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We study inflation in scalar–tensor perturbative quantum gravity driven by a one-loop effective potential. We consider effective potentials generated by three models. The first model describes a single scalar field with a nonvanishing mass. The second model describes a massless scalar field with nonminimal coupling to the Einstein tensor. The third model generalizes the Coleman–Weinberg model for the gravitational case. The first model can be consistent with the observational data for N∼70 e-foldings. The second model can be consistent with the observational data for N∼40 e-foldings. We did not find parameters that make the generalized Coleman–Weinberg model consistent with the observational data. We discuss the implications of these results and ways to improve them with other terms of effective action. [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]

Inspired by Einstein's Strong Principle of Equivalence, we consider the effects of quantum mechanics to the gravity-like phenomena experienced by an observer in a uniformly accelerating motion in flat spacetime. Among other things, our model of quantum gravity, derived from the first principles, predicts the Unruh effect, and a discrete area spectrum for spacelike two-surfaces. [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]

Inspired by an exponential f (R) gravity model studied in the literature, in this work we introduce a new and viable f (Q) gravity model, which can be represented as a perturbation of Λ CDM. Typically, within the realm of f (Q) gravity, the customary approach to investigate cosmological evolution involves employing a parametrization of the Hubble expansion rate in terms of the redshift, H (z) , among other strategies. In this work, we have implemented a different strategy, deriving an analytical approximation for H (z) , from which we deduce approximated analytical expressions for the parameters w DE , w eff and Ω DE , as well as the deceleration parameter q. In order to verify the viability of this approximate analytical solution, we examined the behavior of these parameters in the late-time regime, in terms of the free parameter of the model, b. We find that for b > 0 , w DE shows a quintessence-like behavior, while for b < 0 , it shows a phantom-like behavior. However, regardless of the sign of b, w eff exhibits a quintessence-like behavior. Furthermore, it has been deduced that as the magnitude of the parameter b increases, the present model deviates progressively from Λ CDM. We have also performed a Markov Chain Monte Carlo statistical analysis to test the model predictions with the Hubble parameter, the Pantheon supernova (SN) observational data and the combination of those samples, obtaining constraints on the parameters of the model and the current values of the Hubble parameter and the matter density. Our findings indicate that this f (Q) gravity model is indeed a viable candidate for describing the late-time evolution of the Universe at the background level. [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]

Paradoxes in gravitational physics, such as grandfather paradoxes with closed timelike curves or the AMPS paradox, are often constructed in a weak gravity regime but upon further examination engender strong gravitational responses such as unstable Cauchy horizons or firewalls. In contrast, some proposed paradoxes in nonrelativistic quantum mechanics ignore gravity completely. Such nonrelativistic proposed paradoxes are often not gauge invariant — they use local operators which should be forbidden by diffeomorphism-invariant quantum gravity. Ignoring this complication is reasonable if there is a consistent weak gravity regime where the paradox can be formulated with gauge invariant observables up to some order in Newton's constant. We show that this approach remains inconsistent due to a lack of analyticity of the necessary solutions. As a consequence, there is no consistent weak gravity, diffeomorphism invariant embedding of such paradoxes—just as in other gravitational paradoxes they generate a strong gravitational response and are in principle sensitive to quantum gravity. [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]

In this paper, we discuss the corrections to the photon orbits of a nonrotating black hole due to semiclassical fluctuations of the metric. It is found that the photon orbit impact parameter differences with the critical impact parameter become of the order of the semiclassical fluctuations. We calculate the effect of the semiclassical fluctuations on the photon orbits and show that instead of circulating the black hole infinite number of times at the critical orbit, the photons bounce off the semiclassical geometry. [ABSTRACT FROM AUTHOR]

We have conducted research on the presence of wormholes characterized by a logarithmic shape function within the framework of exponential f (R , T) gravity. This gravity model is defined as f (R , T) = R + 2 e β t , with β representing an arbitrary constant. We find that the logarithmic shape function serves as expected, providing all the necessary conditions for traversable and asymptotically flat wormholes. We have examined the solutions with three distinct sets of physical constraints placed on p r , p t and ρ. viz. Model-I: p r = k p t with k being variable dependent on r and p t = ω ρ with ω being EoS parameter, Model-II: p r = ω 1 ρ and p t = ω 2 ρ with ω 1 and ω 2 being EoS parameters, Model-III: Trace is zero i.e. T = 0 or ρ = p r + 2 p t i.e. f (R) gravity model. From Model-I, we have obtained a new parametrization for EoS parameter ω (r) and from Model-III, we have obtained a new form of shape function b (r) in f (R) gravity theory. [ABSTRACT FROM AUTHOR]

Vacuum spherically symmetric loop quantum gravity in the midi-superspace approximation using inhomogeneous horizon-penetrating slices has been studied for a decade, and it has been noted that the singularity is eliminated. It is replaced by a region of high curvature and potentially large quantum fluctuations. It was recently pointed out that the effective semiclassical metric implies the existence of a shell of matter which violates energy conditions in regions where the curvature is largest. Here, we propose an alternative way of treating the problem that is free from the shells. The ambiguity in the treatment is related with the existence of new observables in the quantum theory that characterize the area excitations, and how the counterpart of diffeomorphisms in the discrete quantum theory is mapped to the continuum semiclassical picture. The resulting spacetime in the high curvature region inside the horizon is approximated by a metric of the type of the Simpson–Visser wormhole and it connects the black hole interior to a white hole in a smooth manner. [ABSTRACT FROM AUTHOR]

We study the periapsis shift of a quasi-circular orbit in general static spherically symmetric spacetimes. We derive two formulae in full order with respect to the gravitational field, one in terms of the gravitational mass m and the Einstein tensor and the other in terms of the orbital angular velocity and the Einstein tensor. These formulae reproduce the well-known ones for the forward shift in the Schwarzschild spacetime. In a general case, the shift deviates from that in the vacuum spacetime due to a particular combination of the components of the Einstein tensor at the radius r of the orbit. The formulae give a backward shift due to the extended-mass effect in Newtonian gravity. In general relativity, in the weak-field and diffuse regime, the active gravitational mass density, ρ A = (ϵ + p r + 2 p t) / c 2 , plays an important role, where ϵ , p r and p t are the energy density, the radial stress and the tangential stress of the matter field, respectively. We show that the shift is backward if ρ A is beyond a critical value ρ c ≃ 2. 8 × 1 0 − 1 5 g/cm3 (m / M ⊙) 2 (r / a.u.) − 4 , while a forward shift greater than that in the vacuum spacetime instead implies ρ A < 0 , i.e. the violation of the strong energy condition, and thereby provides evidence for dark energy. We obtain new observational constraints on ρ A in the Solar System and the Galactic Centre. [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]

The quantum superposition principle states that quantum-mechanical systems such as atoms can be placed in a superposition of mass-energy eigenstates. Inspired by this idea and the seminal conjecture of Bekenstein, who proposed that black holes in quantum gravity must possess a discrete mass eigenspectrum, here we analyze the effects produced by a black hole in a superposition of masses. Analogous to using the electromagnetic field to probe atoms, we consider a quantum scalar field on the spacetime background sourced by the black hole mass superposition. From the resulting spectra, as measured by a hypothetical two-level system interacting with the field, we infer signatures of discretization of the black hole mass in support of Bekenstein's conjecture. [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]

It is now widely believed that if the gravitational field is (perturbatively) quantum, it would entangle two massive objects (in spatial superpositions) which were otherwise unentangled to begin with. Recently, actual table-top experiments have been proposed to test this idea in what would be the first detection of perturbative quantum gravity. In this essay, we devise a thought experiment to prove that such gravity-induced entanglement depends on the spacetime curvature and can, in principle, act as an alternate signature of the expanding background. This will open up new and complementary directions to search for such entanglement in curved spacetime and reveal fresh perspectives on it. [ABSTRACT FROM AUTHOR]

The thermodynamics of local causal horizons has been shown to imply gravitational dynamics. In this essay, we discuss the principles underlying this observation, and its significance in our understanding of (quantum) gravity. We also show why the local thermodynamic methods cannot by themselves recover general relativity. Instead, they lead to the so-called Weyl transverse gravity. Because of this, local thermodynamic approaches avoid huge vacuum energy contributions to the cosmological constant. They even suggest a possible source for its small observed value. We also outline a way in which thermodynamics allows us to study low energy quantum gravitational effects. We arrive at quantum corrections to the gravitational equations which are suppressed by the Planck length squared. [ABSTRACT FROM AUTHOR]

Black holes play a pivotal role in the foundations of physics, but there is an alarming discrepancy between what is considered to be a black hole in observational astronomy and theoretical studies. Despite claims to the contrary, we argue that identifying the observed astrophysical black hole candidates as genuine black holes is not justified based on the currently available observational data, and elaborate on the necessary evidence required to support such a remarkable claim. In addition, we investigate whether the predictions of semiclassical gravity are equally compatible with competing theoretical models, and find that semiclassical arguments favor horizonless configurations. [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]

We show that the repulsive effects associated to the zero-point energies of quantum fields are capable of supporting ultracompact stars that overcome the compactness limits present in general relativity for any object in hydrostatic equilibrium. These objects are exact self-consistent solutions in semiclassical gravity that incorporate the backreaction of the renormalized stress-energy tensor (RSET) of quantum fields in vacuum. We arrive at stars of striking qualitative agreement through two independent modelings of the RSET, evidencing the generality and robustness of this result. The main physical properties of these novel black hole mimickers are reviewed. [ABSTRACT FROM AUTHOR]

In this essay, it is proved that there are black holes that are dangerously cold. In particular, by analyzing the emission spectra of highly charged black holes we reveal the fact that near-extremal black holes whose Bekenstein–Hawking temperatures lie in the regime T BH ≲ m e 6 / e 3 may turn into horizonless naked singularities, thus violating the cosmic censorship principle, if they emit a photon with the characteristic thermal energy ω = O (T BH ) (here { m e , e } are, respectively, the proper mass and the electric charge of the electron, the lightest charged particle). We therefore raise here the conjecture that, in the yet unknown quantum theory of gravity, the temperatures of well-behaved black-hole spacetimes are fundamentally bounded from below by the relation T BH ≳ m e 6 / e 3 . [ABSTRACT FROM AUTHOR]

Hawking showed that a black hole formed by collapse will emit radiation and eventually disappear. We address the challenge to define an objective notion of physical entropy which increases throughout this process in a way consistent with unitarity. We have suggested that (instead of coarse-grained entropy) physical entropy is matter–gravity entanglement entropy and that this may offer an explanation of entropy increase both for the black hole collapse and evaporation system and also for other closed unitarily evolving systems. For this to work, the matter–gravity entanglement entropy of the late-time state of black hole evaporation would have to be larger than the entropy of the freshly formed black hole. We argue that this may possibly be the case due to (usually neglected) photon–graviton interactions. [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]

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]

The experimental successes of quantum-field theory do not justify using it to describe even a finite fraction of the entanglement entropy of a causal diamond with its exterior, in the limit of large diamonds. Susskind and Uglum and Jacobson conjectured that this divergent entropy could be thought of as a renormalization of Newton's constant in the Bekenstein–Hawking formula, if we applied that formula to arbitrary causal diamonds. Jacobson showed that this leads to a derivation of the null projection of Einstein's equations as the hydrodynamic equations of the area law for arbitrary diamonds, a derivation which has the added virtue of demonstrating that the cosmological constant is not an energy density. Using a gauge choice adapted to causal diamond boundaries, we revisit arguments of Carlip and Solodukhin that the proper theory of near horizon states is a (cut-off) (1 + 1)-dimensional conformal field theory, with central charge proportional to the transverse area. This leads to a universal formula for fluctuations of the modular Hamiltonian of a diamond, which we argue is compatible with the explanation of the temperature of de Sitter space in terms of an identification between localized energy and the number of constrained q-bits of the holographic degrees of freedom. [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]

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]

By considering Kehagias–Sfetsos black hole in the framework of the Hořava–Lifshitz gravity, we study the optical appearance of such black holes surrounded by spherical accretion flow. For the static/ infalling spherical accretion flow, we compute the observed specific intensity as a function of impact parameter. We also investigate the effect of the Hořava parameter and accreting matter on the luminosity of shadows and photon rings. It is found that an increase in the Hořava parameter decreases the shadow size, while the shadows and photon rings luminosities increase. Moreover, we constrain the Hořava parameter from the observational data reported by the Event Horizon Telescope for M87* and Sgr A*. [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]

We investigate the behavior of expansion free collapsing fluids, as studied by L. Herrera, A. Di Prisco and J. Ospino [Symmetry 15 (2023) 754], in the framework of f (R) gravity, which represents a modification of Einstein's general relativity by establishing a function of the Ricci scalar R in the gravitational action. We explore dynamical equations from Bianchi identities that demonstrate the motion and evolution of physical systems under the influence of gravitational fields. We match the inner and outer geometries of spacetime on the hypersurface to develop junction conditions by using the Misner–Sharp formalism. This allows us to identify the connection between mass functions for the inner and outer space as well as the relationship for heat flux q and radial pressure ℙ r . We also investigate analytical solutions of dissipative fluid distribution that fulfill the vanishing expansion condition together with the vanishing complexity factor constraint. For this, we introduce new constraints that permit the integration of the complex system in f (R) gravity. Next, we extract a set of differential equations that explain the dynamical structure of the dissipative spheres both in geodesic and non-geodesic fluids. Furthermore, we explore the physical characteristics of the obtained solutions, such as heat flux, energy density, shear stress, fluid's temperature along with tangential and radial pressure, to assess their viability in describing real astrophysical systems. [ABSTRACT FROM AUTHOR]

In this study, we propose a reparametrization of a specific viable f (R) gravity model to represent it as a perturbation of the Λ CDM model. The f (R) gravity model under consideration includes two parameters, b and n , which control how close the proposed model can be to Λ CDM, allowing for arbitrary proximity. Furthermore, it is shown that the Hu–Sawicki (HS) model is a limiting case of this reparametrized model. Following the existing literature, we also derive an analytical approximation for the expansion rate H (z) , which shows an excellent agreement between this analytical approximation and the numerical solution over a wide range of redshifts for realistic values of the deviation parameter b. By appropriately selecting values for the model parameters, we plot the cosmological parameters w DE , w eff , Ω DE and H (z) , as well as the statefinder quantities q , j , s and Om (z). We find that their present values (at z = 0) are consistent with the observations from Planck 2018 and the values predicted by the Λ CDM model. It is important to note that the examined cosmological and statefinder parameters do not exhibit significant oscillations of effective dark energy, which could lead to singular and unphysical solutions at high redshifts. This anomalous behavior has been avoided here by utilizing the approximate analytical solution for H (z). Additionally, we conduct a detailed analysis of the evolution of matter density perturbations within the introduced f (R) gravity model. The results demonstrate that this viable f (R) gravity model is practically indistinguishable from the Λ CDM model at the background level. [ABSTRACT FROM AUTHOR]

This work treats cosmological perturbation in a mixture of standard matter, Chaplygin gas as well as Gauss–Bonnet fluids using a 1+3 covariant approach in the context of modified f (G) gravity. We define the gradient variables to obtain linear perturbation equations. After scalar and redshift transformations, we consider both an original Chaplygin and generalized Chaplygin gas models under Gauss–Bonnet gravity. For pedagogical purposes, the consideration of polynomial f (G) gravity model was used to solve the perturbation equations for short- and long-wavelength modes and investigate the late-time evolution. The numerical solutions were obtained. The results show that the energy overdensity perturbations decay with an increase in redshift. The treatment recovers GR results under limiting cases. [ABSTRACT FROM AUTHOR]

In this work, we investigate two mechanisms for locating fermions on the brane in a modified teleparallel gravity f (T ,) , where T is the torsion scalar and is the trace of the energy–momentum tensor. The first mechanism is a Yukawa-type minimal coupling between the fermion and the background scalar field. The second mechanism is a nonminimal coupling between the fermion and the torsion. The geometric coupling presents a greater sensitivity to the influence of torsion and to the trace of the energy–momentum tensor. [ABSTRACT FROM AUTHOR]

In this paper, we characterize weakly Ricci-symmetric (shortly, (WRS) 4 ) spacetimes and their solutions in f (ℛ) -gravity. It is demonstrated that a (WRS) 4 spacetime represents a stiff matter fluid. In addition, we obtain that a conformally flat (WRS) 4 spacetime is a space of quasi-constant sectional curvature. Moreover, we establish that a Ricci symmetric (WRS) 4 spacetime represents a static spacetime. Finally, we investigate the effect of (WRS) 4 spacetime solutions in f (ℛ) -gravity. [ABSTRACT FROM AUTHOR]

We discuss a cosmological scenario with a stochastic background of gravitational waves sourced by the tensor perturbation due to a hybrid inflationary model with cubic potential. The tensor-to-scalar ratio for the present hybrid inflationary model is obtained as r ≈ 0. 0 0 0 6. Gravitational wave spectrum of this stochastic background, for large-scale CMB modes, 1 0 − 4 M pc − 1 to 1 M pc − 1 is studied. The present-day energy spectrum of gravitational waves Ω 0 g w (f) is sensitively related to the tensor power spectrum and r which is, in turn, dependent on the unknown physics of the early cosmos. This uncertainty is characterized by two parameters: n ̂ t (f) logarithmic average over the primordial tensor spectral index and ŵ (f) logarithmic average over the effective equation-of-state parameter. Thus, exact constraints in the n ̂ t (f) - ŵ (f) plane can be obtained by comparing theoretical constraints of our model on r and Ω 0 g w (f). We obtain a limit on ŵ (1 0 − 1 5 Hz) < 0. 3 3 around the modes probed by CMB scales. [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]

At relativistic energies and finite magnetic fields, the noncommutative relation of distance and momentum, the Heisenberg uncertainty principle, the fundamental theory of quantum mechanics, is conjectured to get modifications. Results from various rigorous approaches to quantum gravity, such as string theory, loop quantum gravity and doubly special relativity support the generalization of the noncommutative relation of the distance and momentum operators and the emergence of a minimal measurable length. With the relativistic four-dimensional generalized uncertainty principle (RGUP) in curved spacetime and Born reciprocity principle, the distance–momentum duality symmetry, we suggest to generalize Riemann to Finsler geometry. The Finsler structure allows the direct implementation of RGUP with its quantum-mechanical nature on a free particle with mass m, so that the Finsler structure F ( x ̂ 0 , ẋ ̂ 0) can be expressed as F ( x ̂ 0 , p ̂ 0) , from which the quantized fundamental tensor can be deduced. We present a systematic analytic and numerical evaluation of the additional geometric structures and connections which exclusively emerged from the proposed quantization approach on three-sphere. When limiting the discussion on the Einstein tensor, we find that the emerged curvatures, i.e. additional sources of gravitation, are dominant almost everywhere on the three-sphere. The nature of those curvatures is radically distinct from the ones of the classical Einstein tensor. For instance, the additional curvatures are no longer smooth or continuous. [ABSTRACT FROM AUTHOR]

Space and matter may both be manifestations of a single fundamental quantum dynamics, as it may become evident during black-hole evaporation. Inspired by the fact that quantum electrodynamics underlies the classical theory of elasticity, that in turn has a natural and well-known geometric description in terms of curvature and torsion, related to topological defects, here we move some necessary steps to find the map from such fundamental quantum level to the emergent level of classical space and quantum matter. We proceed by adapting the boson transformation method of standard quantum field theory to the quantum gravity fundamental scenario and successfully obtain the emergence of curvature and torsion, our main focus here. In doing so, we have been able to overcome difficult issues of interpretation, related to the Goldstone modes for rotational symmetry. In fact, we have been able to apply the boson transformation method to disclinations, to relate them to the spin structure and to give an heuristic derivation of the matter field equation on curved space. We also improve results of previous work on the emergence of geometric tensors from elasticity theory, as the non-Abelian contributions to the torsion and curvature tensors, postulated in those papers, here emerge naturally. More work is necessary to identify the type of gravity theories one can obtain in this way. [ABSTRACT FROM AUTHOR]

In this paper, the asymptotically safe gravity has been taken in the gravitational action to derive the wormhole solutions. The paper is aimed at the study of traversable wormhole solutions having repulsive geometry at the throat. The wormhole solutions are produced in two cases: the first case includes the logarithmic form of the shape function while in the second case, the shape function is derived numerically so that it satisfies necessary properties. In both cases, the nature of the matter supporting the wormhole solution is investigated by testing the energy conditions, and the stability of the solutions is determined. The energy conditions are respected in the region near the throat in the first case and everywhere in the second case. The stability of solutions is obtained in the second case along with the repulsive geometry near the throat and attractive away from it. [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]