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We introduce a theoretical framework to alleviate the Hubble tension. This framework is based on dynamics of a minimally coupled scalar field which either belongs to the Brans–Dicke theory with a self-interacting potential or is the scalar partner of f(R) gravity. These two theories are dynamically equivalent when the Brans–Dicke parameter is zero. We will use the dynamical equivalence to interpret the Hubble tension in the same theoretical framework. For both theories, we write one set of field equations in which the value of a parameter distinguishes between the two theories. We will show that H0 actually evolves with redshift so that its value is consistent with that measured from the local distance ladder and then drops to the value measured from CMB at high redshift. We argue that even though both theories exhibit this behavior, Brans–Dicke theory with an exponential potential is more successful than power-law f(R) gravity to relieve the Hubble tension. [ABSTRACT FROM AUTHOR]

If entanglement builds spacetime, then conversely, disentanglement ought to destroy spacetime. From the quantum null energy condition and quantum focusing conjecture, we obtain disentanglement criteria which necessitate infinite energies and strong spacetime singularities. We apply our results to the strong cosmic censorship proposal, where singularities at the Cauchy horizons in black holes are desirable. Using our disentanglement criteria and without resorting to any detailed calculations, we provide an exceedingly general and physically transparent discussion of strong cosmic censorship in semiclassical black holes. We argue that strong cosmic censorship is enforced in asymptotically flat and de Sitter black holes by disentanglement and describe how similar disentanglement might be avoided in some anti-de Sitter cases. [ABSTRACT FROM AUTHOR]

In this paper, we will manifest how polytropic fluids produce effects on an alternate gravitational source, regardless of its features, for spherically symmetric and static spacetimes. For this purpose, we use f (T ,) modified gravity, which is characterized by the trace of an energy–momentum tensor as and torsion scalar T , Additionally, using the polytropic equation of state, we will speculate about the distribution of energy among fluids within an astrophysical object. We offer a thorough methodology for identifying generic relativistic polytropes accompanied by the minimal geometric decoupling technique. Ultimately, a few tangible attributes of polytropes along with perfect fluids are observed using the Tolman IV solution, including total pressure, energy interchange, and thermal characteristics. [ABSTRACT FROM AUTHOR]

We expect the final theory of gravity to have more symmetries than we suspect and our research points in this direction. To start with, standard general coordinate invariance can be extended to complex holomorphic general coordinate transformations. This is possible by introducing a non-Riemannian measure of integration (NRMI) and where we avoid the nonholomorphic standard −g measure of integration. Second, locally signed coordinate transformations where the Jacobian changes sign locally but the Jacobian approaches one asymptotically should be symmetries of Nature. This is unlike globally signed transformations that produce a change of boundary conditions, like in the cases of global parity and global time reversal, which are not symmetries of Nature. The holomorphic extension can regularize the regions of spacetime where the Jacobian changes sign. Consequences for Quantum Gravity are discussed. [ABSTRACT FROM AUTHOR]

We introduce a new class of two-dimensional gravity models using ideas motivated by the Teleparallel Equivalent of General Relativity. This leads to a rather natural formulation of a theory that has close links with Jackiw–Teitelboim gravity. After introducing the theory and discussing its vacuum solutions, we present the Hamiltonian analysis. This implies the presence of a single dynamical degree of freedom, which is in sharp contrast to General Relativity, where there are no degrees of freedom in two spacetime dimensions. Our approach can be extended to various other lower-dimensional gravity theories and thus could be of wider interest. [ABSTRACT FROM AUTHOR]

In this work, we have proposed a simple parametrization for the pressure component p (z) of the dark energy model and have studied the cosmological implications of this model in the framework of f (Q) modified gravity theory, aka, the symmetric teleparallel gravity theory, where Q is known as the nonmetricity scalar. By considering a particular parametric form of p (z) , we obtained the Hubble solution for the f (Q) modified gravity model. In order to see whether this model is consistent with or challenges the Λ CDM limits, we tried to put constraints on the model parameters using the recent observational datasets like Hubble data, Cosmic Chronometer data, Type Ia Supernovae (SNIa) data, baryonic acoustic oscillations (BAO) data. We have employed the χ 2 minimization technique and have carried out the Markov Chain Monte Carlo (MCMC) analysis using emcee package. We have found that the deceleration parameter shows a smooth transition from positive to negative value in recent past which is essential for the structure formation of the Universe. It has been found that the parametric form of the dark energy pressure parameter is consistent with current cosmological scenario. [ABSTRACT FROM AUTHOR]

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]

This research work provides an exhaustive investigation of the viability of different coupled wormhole (WH) geometries with the relativistic matter configurations in the f (R , G , T) extended gravity framework. We consider a specific model in the context of f (R , G , T) -gravity for this purpose. Also, we assume a static spherically symmetric spacetime geometry and a unique distribution of matter with a set of shape functions (β (r)) for analyzing different energy conditions. In addition to this, we examined WH-models in the equilibrium scenario by employing anisotropic fluid. The corresponding results are obtained using numerical methods and then presented using different plots. In this case, f (R , G , T) gravity generates additional curvature quantities, which can be thought of as gravitational objects that maintain irregular WH-situations. Based on our findings, we conclude that in the absence of exotic matter, WH can exist in some specific regions of the parametric space using modified gravity model as f (R , G , T) = R + α R 2 + β G n + γ G ln (G) + λ T. [ABSTRACT FROM AUTHOR]

This study aims to investigate the impact of dark energy in cosmological scenarios by exploiting f (R ̄ , T ̄) gravity within the framework of a nonstandard theory, called K-essence theory, where R ̄ represents the Ricci scalar and T ̄ denotes the trace of the energy–momentum tensor associated with the K-essence geometry. The Dirac–Born–Infeld (DBI) nonstandard Lagrangian has been employed to generate the emergent gravity metric ( G ̄ μ ν) associated with the K-essence. This metric is distinct from the usual gravitational metric (g μ ν). It has been shown that under a flat Friedmann–Lemaître–Robertson–Walker (FLRW) background gravitational metric, the modified field equations and the Friedmann equations of the f (R ̄ , T ̄) gravity are distinct from the usual ones. In order to get the equation of state (EoS) parameter ω , we have solved the Friedmann equations by taking into account the function f (R ̄ , T ̄) ≡ f (R ̄) + λ T ̄ , where λ represents a parameter within the model. We have found a relationship between ω and time for different kinds of f (R ̄) by treating the kinetic energy of the K-essence scalar field ( ϕ ̇ 2 ) as the dark energy density which fluctuates with time. Surprisingly, this result meets the condition of the restriction on ϕ ̇ 2 . By presenting graphical representations of the EoS parameter with time, we show that our model is consistent with the data of SNIa + BAO + H (z) within a certain temporal interval. [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]

In this work, we have investigated the cosmology of holographic origin within modified f (Q , T) gravity framework by employing different forms of the scale factor viz. power law, intermediate, emergent, and logamediate forms. The reconstruction of f (Q , T) gravity is done by assuming the generalized Holographic Dark Energy as the background fluid where the IR cut-off is considered (α H 2 + β Ḣ). We consider two cosmological models corresponding to the intermediate and emergent scale factors. By choosing a linear combination of Q and T in each case, we analyze the equation of state (EoS) parameter. Considering the intermediate and logamediate scale factors, the reconstruction of f (Q , T) gravity is achieved using the holographic fluid as background fluid without any functional form assumption for f. In some cases, it is observed that the EoS parameter could cross phantom boundary in later stages and thus indicate the possibility of avoidance of big rip. [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 unimodular-like theories, the constants of nature are demoted from pre-given parameters to phase space variables. Their canonical duals provide physical time variables. We investigate how this interacts with an alternative approach to varying constants, where they are replaced by dynamical scalar fields. Specifically, we investigate the Brans–Dicke theory of gravity and its interaction with clocks dual to the cosmological constant, the Planck mass, etc. We crucially distinguish between the different role of Newton's G in this process, leading to the possibility of local Lorentz invariance violation. A large number of possible theories emerge, for example where the Brans–Dicke coupling, ω , depends on unimodular-like times (in a generalization of scalar-tensor theories), or even become the dual variable to unimodular-like clocks ticking variations in other demoted constants, such as the cosmological constant. We scan the space of possible theories and select those most interesting regarding the joint variations of the Brans–Dicke ω and other parameters, (such as the cosmological constant); and also regarding their energy conservation violation properties. This ground work is meant to provide the formalism for further developments, namely regarding cosmology, black holes and the cosmological constant problem. [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]

We examine the cosmic scenario of interacting Kaniadakis holographic dark energy in dynamical Chern–Simons modified gravity and the fractal universe. For this purpose, the Hubble, deceleration, coincidence, equation-of-state and jerk parameters have been evaluated in view of the redshift parameter. It is observed that deceleration parameter (q) evaluates the accelerated expansion of the universe in both gravities. The coincidence parameter (u) exhibits the transition of behavior from dark matter to dark energy era of the universe. The behavior of the equation-of-state parameter (ω θ) describes the quintessence and vacuum regions of the universe in both gravities for maximum choices of the interacting parameters. The jerk parameter shows the correspondence of the given model with Λ CDM model and other standard models in both gravities. All the parameters exhibit consistent behavior with the Planck 2018 data. We also analyze the dynamical stability in both frameworks by formulating dynamical models in the form of a system of differential equations and evaluating their corresponding critical points. Critical points of both models are stable and phase plots indicate attractor behavior that implies stability of the models. Further, stability conditions of both models signify the accelerating expansion of the universe. In addition, we discuss the thermodynamics of this model with the generalized second law and find its validity in both frameworks. [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]

In general relativity, the allowed set of diffeomorphisms or gauge transformations at asymptotic infinity forms the BMS group, an infinite-dimensional extension of the Poincaré group. We focus on the structure of the BMS group in two distinct forms of Hamiltonian dynamics — the instant and front forms. Both similarities and differences in these two forms are examined while emphasizing the role of noncovariant approaches to symmetries in gravity. [ABSTRACT FROM AUTHOR]

Although it's widely believed that gravity should have a quantum nature like every other force, the conceptual obstacles to constructing a quantum theory of gravity compel us to explore other perspectives. Gravity is not like any other force. It alone defines a universal spacetime geometry upon which quantum fields evolve. We feel gravity because matter causes spacetime to bend. Time flows at unequal rates at different locations. The rate at which time flows, and the causal structure it provides, may be required to have a classical description in order for quantum theory to be well-formulated. I discuss arguments for this proposition, but ultimately conclude that we must turn to experiment to guide us. [ABSTRACT FROM AUTHOR]

Galaxy Clusters are huge physical systems having a generic size of tens of millions of light years. Thus any modification at the center of the cluster will affect the outskirts only tens of millions of years afterwards. Those retardation considerations seem to be neglected in present day analysis used to estimate the total mass of the galaxy cluster, including those estimates which are based on the virial theorem. The significant differences between the velocities predicted by Newtonian action at a distance theory and observations are usually dealt with by either assuming an unobservable type of matter or by modifying gravity. Here we demonstrate that considering general relativistic effects one can explain the apparent excess matter appearing in galaxy clusters virial estimates. [ABSTRACT FROM AUTHOR]

Neutron stars are one of the most mysterious wonders in the universe. Their extreme densities hint at new and exotic physics at work within. Gravitational waves could be the key to unlocking their secrets. In particular, a first detection of gravitational waves from rapidly-spinning, deformed neutron stars could yield new insights into the physics of matter at extreme densities and under strong gravity. Once a first detection is made, a critical challenge will be to robustly extract physically interesting information from the detected signals. In this essay, we describe initial research towards answering this challenge, and thereby unleashing the full power of gravitational waves as an engine for the discovery of new physics. [ABSTRACT FROM AUTHOR]

Nonlocal gravity (NLG), a classical extension of Einstein's theory of gravitation, has been studied mainly in linearized form. In particular, nonlinearities have thus far prevented the treatment of cosmological models in NLG. In this essay, we discuss the local limit of NLG and apply this limit to the expanding homogenous and isotropic universe. The theory only allows spatially flat cosmological models; furthermore, de Sitter spacetime is forbidden. The components of the model will have different dynamics with respect to cosmic time as compared to the standard Λ CDM model; specifically, instead of the cosmological constant, the modified flat model of cosmology involves a dynamic dark energy component in order to account for the accelerated phase of the expansion of the universe. [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]

The (1 + 1)-dimensional dilatonic background geometry of [C. Callan, S. Giddings, J. Harvey and A. Strominger, Phys. Rev. D 45 (1992) R1005] admits an infinite class of colliding wave solutions. We give examples from real and ghost fields with bounded/ unbounded profiles. In particular, colliding ghost fields in the "lineland" geometry create particle-like wormholes. Such wormholes, however, collapse to a black hole whenever the supporting ghosts turn off. Lastly, it is shown that the vacuum collision of null fields yields a ghost wormhole which is unstable. [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 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]

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]

In this study, we consider a cosmological model for the Maxwell gravity which is constructed by gauging the semi-simple extended Poincaré algebra. Inspired by the Einstein–Yang–Mills theory, we describe the Maxwell gauge field in terms of two additional time-dependent scalar fields. Within the context of a homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker universe, we derive the Friedmann equations together with new contributions. Additionally, we examine the modified Friedmann equation to demonstrate how diverse cosmological scenarios can be achieved within this framework. Moreover, we investigate the gauge theory of gravity based on the Maxwell algebra and show that this model leads to the (anti)-de Sitter universe as well as a non-accelerated universe model. [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]

We investigated the stability condition of f (Q) gravity theory with interacting and noninteracting models by using dynamical system. We assume the f (Q) function as f (Q) = Q + M 2 2 , where M is the free parameter. We evaluated the critical points for this model and examined the stability behavior. We found two stable critical points for interacting model. The phase plots for this system are examined and the physical interpretation is discussed. We illustrate all the cosmological parameters such as Ω m , Ω ϕ , q and ω Tot at each fixed point and compare the parameters with observational values. Further, we assume hybrid scale factor and the equation of redshift and time is t (z) = μ ν W [ ν (1 + z) − m μ μ ]. We transform all the parameters in terms of redshift by using this equation and examine the behavior of these parameters. Our model represents the accelerated expansion of the universe. The energy conditions are examined in terms of redshift and strong energy conditions are not satisfied for the model. We also find the statefinder parameters { r , s } in terms of z and discuss the nature of r–s and r–q plane. For both pairs { r , s } and { r , q } our model represents the Λ CDM model. Hence, we determine that our f (Q) model is stable and it satisfies all the observational values. [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]

In this work, spherically symmetric thin-shell wormholes with a conformally invariant Maxwell field for N -dimensional F (R) gravity and constant scalar curvature R are built. Two cases are considered: wormholes symmetric across the throat and asymmetric ones having different values of the scalar curvature across the throat. Their stability under radial perturbations is analyzed, finding that unstable and stable solutions are possible for suitable values of the parameters, always made of exotic matter. The stable solutions are found for a short range, slightly over a large critical value of charge. [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]

We investigated the viability of a logarithmic f (R) gravity in calculating the reheating temperature and describing dark matter. We discovered that the logarithmic model possessed a chameleon problem, and we addressed this by modifying the model to incorporate the R 2 term from the Starobinsky model. We examined the viability of the corrected model and revealed that it provided the same reheating temperature as the original logarithmic model, approximately 1 0 6. 5 GeV. However, the constraints on the parameter α are significantly different in the R 2 -Logarithmic f (R) gravity, in which we obtained α ≪ 3 instead of 5 0 0 < α < 4 0 0 0 in the original model. Furthermore, the R 2 -Logarithmic f (R) model avoids the chameleon problem, and the scalaron derived from the model can be a potential dark matter candidate with y ∈ (0 , α − 2). [ABSTRACT FROM AUTHOR]

We obtain new regular black hole solutions for an action in 2+1 dimensions with a bilocal Ricci scalar and a negative cosmological constant. Besides their connection to the cosmological constant, these solutions depend on a fundamental length due to their nonlocal nature. The effective profile densities that result from the nonlocal geometries have quasi-localized mass/energy since they are finite at the origin and their integration in all space is convergent. The black holes obtained are free of singularities and present one, two, or none horizons depending on the values of the involved parameters. The new solutions can have either an AdS, dS, or even a flat core. In the case of a de-Sitter core, it could represent a repulsive force coming from quantum effects. Although the resulting (effective) cosmological constant is positive near the origin, the classical (naked) counterpart is still negative thus precluding a cosmological horizon. We investigate the energy conditions of the effective source and determine the region where exotic energy should be found. Thermodynamic quantities are also computed. On the one hand, the Gibbs's potential shows that both solutions are globally unstable, as in the BTZ case. On the other hand, we show that for small values of the horizon radius the Hawking temperature is negatively divergent but a finite size remnant can be defined where T H crosses zero. At this point, the heat capacity sign changes from negative to positive, indicating that the black holes are locally stable while irradiating. Thus, such a quantity, along with T H , presents crucial differences with the BTZ black hole for small horizon radii where quantum effects become relevant. Finally, we analyze the bilocal black hole geodesics and find stable circular orbits for massless and massive particles, another feature absent in the BTZ case. [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]

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]

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]

Modified General Relativity (MGR) is the natural extension of General Relativity (GR). MGR explicitly uses the smooth regular line element vector field (X , − X) , which exists in all Lorentzian spacetimes, to construct a connection-independent symmetric tensor that represents the energy–momentum of the gravitational field. It solves the problem of the nonlocalization of gravitational energy–momentum in GR, preserves the ontology of the Einstein equation, and maintains the equivalence principle. The line element field provides MGR with the extra freedom required to describe dark energy and dark matter. An extended Schwarzschild solution for the matter-free Einstein equation of MGR is developed, from which the Tully–Fisher relation is derived, and the gravitational energy density is calculated. The mass of the invisible matter halo of galaxy NGC 3198 calculated with MGR is identical to the result obtained from GR using a dark matter profile. Although dark matter in MGR is described geometrically, it has an equivalent representation as a particle with the property of a vector boson or a pair of fermions; the geometry of spacetime and the quantum nature of matter are linked together by the unit line element covectors that belong to both the Lorentzian metric and the spin-1 Klein–Gordon wave equation. The three classic tests of GR provide a comparison of the theories in the solar system and several parts of the cosmos. MGR provides the flexibility to describe inflation after the Big Bang and galactic anisotropies. [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 work, we consider static quark star (QS) within the framework of Rastall gravity. Rastall gravity purports to be the nonconservative theory of gravity and an unusual nonminimal coupling between matter and geometry. In our study, we consider a Quantum chromodynamics (QCD)-motivated Equation of State (EoS) to determine the properties of QSs in Rastall gravity. Depending on the values of parameters, we seek to determine the mass–radius relations for QSs in Rastall gravity, identifying the deviation from standard general relativity (GR) counterparts. Interestingly, we find the value of the maximum gravitational mass to be more than 2 M ⊙ for the given equation of state (EoS). We present the essential features regarding the stability of QSs. [ABSTRACT FROM AUTHOR]

General Relativity (GR) has been tested extensively in the solar system and is being tested in the new environment of the Galactic Center (GC) black hole where the dimensionless gravitational potential (G M / c 2 r) is 100 times stronger than the one encountered in solar system. Therefore, the neighborhood of the GC black hole is a naive opportunity to test modified theories of gravity. In this work, effect of f (R) gravity near the black hole is studied. The difference of pericenter shift between GR and f (R) gravity is studied for compact orbits having semi-major axis equal to and below a = 1 0 0 0 au (S0-2 like orbits). In a model-dependent approach, we choose f (R) ∝ R 2 (power law gravity) model which is cosmologically motivated and study the deviation in orbital pericenter shift for both zero spin and non-zero spin of the black hole. It is found that effect of f (R) gravity becomes prominent for compact orbits. In model-independent approach to f (R) gravity with the generic scalaron fields (ψ = f ′ (R)), we extract the parameters of f (R) gravity from the current bounds on Parametrized Post-Newtonian (PPN) parameters (γ , β) near the GC black hole. The screening of f (R) gravity is also investigated for these bounds on PPN parameters. It has been found that sufficiently massive scalarons (1 0 − 1 6 eV) are completely screened but light and intermediate mass scalarons (1 0 − 2 2 eV and 1 0 − 1 9 eV) are unscreened towards S0-2 like orbits as well as in the orbit of the newly discovered short period star S4716 (a = 4 0 7 au). The possibility of detection of the f (R) gravity effects due to these unscreened scalarons is forecasted with existing and upcoming astrometric capabilities of Extremely Large Telescopes (ELTs). [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 the teleparallel f (T) gravity scenario, we consider a five-dimensional thick brane. This scenario is interesting because this theory can provide explanations for inflation, radiation and dark matter under certain conditions. It is convenient to assume, for our study, a polynomial profile of the function f (T). Indeed, some polynomial profiles can produce internal structures for which a brane splitting occurs. For functions f (T) with this capability, geometrically contracted matter field configurations are obtained. These contractions of the matter field for the profiles of f (T) reproduce compact-like settings. To complement the study, we analyze the stability of the brane using the concept of Configurational Entropy (CE). The CE arguments are interesting because they tell us the most stable and likely configurations from the brane in this gravitational background. Therefore, we can indicate the best profile of the function f (T). [ABSTRACT FROM AUTHOR]

Recent research works have shown the existence of super-massive neutron stars (NSs) with mass about 2. 2 M ⊙ or even more. The query about those super-massive NSs inspires the researchers to analyze their features and structures immensely. Here, we have inspected the behavior and properties of some of those super-massive NSs in f (T) modified gravity with T = T + α T 2 , where T is the torsional scalar and α is a regulatory parameter. In this framework of teleparallel formalism of modified gravity, we obtain the equations of motion by considering quintessence field, modified Chaplygin gas (MCG) and electromagnetic field. For our model, we use matching conditions under spherical symmetry, in order to find out the numerical values of different unknown constants of our model. This helps us to acquire various physical quantities thoroughly and to understand about the nature of those super-massive NSs deeply and quite clearly. Moreover, from our work, we can also explain the role of quintessence field and MCG in case of massive compact stars. The mass–radius relationship curve of this model can effectively describe the mass of the heaviest NS (about 2. 6 M ⊙ ) ever detected via gravitational wave detection. Again, we overall investigate the anisotropic behavior, density profile, pressure profile, core repulsive force, stability, equilibrium and energy conditions of those massive compact objects. We further analyze different important parameters like anisotropic stress, surface redshift, adiabatic behavior, compactness factor, sound speed, etc. in case of super-massive NSs for better realization and future study. [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]

Through this study, we intend to present a comprehensive summary of the most recent research on complex systems or complexity factor ( f) in the background of modified gravity theories (MGT). In this way, we examined f for the novel MGT named f (, T 2) gravity under the anisotropic static fluid configuration. f is defined as one of the structure scalars (Y T F) which is trace-free component obtained from the orthogonal splitting of the Riemann tensor. Two mass approaches are used which have immense influence on f . Furthermore, vanishing complexity condition is used with an eye towards finding the solution of the nonlinear system. Two extra models are explored which ensure one more condition for the purpose of comprehensive results. [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]

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]

By using the Kaniadakis statistics, we discuss the modifications of Newtonian gravity and radial velocity profile in the light of Verlinde's formalism for gravitational entropy. After considering the implications of κ -statistics on the gravitational potential, it is shown that an accelerated universe may be obtained by considering the Friedmann first equation in this nonextensive statistics. [ABSTRACT FROM AUTHOR]