inflation roll rate, defined by ϕ ̈ H ϕ ̇ is supposed to be constant and study constant-roll inflation by a scalar field with minimal coupling to gravity in the presence of non-commutative (NC) parameters. For this purpose, we use the deformation approach and obtain the corresponding Lagrangian to the model by employing new variables. Then, we find exact solutions for the inflation potential, which include power-law, quadratic hilltop, and natural inflation. Further, we discuss the effect of parameters on the inflation model in NC space. [ABSTRACT FROM AUTHOR]

In this paper, a modified Born–Infeld gravitation theory with an f (R) function being added to the determinant action is analyzed from a cosmological viewpoint. The corresponding accelerating dynamics are studied in a simplified conformal approach without matter. Three different structures for the auxiliary metric function are analyzed, with the aim to establish a deeper understanding of the role of this function in cosmology. After performing the analysis, it is seen that by modifying the auxiliary metric function, a Big Rip singularity or either a Little Rip dark energy model may arise. [ABSTRACT FROM AUTHOR]

Mimetic gravity analysis has been studied as a theory in various types of general relativity extensions, such as mimetic f (R) gravity, mimetic f (R , T) gravity, mimetic f (R , G) gravity, etc. in the literature. This paper presents a set of equations arising from mimetic conditions and studies cosmic inflation with a combination of mimetic f (G) gravity and swampland dS conjectures. We analyze and evaluate these results. Therefore, we first thoroughly introduce the mimetic f (G) gravity and calculate some cosmological parameters such as the scalar spectral index, the tensor-to-scalar ratio, and the slow-roll parameters. Also, we investigate the potential according to the mimetic f (G) gravity. Then we will challenge the swampland dS conjectures with this condition. By expressing the coefficient of swampland dS conjectures viz C 1 and C 2 in terms of n s and r , we plot some figures and determine the allowable range for each of these cosmological parameters and these coefficients, and finally, compare these results with observable data such as Planck and BICEP2/Keck array data. We show C 1 and C 2 are not (1) , so the refining swampland dS conjecture is not satisfied for this inflationary model. Then we examine it with further refining swampland dS conjecture, which has a series of free parameters such as a , b > 0 , q > 2 and a + b = 1. By adjusting these parameters, the compatibility of the mentioned conjecture with the inflationary model can be discussed. We determine the further refining swampland dS conjecture is satisfied. when a < 1 1. 0 0 4 8 9 = 0. 9 9 5 1 3 , we can always find a , b and q whose value is larger than 2, viz for q = 2. 4 , we find 0. 9 9 1 8 5 ≤ a < 1 , which we can choose a = 0. 9 9 2 3 5 according to the condition a < 0. 9 9 5 1 3. Also we know b = 1 − a , so we will have 1 − 0. 9 9 2 3 5 = 0. 0 0 7 6 5 > 0. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to uncover a connection between the dynamics associated with cosmological models and the radially symmetric nonlinear Schrödinger (NLS) equation. The link is made possible due to the central role played by a nonlinear, second-order ordinary differential equation (ODE) known as Milne–Ermakov–Pinney (EMP) equation. To this end, we describe the standard Friedmann–Robertson–Walker (FRW) cosmology of canonical scalar field (CSF) with two barotropic fluids (BFs) in terms of time-independent NLS type formulation within Rastall's theory of gravity (RTG). We derive all cosmological dynamical quantities including scalar field ϕ (x) , scalar potential V (x) , total Schrödinger energy E (x) , wave function u (x) and NLS potential P (x) , of EMP equation in terms of Schrödinger quantities. We consider bouncing and power-law models to find the exact solutions to these quantities in terms of cosmic time. It is found that RTG is a good candidate to discuss NLS type FRW formulation as the obtained results are in good compatibility with the standard theory of gravity. [ABSTRACT FROM AUTHOR]

A consistent description of gravity in quantum mechanics and general relativity is becoming increasingly accessible to table-top experiments. In this paper, I introduce the experimental technique of large momentum transfer optics as a means to probe gravity at microscopic scales. I argue, with the help of recent experimental observations, that large momentum transfer optics is the best experimental technique to do so. I conclude with possible future directions using large momentum transfer optics. [ABSTRACT FROM AUTHOR]

In this paper, we present an overview of recent developments in the numerical solution of Horndeski gravity theories, which are the class of all scalar–tensor theories of gravity that have second-order equations of motion. We review several methods that have been used to establish well-posed initial value problems for these theories, and discuss well-posed formulations of the constraint equations. We also discuss global aspects of exact, strongly coupled solutions to some of the Horndeski gravity theories: the formation of shocks, the loss of hyperbolicity, and the formation of naked curvature singularities. Finally, we discuss numerical solutions to binary black hole and neutron star systems for several Horndeski theories. [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]

This paper investigates the possibility of reconstruction of the generic function in F (ℛ , T) gravitational theory by considering some well-known cosmological bouncing models, namely, exponential evaluation, oscillatory, power law and matter bounce model, where ℛ and T are Ricci scalar and trace of energy–momentum tensor, respectively. Due to the complexity of dynamical field equations, we propose some ansatz forms of function F (ℛ , T) in perspective models and examine which type of Lagrangian is capable of reproducing bouncing solution via analytical expression. It is seen that for some cases of exponential, oscillatory and matter bounce models, it is possible to get analytical solution while in other cases, it is not possible to achieve exact (general) solutions so only complementary solutions can be discussed. However, for power-law model, all forms of generic function can be reconstructed analytically. Next we analyze the energy conditions and stability of these reconstructed cosmological bouncing models which have analytical forms. It is found that these models are stable for linear forms of Lagrangian only but the reconstructed solutions for power law are unstable for some nonlinear forms of Lagrangian. Further, we determine the observable quantities like spectral index ( n s ) and tensor-to-scalar ratio (r) for the simplest reconstructed form of f (R , T) function. As a result, we directly confront the reconstructed linear form of Lagrangian in F (ℛ , T) model with 2018 Planck observations. Furthermore, we analyze that F (ℛ , T) gravity with dark energy epoch is consistent with Sne-Ia+BAO+H(z)+CMB data and show that bounce can unify with dark energy epochs in F (ℛ , T) gravity. [ABSTRACT FROM AUTHOR]

In this paper, the wormholes have been constructed in the framework of the linear f (R , T) gravity model with noncommutative geometry. The wormhole solutions are obtained in the context of Gaussian and Lorentzian distributions of matter using f (R , T) gravity. The energy conditions such as null, weak, strong, and dominant energy conditions are explored and the regions satisfying these conditions are examined for both forms of matter distributions. Further, the stability of wormhole solutions is studied with the help of TOV (Tolman–Oppenheimer–Volkoff) equations. It is found that the wormhole solutions are stable and free from exotic matter for the suitable choice of parameters. [ABSTRACT FROM AUTHOR]

In this paper, we consider static spherical structure to develop some anisotropic solutions by employing the extended gravitational decoupling scheme in the background of f (ℛ , , ℛ γ χ γ χ) gravity, where ℛ and indicate the Ricci scalar and trace of the energy–momentum tensor, respectively. We transform both radial as well as temporal metric functions and apply them on the field equations that produce two different sets corresponding to the decoupling parameter ξ. The first set is associated with isotropic distribution, i.e. modified Krori–Barua solution. The second set is influenced from anisotropic factor and contains unknowns which are determined by taking some constraints. The impact of decoupling parameter is then analyzed on the obtained physical variables and anisotropy. We also investigate energy conditions and some other parameters such as mass, compactness and redshift graphically. It is found that our solution corresponding to pressure-like constraint shows stable behavior throughout in this gravity for the considered range of ξ. [ABSTRACT FROM AUTHOR]

In this paper, metric-affine theories in which the gravity Lagrangian is built using (projectively invariant) contractions of the Ricci tensor with itself and with the metric (Ricci-based gravity theories, or RBGs for short) are reviewed. The goal is to provide a contextualized and coherent presentation of some recent results. In particular, we focus on the correspondence that exists between the field equations of these theories and those of general relativity, and comment on how this can be used to build new solutions of physical interest. We also discuss the formalism of junction conditions in the f (R) case, and provide a brief summary on current experimental and observational bounds on model parameters. [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 deals with stability analysis of anisotropic-charged cylindrical geometry coupled with f (R , T) modified gravity theory (ℳ ). In this ℳ , the Lagrangian is a varying function of traces of Ricci and stress tensors. In order to examine the stable behavior of the anisotropic cylindrical cosmic object under the electromagnetic effects, we calculated the modified equations of motion and equations of continuity. in the background of f (R , T) theory. By utilizing the perturbation technique, we constitute a modified collapse equation (ℳ ℰ), which contains geometrical and physical variables in the charged scenario. Our investigation continues under the nonminimally coupled function of ℳ . Furthermore, we investigate the dynamically stable phases of charged geometry at Newtonian () and post-Newtonian (p ) epochs with the help of the rigidity parameter of fluid (Γ) by imposing some constraints on physical parameters. [ABSTRACT FROM AUTHOR]

Hamilton–Jacobi ansatz is used to analyze boson charged particles tunneling in Rastall gravity Kerr–Newman black hole (BH) surrounded by perfect fluid matter through the horizon. The geometrical BH parameters are observed, how these are affecting the radiation utilizing the Lagrangian gravity equation. In this paper, the corrected Hawking temperature is computed by considering the effects of quantum gravity. In our analysis, the Rastall gravity BH solution surrounded by perfect fluid matter effects on Hawking radiation is analyzed graphically. Moreover, the stability and instability of Rastall gravity BH are investigated. The influence of quantum gravity and rotation parameters on BH radiation are also observed by graphical representation. [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]

The motive of this paper is to investigate the evolution of dissipative axially symmetry collapsing fluid in the presence of dark sources. For this purpose, we use the modified Gauss–Bonnet gravity as an exotic energy candidate. We formulate the dynamical variables and investigate the impacts of dark sources upon the heat dissipation and pressure anisotropy. We find scalar functions through orthogonal decomposition of Riemann tensor. The physical behavior of these scalars has been examined for matter as well as dark source configurations. Moreover, we develop a set of EEs related to the evolution of dynamical variables, heat transport equation, Weyl tensor and super-Poynting vector. These equations discuss collapse features like thermodynamics, density inhomogeneity and gravitational radiations in presences of exotic terms. It has been shown that the dark source terms have an effect on the thermodynamics of the system, evolution of kinematical variables and density inhomogeneity. The exotic terms can generate repulsive radiations which can induce cosmic expansion. Finally, the physical significance of modified Gauss–Bonnet gravity is explored in association to the stellar 4U1820-30. [ABSTRACT FROM AUTHOR]

In this paper, we study the properties of gravity and bulk fields living in a torsion warped braneworld. The torsion is driven by a background vector whose norm provides a source for the bulk cosmological constant. For a vector as the derivative of a scalar field, we find new isotropic thick brane geometry. We analyze the features of bosonic and fermionic fields in this scenario. The background vector provides nonminimal coupling between the fields and the geometry leading to modifications in the Kaluza–Klein (KK) states. For a scalar field, the background vector breaks the Z 2 symmetry, whereas for the vector gauge field, a geometric coupling to the torsion tensor traps the massless mode. The spinor connection is modified by the torsion and a geometric derivative Yukawa-like coupling is proposed. The effects of these new couplings are investigated. [ABSTRACT FROM AUTHOR]

In this paper, we study the sparsity of Hawking radiation from nonrotating singularity-free black holes in conformal gravity. We give a rigorous bound on the greybody factor for massless scalar field and calculate the sparsity of Hawking radiation from the black hole. Besides, we investigate the dependence of the bound of the greybody factor and the sparsity of Hawking radiation on the conformal parameters, respectively. Our study shows that when the conformal parameters are large, the increase of conformal parameters will lead to a more sparse Hawking radiation, while to a less sparse Hawking radiation if the conformal parameters are small. [ABSTRACT FROM AUTHOR]

In this paper, we consider two interesting cosmological models characterized by two expressions of the Hubble parameter and as goal, we realize the unification of the early-time and the late-time acceleration eras with the radiation- and matter-dominated eras, and this, in the context of f (T) theory of gravity. The particularity here is that the first model ensures just the unification of the early- and late-time eras with the matter-dominated era without the account of the radiation era, while the second model guarantees the unification of all the eras, namely, the early time, the radiation- and matter-dominated eras, and the late-time acceleration era. For each model, we point out two type IV singularities, the first occurring at the end of the inflationary era and the second at the end of the matter-dominated era. For the first model, the Hubble radius is introduced and three different expressions are taken into account: the first for the inflationary era, the second for the matter-dominated era and the third for the dark energy-dominated era. Hence, the algebraic f (T) is reconstructed for the three eras. Attention is attached to the inflationary era by calculating the corresponding two first slow-roll parameters and challenging them with recent Planck and BICEP2/Keck-Array observational data, and the results show the consistency of the reconstructed inflationary model. For the second model, we note a similarity with the first model, concerning the inflationary, matter-dominated and late-time acceleration eras. Then, we just perform the reconstruction of f (T) , which is able to realize the radiation-dominated era. [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]

Morris and Thorne [Am. J. Phys.56 (1988) 395] developed wormhole solutions in General Relativity which requires the dissatisfaction of null energy condition at the throat. The current scenario of the universe has been explained by using the background of modified gravity and it has inspired the researchers to explore the wormhole solutions in modified versions of gravity. In this paper, the f (R , T) theory of gravity proposed by Harko et al. [Phys. Rev. D84 (2011) 024020] is taken to investigate traversable wormhole solutions. These solutions have been developed in three cases. In each case, the shape function is derived, its characteristics are studied and the embedding surface is constructed. Further, in each case, the wormhole solutions are constructed and energy conditions are tested to know about the matter sustaining the wormhole geometries. [ABSTRACT FROM AUTHOR]

In this paper, we study the traversable wormhole solutions for a logarithmic corrected f (R) model by considering two different statements of shape b (r) and redshift Φ (r) functions. We calculate the parameters of the model including energy density ρ , tangential pressure P t and radial pressure P r for the corresponding forms of the functions. Then, we investigate different energy conditions such as null energy condition, weak energy condition, dominant energy condition and strong energy condition for our considered cases. Finally, we explain the satisfactory conditions of energy of the models by related plots. [ABSTRACT FROM AUTHOR]

In this work, we shall study a class of f (R , ϕ) gravity models which during the inflationary era, which is the large curvature regime, result to an effective inflationary Lagrangian that contains a rescaled Einstein–Hilbert term α R in the presence of a canonical minimally coupled scalar field. The dimensionless parameter α is chosen to take values in the range 0 < α < 1 and the main motivation for studying these rescaled Einstein–Hilbert f (R , ϕ) gravities, is the fact that the rescaled action may render an otherwise incompatible canonical scalar field theory with the Swampland criteria, to be compatible with the Swampland criteria and specifically the distance conjecture and the de Sitter conjecture, without considering the refined de Sitter conjecture however. As we will show, by studying a large number of inflationary potentials appearing in the 2018 Planck collaboration paper for the constraints on inflation, the simultaneous compatibility with both the Planck constraints and the aforementioned two Swampland criteria is achieved for some models, and the main characteristic of the models for which this is possible is the small value that the parameter α must take. [ABSTRACT FROM AUTHOR]

In this paper, we develop the Ostrogradsky–Hamilton formalism for geodetic brane gravity, described by the Regge–Teitelboim geometric model in higher codimension. We treat this gravity theory as a second-order derivative theory, based on the extrinsic geometric structure of the model. As opposed to previous treatments of geodetic brane gravity, our Lagrangian is linearly dependent on second-order time derivatives of the field variables, the embedding functions. The difference resides in a boundary term in the action, usually discarded. Certainly, this suggests applying an appropriate Ostrogradsky–Hamiltonian approach to this type of theories. The price to pay for this choice is the appearance of second-class constraints. We determine the full set of phase space constraints, as well as the gauge transformations they generate in the reduced phase space. Additionally, we compute the algebra of constraints and explain its physical content. In the same spirit, we deduce the counting of the physical degrees of freedom. We comment briefly on the naive formal canonical quantization emerging from our development. [ABSTRACT FROM AUTHOR]

The assumptions of the Hawking–Penrose singularity theorem are not covariant under field redefinitions. Following the works on the covariant formulation of quantum field theory initiated by Vilkovisky and DeWitt in the 80s, we propose to study singularities in field space, where the spacetime metric is treated as a coordinate along with the other fields in the theory. From this viewpoint, a spacetime singularity might be just a singularity in the field-space coordinates, analogously to the standard coordinate singularities in General Relativity. Objects invariant under field-space coordinate transformations can then reveal whether certain spacetime singularity is indeed singular. We recall that observables in quantum field theory are scalar functionals in field space. Therefore, in principle, spacetime singularities corresponding to regular field-space curvature invariants would not affect physical observables. In this paper, we show that the field-space Kretschmann scalar for a certain choice of the DeWitt field-space metric is everywhere finite. This fact could be interpreted as an indication that no singularities actually exist in pure gravity for any gravitational action. In particular, all vacuum singularities of General Relativity result from an unhappy choice of field variables. The extension to the case in which matter fields are present, as required by singularity theorems, is left for future development. [ABSTRACT FROM AUTHOR]

In this paper, two numerical methods are used to calculate quasi-normal modes (QNMs) of near-extremal black holes/strings in the generalized spherically/cylindrically symmetric background, the Asymptotic Iteration Method (AIM) and the Spectral Method. The numerical results confirm the accuracy of the approximate analytic formula using the Pöschl–Teller potential. Our analytic formula is used to investigate the Strong Cosmic Censorship conjecture of extremal and near-extremal black holes in (P. Burikham, S. Ponglertsakul and T. Wuthicharn, Eur. Phys. J. C80 (2020) 954, arXiv:2010.05879 [gr-qc]). [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, we investigate the cosmological inflation in the context of a minimal matter–geometry coupling, which is based on the f (R , T) gravity theory, where f is a generic function of the curvature scalar R and the trace T of the energy–momentum tensor. Assuming that the slow-roll inflation conditions hold true in f (R , T) gravity, we obtain the various inflation-related observables such as the tensor-to-scalar ratio r , the scalar spectral index n s , the running α s of the spectral index and the tensor spectral n T for two specific f (R , T) models from the Hubble slow-roll parameters. To observe the viability of these models, a numerical analysis of such parameters has been done. The results showed that, by using the various values of free parameters, it is possible to obtain a viable compatible with the observational data. [ABSTRACT FROM AUTHOR]

Wormholes are tunnels connecting different regions in spacetime. They were obtained originally as a solution for Einstein's General Theory of Relativity and according to this theory they need to be filled by an exotic kind of anisotropic matter. In the present sense, by "exotic matter" we mean matter that does not satisfy the energy conditions. In this paper, we propose the modeling of traversable wormholes (i.e. wormholes that can be safely crossed) within an alternative gravity theory that proposes an extra material (rather than geometrical) term in its gravitational action, namely the traceless f (R , T) theory of gravitation, with R and T being, respectively, the Ricci scalar and trace of the energy–momentum tensor. Our solutions are obtained from well-known particular cases of the wormhole metric potentials, namely redshift and shape functions. In possession of the solutions for the wormhole material content, we also apply the energy conditions to them. The features of those are carefully discussed. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the holographic inflation in the framework of two gravitational theories like f (T) gravity, where T corresponds to the torsion scalar and DGP braneworld model which is based on the idea that our four-dimensional FRW universe located on five-dimensional manifold. The modified infrared cutoff is used to explore all the inflationary parameters as slow-roll parameters ( 1 , 2 , 3) , number of e -folds N , scalar spectral index n s , tensor-to-scalar ratio r and running of the scalar spectral index α s . Also, we investigate the behavior of inflationary parameters n s − N , α s − N , r − N , α s − n s and r − n s through graphical presentation. The behavior of these parameters shows the consistency with Planck data 2 0 1 8. [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]

In this paper, we carry a dynamical system analysis of a holographic dark energy model of a modified theory of gravity based on braneworld scenario. The infra-red cutoff of the holographic dark energy is considered to be the Hubble scale in a first approach and the Ricci scalar curvature scale in a second approach. We present a detailed analysis of the critical points, their stability and the late-time cosmological viability of the solutions in the phase space. For both holographic dark energy models, we investigate the case of a noninteracting braneworld and extend our analysis to the interacting case between dark energy and cold dark matter by considering some specific forms of interaction. [ABSTRACT FROM AUTHOR]