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We examine the accretion process in a thin disk surrounding a supermassive black hole within the framework of Einstein–Maxwell-scalar (EMS) gravity. Our investigation aims to elucidate how variations in model parameters affect different physical properties of the disk. When keeping EMS parameters β and q constant, we observe a reduction in radiation flux and temperature as α increases. However, the luminosity and radiative efficiency exhibit relatively minor variation. Conversely, under fixed α and q, an escalation in β leads to heightened levels of radiation flux, temperature, luminosity, and radiative efficiency. These results underscore the diverse influences of model parameters on observable metrics, providing valuable insights for the astronomical study of distinct black holes. [ABSTRACT FROM AUTHOR]

In this short note we perform canonical analysis of the theory invariant under restricted diffeomorphism so that the action contains kinetic term for determinant of metric. We find corresponding Hamiltonian and determine structure of constraints. [ABSTRACT FROM AUTHOR]

In this paper we consider a high energy scattering of free scalar particles through a gravitational field. The one particle t-channel amplitude of the scattering in this limit is governed by reggeized graviton. Therefore, we discuss an appearance of the reggeized gravitons in the framework of Einstein–Hilbert gravity and consider Lipatov's effective action for the reggeized gravitons. We calculate the trajectory of the corresponding t-channel amplitude in the framework and thereafter define the leading order amplitude of scattering of two massive scalar particles. An impact factor of the interaction of scalar particle with the reggeized gravitons is also calculated and possible applications of the approach are discussed as well. [ABSTRACT FROM AUTHOR]

In this work, we study metric-Palatini gravity extended by the antisymmetric part of the affine curvature. This gravity theory leads to general relativity plus a geometric Proca field. Using our previous construction of its static spherically-symmetric AdS solution (Eur Phys J. C 83(4):318, 2023), we perform a detailed analysis in this work using the observational quasiperiodic oscillations (QPOs) data. To this end, we use the latest data from stellar-mass black hole GRO J1655-40, intermediate-mass black hole in M82-X1, and the super-massive black hole in SgA* (our Milky Way) and perform a Monte-Carlo-Markov-Chain (MCMC) analysis to determine or bound the model parameters. Our results shed light on the allowed ranges of the Proca mass and other parameters. The results imply that our solutions can cover all three astrophysical black holes. Our analysis can also be extended to more general metric-affine gravity theories. [ABSTRACT FROM AUTHOR]

We derive novel black hole solutions in a modified gravity theory, namely the Hu–Sawicki model of f(R) gravity. After obtaining the black hole solution, we study the horizon radius of the black hole from the metric and then analyse the dependence of the model parameters on the horizon. We then use the 6th order WKB method to study the quasinormal modes of oscillations (QNMs) of the black hole perturbed by a scalar field. The dependence of the amplitude and damping part of the QNMs are analysed with respect to variations in model parameters and the error associated with the QNMs are also computed. After that we study some thermodynamic properties associated with the black hole such as its thermodynamic temperature as well as greybody factors. It is found that the black hole has the possibility of showcasing negative temperatures and is thermodynamically unstable for feasible values of model parameters. Then we analyse the geodesics and derive the photon sphere radius as well as the shadow radius of the black hole. The photon radius is independent of the model parameters while shadow radius showed fair amount of dependence on the model parameters. We tried to constrain the parameters with the help of Keck and VLTI observational data and obtained some bounds on m and c 2 parameters. [ABSTRACT FROM AUTHOR]

We consider the more general geometrical flow in the space of metrics for three-manifolds that consists of a combination of two flows, the Cotton flow and the Ricci flow; by playing a fundamental role in the detailed balance principle of the four dimensional Hořava–Lifshitz gravity, this generalized flow reveals another difficulty with this theory that attempts to be a candidate for an UV completion of Einstein general relativity, namely, the supposed emergency of the speed of light, the Newton constant, and the cosmological constant, from a deeply nonrelativistic theory of gravity. Respecting that principle, the generalized flow shows the proliferation of different limits of the theory with an unwanted behavior at both the IR and UV regimes. [ABSTRACT FROM AUTHOR]

We investigate a family of axial symmetry solution constructed in general relativity (GR) within the framework of Ricci-inverse (RI) gravity theory. In GR, these solutions admitted closed time-like curves at an instant of time from an initial spacelike hypersurface in a causally well-behaved manner, thus, violates the causality condition. Our aim is to examine these axial symmetry solutions within the context of Ricci-inverse gravity theory to determine whether closed time-like curves still appear in this new gravity theory. We consider two Classes of RI-gravity models: (i) Class-II models defined by a function f = f (R , A μ ν A μ ν) gravity and (ii) Class-III models defined by f = f (R , A , A μ ν A μ ν) , where A μ ν is the anti-curvature tensor, A = g μ ν A μ ν as its scalar, and R μ ν is the Ricci tensor. We are able solved the modified field equations considering these axial symmetry solutions as background in RI-gravity with null radiation as the matter content and the cosmological constant. This confirms that the chosen family of axial symmetry solutions are valid solutions in RI-gravity theory and, consequently, closed time-like curves is still form, analogous to their formation in GR. [ABSTRACT FROM AUTHOR]

We study self-gravitating solutions of three-dimensional massive gravity coupled to the Yang–Mills–Chern–Simons gauge theory. Among these, there is a family of asymptotically Warped-Anti de Sitter black holes that come to generalize previous solutions found in the literature and studied in the context of WAdS 3 /CFT 2 . We also present self-gravitating solutions to three-dimensional Einstein–Yang–Mills theory, as well other self-gravitating solutions in the presence of higher-curvature terms. [ABSTRACT FROM AUTHOR]

We consider the third law of thermodynamics for families of 4- and n-dimensional Vaidya black holes, including many of interest. Since there are several versions of the definition of surface gravity as well as the extremality condition for dynamical black holes, we first show that for the considered 4- and n-dimensional Vaidya families, these definitions are consistent with each other. We assume, first, that a non-extremal black hole evolves to an extremality state after a finite time and second, that the weak energy condition for the source holds at all times. We then compare the results of these assumptions and investigate whether there are ranges of black hole parameters where these two assumptions are in conflict. [ABSTRACT FROM AUTHOR]

We study how the number of degrees of freedom in Weyl conformal gravity runs with the energy scale from the UV fixed point. To this end we employ two approaches. First, we utilize the Fradkin–Tseytlin prescription for the number of degrees of freedom and demonstrate that the one-loop result is highly dependent on the selected background. We then employ the counting methodology based on the a- and c-function, which are typically used to characterize the trace anomaly of conformal field theories in four dimensions. We compute these in the enhanced one-loop approximation and demonstrate that the degrees of freedom decrease monotonically from six degrees in the UV regime. This behavior is independent of the backgrounds considered. Further salient issues, such as the connection between the Fradkin–Tseytlin prescription and counting based on the a- and c-function, or the applicability of both methods in Einstein's gravity, are also addressed. [ABSTRACT FROM AUTHOR]

In this paper, we construct the Dirac stars model composed of two Dirac fields and Einstein gravity in four-dimensional Anti-de Sitter spacetime. We consider the Dirac stars with free field (no self-interacting). Our investigation focuses on understanding the relationship between Arnowitt–Deser–Misner (ADM) mass and binding energy in relation to the cosmological constant. Furthermore, we extend the study to the Dirac stars with self-interacting potentials. For the self-interacting Dirac stars, three types of interactions are examined: only quartic, only sextic, quartic and sextic interactions that is kind of Q-ball type interactions. In these three distinct scenarios, it is essential to investigate the impact of self-interaction constants on Dirac stars. Additionally, we study the Dirac solitons in the AdS background. Finally, we discussed the dependence of the spectral flow of the Dirac Hamiltonian on the model parameters. [ABSTRACT FROM AUTHOR]

This paper investigates the disparity between matter and antimatter in the universe with the help of gravitational baryogenesis. This phenomenon commenced shortly after the big bang, resulting in a predominance of matter over antimatter. We analyze the mechanism of gravitational baryogenesis (baryon to entropy ratio) under the framework of f(Q, C) gravity, where Q indicates non-metricity scalar and C denotes the boundary term. This Phenomenon depends on the charge parity violation interaction and for this paper we produce it with the coupling between baryon matter current ( j ν ) and ∂ ν (Q + C) . In the present work, we evaluate he baryon to entropy ratio ( η B S ) by proposing two models of f(Q, C) with the assumption of power-law scale factor for each model and the universe contains perfect fluid throughout. We find that under optimal choice of model parameters, the results of η B S of propose models in f(Q, C) are compatible with the observational bound. The crux of the current work is that the outcomes of our propose models for generalized case of gravitational baryogenesis are consist with its observational constraint in different eras of the Universe. [ABSTRACT FROM AUTHOR]

We explore superoscillations within the context of classical and quantum field theories, presenting novel solutions to Klein–Gordon's, Dirac's, Maxwell's and Einstein's equations. In particular, we illustrate a procedure of second quantization of fields and how to construct a Fock space which encompasses Superoscillating states. Furthermore, we extend the application of superoscillations to quantum tunnelings, scatterings and mixings of particles, squeezed states and potential advancements in laser interferometry, which could open new avenues for experimental tests of quantum gravity effects. By delving into the relationship among superoscillations and phenomena such as Hawking radiation, the black hole (BH) information and the Firewall paradox, we propose an alternative mechanism for information transfer across the BH event horizon. [ABSTRACT FROM AUTHOR]

This study explores the potential existence of traversable wormholes influenced by a global monopole charge within the f(Q) gravity framework. To elucidate the characteristics of these wormholes, we conducted a comprehensive analysis of wormhole solutions employing three different forms of redshift function under a linear f(Q) model. Wormhole shape functions were derived for barotropic, anisotropic, and isotropic Equations of State (EoS) cases. However, in the isotropic EoS case, the calculated shape function failed to satisfy the asymptotic flatness condition. Additionally, we observed that our obtained shape functions adhered to the flaring-out conditions under an asymptotic background for the remaining EoS cases. Furthermore, we examined the energy conditions at the wormhole throat with a radius r 0 . We noted the influences of the global monopole's parameter η , the EoS parameter ω , and n in violating energy conditions, particularly the null energy conditions. Finally, we conducted a stability analysis utilizing the Tolman–Oppenheimer–Volkov (TOV) equation and found that our obtained wormhole solution is stable. [ABSTRACT FROM AUTHOR]

We study a massless real self-interacting scalar field φ non-minimally coupled to Einstein gravity with torsion in (2+1) space-time dimensions in the presence of a cosmological constant. The field equations with a self-interaction potential V (φ) including φ n terms are derived by a variational principle. By numerically solving these field equations with the 4th Runge–Kutta method, the circularly symmetric rotating solutions for (2+1) dimensions Einstein gravity with torsion are obtained. Exact analytical solutions to the field equations are derived for the proposed metric in the absence of both torsion and angular momentum. We find that the self-interacting potential only exists for n = 6 , as requested by conformal symmetry. We also study the motion of massive and massless particles in (2+1) Einstein gravity with torsion coupled to a self-interacting scalar field. The effect of torsion on the behavior of the effective potentials of the particles is analyzed numerically. [ABSTRACT FROM AUTHOR]

In the present paper, we investigate some exact cosmological models in F (R , T ¯) gravity theory. We have considered the arbitrary function F (R , T ¯) = R + λ T ¯ where λ is an arbitrary constant, R , T ¯ are respectively, the Ricci-scalar curvature and the torsion. We have solved the field equations in a flat FLRW spacetime manifold for Hubble parameter and using the MCMC analysis, we have estimated the best fit values of model parameters with 1 - σ , 2 - σ , 3 - σ regions, for two observational datasets like H(z) and Pantheon SNe Ia datasets. Using these best fit values of model parameters, we have done the result analysis and discussion of the model. We have found a transit phase decelerating-accelerating universe model with transition redshifts z t = 0. 4438 - 0.0790 + 0.1008 , 0. 3651 - 0.0904 + 0.1644 . The effective dark energy equation of state varies as - 1 ≤ ω de ≤ - 0.5176 and the present age of the universe is found as t 0 = 13. 8486 - 0.0640 + 0.1005 , 12. 0135 - 0.2743 + 0.6206 Gyrs, respectively for two datasets. [ABSTRACT FROM AUTHOR]

We discuss spherically symmetric dynamical systems in the framework of a general model of f (R) gravity, i.e. f (R) = R e ζ R , where ζ is a dimensional quantity in squared length units [L 2 ]. We initially assume that the internal structure of such systems is governed by the Krori–Barua ansatz, alongside the presence of fluid anisotropy. By employing astrophysical observations obtained from the pulsar SAX J1748.9-2021, derived from bursting X-ray binaries located within globular clusters, we determine that ζ is approximately equal to ± 5 km 2 . In particular, the model is capable of producing stable configurations for SAX J1748.9-2021, encompassing both its geometric and physical characteristics. We show that, within the framework of f (R) gravity, the Krori–Barua ansatz establishes semi-analytical connections between the radial ( p r ) and tangential ( p t ) pressures, and the density (ρ ). These relations are described as p r ≈ v r 2 (ρ - ρ I) and p t ≈ v t 2 (ρ - ρ II) . In this context, v r and v t denote the sound speeds in the radial and tangential directions, respectively. Meanwhile, ρ I pertains to the surface density, and ρ II is derived from the model parameters. These connections are consistent with the equations of state derived from the best-fit solutions identified in the ongoing investigation. Notably, within the framework of f (R) gravity where ζ is negative, the maximum compactness, denoted as C, is inherently limited to values that do not exceed the Buchdahl limit. This contrasts with general relativity or f (R) gravity with ζ positive, where the compactness has the potential to asymptotically reach the black hole threshold ( C → 1 ). The model predictions suggest a central density that largely exceeds the saturation nuclear density, which is ρ nuc = 3 × 10 14 g/cm 3 . Also the surface density ρ I surpasses ρ nuc . We obtain a mass-radius diagram, corresponding to the boundary density, which is consistent with other observational data. [ABSTRACT FROM AUTHOR]

Einsteinian cubic gravity (ECG) is the most general theory up to cubic order in curvature, which has the same graviton spectrum as the Einstein theory. In this paper, we investigate the geodesic motions of timelike particles around the four dimensional asymptotically flat black holes in ECG, and discuss their potential applications when connecting them with recent observational results. We first explore the effects of the cubic couplings on the marginally bound orbits (MBO), innermost stable circular orbits (ISCO) and on the periodic orbits around the Einsteinian cubic black hole. We find that comparing to Schwarzschild black hole in general relativity, the cubic coupling enhances the energy as well as the angular momentum for all the bound orbits of the particles. Then, we derive the relativistic periastron precessions of the particles and give a preliminary bound on the cubic coupling employing the observational result of the S2 star's pericenter precession in SgrA*. Finally, after calculating the periodic orbits' configurations, we preliminarily evaluate the gravitational waveform radiated from several periodic orbits in one complete period of a test object which orbits a supermassive Einsteinian cubic black hole. Our studies could be helpful for us to better understand the gravitational structure of the theory with high curvatures. [ABSTRACT FROM AUTHOR]

Although General Relativity (GR) is a very successful theory of gravity, it cannot explain every observational phenomenon. People have tried many kinds of modified gravity theory to explain these phenomena which GR cannot explain very well, such as string theory. In recent years Double Field Theory (DFT) has been an exciting research area in string theory. The most general, spherically symmetric, asymptotically flat, static vacuum solution to D = 4 double field theory has been derived by S.M. Ko, J.H. Park and M. Suh. In this article, we calculate the minor corrections to the three predictions in GR: optical deflation, planet precession and gravitational redshift. These three predictions should be able to tested by observations and find the discrepancies between GR and DFT in the future. [ABSTRACT FROM AUTHOR]

We analyze inhomogeneous cosmological models in the local Universe, described by the Lemaître–Tolman–Bondi (LTB) metric and developed using linear perturbation theory on a homogeneous and isotropic Universe background. Focusing on the different evolution of spherical symmetric inhomogeneities, we compare the Λ LTB model, in which the cosmological constant Λ is included in the LTB formalism, with inhomogeneous cosmological models based on f R modified gravity theories viewed in the Jordan frame. We solve the system of field equations for both inhomogeneous cosmological models adopting the method of separation of variables: we integrate analytically the radial profiles of local perturbations, while their time evolution requires a numerical approach. The main result of the analysis concerns the different radial profiles of local inhomogeneities due to the presence of a non-minimally coupled scalar field in the Jordan frame of f R gravity. While radial perturbations follow a power-law in the Λ LTB model, Yukawa-like contributions appear in the f R theory. Interestingly, this latter peculiar behavior of radial profile is not affected by the choice of the f R functional form. The numerical solution of time-dependent perturbations exhibits a non-diverging profile. This work suggests that investigations about local inhomogeneities in the late Universe may allow us to discriminate if the present cosmic acceleration is caused by a cosmological constant term or a modified gravity effect. [ABSTRACT FROM AUTHOR]

We study the Hamiltonian structure of unimodular-like theories, where the cosmological constant (or other supposed constants of nature) are demoted from fixed parameters to classical constants of motion. No new local degrees of freedom are present as a result of a U(1) gauge invariance of the theory. Hamiltonian analysis of the action reveals that the only possible gauge fixing that can be enforced is setting the spatial components of the four-volume time vector T i ≈ 0 . As a consequence of this, the gauge-fixed unimodular path integral is equivalent to the minisuperspace unimodular path integral. However, should we break the U(1) gauge invariance, two things happen: a massless propagating degree of freedom appears, and the (gauge-invariant) zero-mode receives modified dynamics. The implications are investigated, with the phenomenology depending crucially on the target "constant". [ABSTRACT FROM AUTHOR]

In this work, we investigate the BRST quantization of the massive N = 4 supersymmetric spinning particle, with a twofold purpose: exploring different approaches to give mass to spinning particle models and formulating a first-quantized theory for massive gravity on both flat and curved spacetime. The main contribution of this study is the development of a worldline formulation of the linear theory of massive gravity, namely of the Fierz–Pauli theory, on a curved spacetime; such a theory describes the propagation of massive spin 2 particle on a non-flat background. Our results suggest that achieving the nilpotency of the BRST charge requires an Einstein spacetime with vanishing cosmological constant as the only viable consistent background. In the course of the analysis, we take the N = 2 supersymmetric worldline as an exemplificative model, correctly producing the Proca theory on curved spacetime. Our analysis shows that the associated BRST system uniquely selects the minimal coupling to the background curvature. [ABSTRACT FROM AUTHOR]

In this work we explore the properties of the spacetime around the Schwarzschild-like black hole in the Starobinsky–Bel–Robinson gravity through a test particle motion and its quasi periodic oscillations (QPOs) in the circular orbits in the close vicinity of the black hole horizon. We show how the extra spacetime parameter β affects the effective potential, energy, angular momentum, inner most circular orbit (ISCO) radius, and trajectory of a test particle. Next, we focus on the QPOs of particles and explore it in the relativistic precision model. Finally, we apply our analysis to explain the observational data obtained for several black hole candidates including GRO J1655-40, GRS 1915+105, XTE J1859+226, and XTE J1550-564 and get constraints on their spacetime parameters by employing the Markov Chain Monte Carlo analysis methodology. [ABSTRACT FROM AUTHOR]

In this work, we study the effect of diffusion of ultra-high energy cosmic ray (UHECR) protons in the presence of turbulent magnetic fields (TMFs) in the light of the f(R) theory of gravity. The f(R) theory of gravity is a successful modified theory of gravity in explaining the various aspects of the observable Universe including its current state of expansion. For this work, we consider two most studied f(R) gravity models, viz., the power-law model and the Starobinsky model. With these two models, we study the diffusive character of the propagation of UHECR protons in terms of their density enhancement. The density enhancement is a measure of how the density of CRs changes due to their diffusion in the intergalactic medium and interaction with the cosmic microwave background (CMB) radiation. Ankle, instep and Greisen–Zatsepin–Kuzmin (GZK) cutoff are all spectrum characteristics that extragalactic UHECRs acquire when they propagate through the CMB. We analyse all these characteristics through the diffusive flux as well as its modification factor. Model dependence of the modification factor is minimal compared to the diffusive flux. We compare the UHECR proton spectra calculated for the considered f(R) gravity models with the available data of the Telescope Array (TA) and Pierre Auger Observatory (PAO). We see that both models of f(R) gravity predict energy spectra of UHECRs with all experimentally observed features, which lay well within the range of combined data of both experiments throughout the energy range of concern. It is to be noted that our present work is only to investigate the possible effects of f(R) gravity theory on the UHECRs propagation, using pure proton composition as a simplified case study since protons are least affected by magnetic fields. Hence, at this stage, our results cannot be used to favor or disfavor f(R) cosmology over Λ CDM cosmology as more work is needed in this regard. [ABSTRACT FROM AUTHOR]

In this paper, we present a generalized non-relativistic Chern–Simons gravity model in three spacetime dimensions. We first study the non-relativistic limit of the Mielke–Baekler gravity through a contraction process. The resulting non-relativistic theory contains a source for the spatial component of the torsion and the curvature measured in terms of two parameters, denoted by p and q. We then extend our results by defining a Newtonian version of the Mielke–Baekler gravity theory, based on a Newtonian like algebra which is obtained from the non-relativistic limit of an enhanced and enlarged relativistic algebra. Remarkably, in both cases, different known non-relativistic and Newtonian gravity theories can be derived by fixing the p , q parameters. In particular, torsionless models are recovered for q = 0 . [ABSTRACT FROM AUTHOR]

We consider Klein–Gordon (KG) particles in a global monopole (GM) spacetime within Eddington-inspired Born–Infeld gravity (EiBI-gravity) and in a Wu–Yang magnetic monopole (WYMM). We discuss a set of KG-oscillators in such spacetime settings. We propose a textbook power series expansion for the KG radial wave function that allows us to retrieve the exact energy levels for KG-oscillators in a GM spacetime and a WYMM without EiBI-gravity. We, moreover, report some conditionally exact, closed form, energy levels (through some parametric correlations) for KG-oscillators in a GM spacetime and a WYMM within EiBI-gravity, and for massless KG-oscillators in a GM spacetime and a WYMM within EiBI-gravity under the influence of a Coulomb plus linear Lorentz scalar potential. We report the effects of the Eddington parameter κ , GM-parameter α , WYMM strength σ , KG-oscillators' frequency Ω , and the coupling parameters of the Coulomb plus linear Lorentz scalar potential, on the spectroscopic structure of the KG-oscillators at hand. Such effects are studied over a vast range of the radial quantum number n r ≥ 0 and include energy levels clustering at κ > > 1 (i.e., extreme EiBI-gravity), and at | σ | > > 1 (i.e., extreme WYMM strength). [ABSTRACT FROM AUTHOR]

In this work, we will study the late-time dynamic evolution of the F(R) gravity dark energy model of an interaction between dark radiation and dark matter. We explore the possibility of the apparent extra dark radiation being linked directly to the physics of cold dark matter (CDM). In particular, we consider a generic scenario where dark radiation, as a result of an interaction, is produced directly by a fraction of the dark matter density effectively decaying into dark radiation. The decay process is controlled by its rate Q = α H ρ dm , where α is the (constant) dimensionless parameter quantifying the strength of the decay mechanism, for the weak coupling between dark matter and dark radiation α ≪ 1 . We will solve the field equations numerically by using the statefinder function y H to appropriately represent the field equation. The F(R) gravity model we consider is described by a nearly R 2 -model at early epochs and at late-time the model mimic the Λ -Cold-Dark-Matter model ( Λ C D M ), and we fine tune the parameters to achieve viability and compatibility with the latest Planck constraints at late-time. Furthermore, we consider the behavior of several well-known statefinder quantities, we find that statefinder diagnostic and Om diagnostic can not only break the degeneracy of different parameter values in the F(R) gravity dark energy model, but also effectively distinguish the difference between the F(R) gravity model and the Λ C D M model. At last, we compared the theoretical results with the SNIa data. [ABSTRACT FROM AUTHOR]

In the current article, we discuss the wormhole geometries in two different gravity theories, namely F (Q , T) gravity and F (R , T) gravity. In these theories, Q is called a non-metricity scalar, R stands for the Ricci scalar, and T denotes the trace of the energy–momentum tensor (EMT). The main goal of this study is to comprehensively compare the properties of wormhole solutions within these two modified gravity frameworks by taking a particular shape function. The conducted analysis shows that the energy density is consistently positive for wormhole models in both gravity theories, while the radial pressure is positive for F (Q , T) gravity and negative in F (R , T) gravity. Furthermore, the tangential pressure shows reverse behavior in comparison to the radial pressure. By using the Tolman-Oppenheimer-Volkov (TOV) equation, the equilibrium aspect is also described, which indicates that hydrostatic force dominates anisotropic force in the case of F (Q , T) gravity theory, while the reverse situation occurs in F (R , T) gravity, i.e., anisotropic force dominates hydrostatic force. Moreover, using the concept of the exoticity parameter, we observed the presence of exotic matter at or near the throat in the case of F (Q , T) gravity while matter distribution is exotic near the throat but normal matter far from the throat in F (R , T) gravity case. In conclusion, precise wormhole models can be created with a potential NEC and DEC violation at the throat of both wormholes while having a positive energy density, i.e., ρ > 0 . [ABSTRACT FROM AUTHOR]

We present the formal solution for a quantum mechanical particle responding to gravitational wave (GW) in the framework of modified theories of gravity (MTG) where, apart from the two standard tensorial modes of polarization of GW, we have considered an additional scalar mode. The presence of this longitudinal scalar mode makes it necessary to extend the usual two-dimensional description of matter-GW interaction to a three-dimensional one. Naturally this requires non-trivial changes in the quantum mechanical treatment which we elaborate in this paper. [ABSTRACT FROM AUTHOR]

Similar to Rastall gravity we introduce matter-geometry nonminimal coupling which is proportional to the gradient of quadratic curvature invariants. Those are mimicking the conformal trace anomaly when backreaction of the quantum fields to a curved spacetime geometry is considered. We consider a static spherically symmetric stellar structure with anisotropic fluid and Krori-Barua metric potentials model to examine the theory. Confronting the model with NICER + XMM-Newton observational constraints on the pulsar PSR J0740 + 6620 quantifies the amount of the nonminimal coupling via a dimensionless parameter ϵ ≃ - 0.01 . We verify that the conformal symmetry is broken everywhere inside the pulsar as the trace anomaly Δ > 0 , or equivalently the trace of the stress-energy tensor T < 0 , whereas the adiabatic sound speed does not violate the conjecture conformal upper limit v r 2 / c 2 = 1 / 3 . The maximum compactness accordingly is C max = 0.752 which is 4 % higher than GR. Notably, if the conformal sound speed constraint is hold, observational data excludes ϵ ≥ 0 up to ≥ 1.6 σ . The stellar model is consistent with the self-bound structure with soft linear equation of state. Investigating possible connection with MIT bag model of strange quarks sets physical bounds from microscopic physics which confirm the negative value of the parameter ϵ . We estimate a radius R = 13.21 ± 0.96 km of the most massive observed compact star PSR J0952−0607 with M = 2.35 ± 0.17 M ⊙ . Finally, we show that the corresponding mass-radius diagram fits well lowest-mass pulsar HESS J1731−347 and highest-mass pulsar PSR J0952−0607 ever observed as well as the intermediate mass range as obtained by NICER and LIGO/Virgo observations. [ABSTRACT FROM AUTHOR]

We investigate the propagating degrees of freedom of f(Q)-gravity in a 4-dimensional space-time under the imposition of the coincident gauge by performing the Dirac–Bergmann analysis. In this work, we start with a top-down reconstruction of the metric-affine gauge theory of gravity based only on the concept of a vector bundle. Then, the so-called geometrical trinity of gravity is introduced and the role of the coincident GR is clarified. After that, we reveal relationships between the boundary terms in the variational principle and the symplectic structure of the theory in order to confirm the validity of the analysis for our studied theories. Then, as examples, we revisit the analysis of GR and its f (R ∘) -extensions. Finally, after reviewing the Dirac–Bergmann analysis of the coincident GR and that of f(T)-gravity, we perform the analysis of coincident f(Q)-gravity. Under the imposition of appropriate spatial boundary conditions, we find that, as a generic case, the theory has five primary, three secondary, and two tertiary constraint densities and all these constraint densities are classified into second-class constraint density; the number six is the propagating degrees of freedom of the theory and there are no longer any remaining gauge degrees of freedom. We also discuss the condition of providing seven pDoF as a generic case. The violation of diffeomorphism invariance of coincident f(Q)-gravity make it possible to emerge such several sectors. [ABSTRACT FROM AUTHOR]

The enforcement of the unimodularity condition in a gravity theory by means of a Lagrange multiplier leads, in general, to inconsistencies upon quantization. This is so, in particular, when the classic linear splitting of the metric between the background and quantum fields is used. To avoid the need of introducing such a Lagrange multiplier while using the classic linear splitting, we carry out the quantization of unimodular gravity with extra Weyl symmetry by using Becchi–Rouet–Stora–Tyutin (BRST) techniques. Here, two gauge symmetries are to be gauge-fixed: transverse diffeomorphisms and Weyl transformations. We perform the gauge-fixing of the transverse diffeomorphism invariance by using BRST transformations that involve antisymmetric ghost fields. We show that these BRST transformations are compatible with the BRST transformations needed to gauge-fix the Weyl symmetry, so that they can be combined in a set of transformations generated by a single BRST operator. Newton's law of gravitation is derived within the BRST formalism we put forward as well as the Slavnov–Taylor equation. [ABSTRACT FROM AUTHOR]

The paper studies different observational features in the case of a specific cubic gravity theory, based on third order contractions of the Riemann tensor. Considering viable cosmic chronometers data, baryon acoustic oscillations, and supernovae, we analyze the viability of such a theoretical model, obtaining specific constraints for different parameters in the current scenario. It is shown that the present extension of the Λ CDM cosmological model is compatible with recent data sets. The results indicate that the dark energy equation of state is exhibiting a phantom regime in the near past in the case of the best fitted values, a behavior which is in agreement with various phenomenological studies. [ABSTRACT FROM AUTHOR]

We investigate sufficient conditions under which cubic gravity is healthy and viable at the perturbation level. We perform a detailed analysis of the scalar and tensor perturbations. We impose the requirement that the two scalar potentials, whose ratio is the post-Newtonian parameter γ , should deviate only minimally form general relativity. Additionally, concerning tensor perturbations we impose satisfaction of the LIGO-VIRGO and Fermi Gamma-ray Burst observations, and thus we result to a gravitational-wave equation with gravitational-wave speed equal to the speed of light, and where the only deviation from general relativity appears in the dispersion relation. Furthermore, we show that cubic gravity exhibits an effective Newton's constant that depends on the model parameter, on the background evolution, and on the wavenumber scale. Hence, by requiring its deviation from the standard Newton's constant to be within observational bounds we extract the constraints on the single coupling parameter β . [ABSTRACT FROM AUTHOR]

In this paper, the properties of higher dimensional holographic superconductors are studied in the background of f(R) gravity and Born–Infeld electrodynamics. A specific model of f(R) gravity is considered, allowing a perturbative approach to the problem. The Sturm–Liouville eigenvalue problem is used to analytically calculate the critical temperature and the condensation operator. An expression for the critical temperature in terms of the charge density including the correction from modified gravity is derived. It is seen that the higher values of the Born–Infeld coupling parameter make the condensation harder to form. In addition, the limiting values of this parameter, above which Born–Infeld electrodynamics cannot be applied, are found for different dimensions. Another interesting property is that the increasing modifications of f(R) gravity lead to larger values of the critical temperature and a decrease in the condensation gap, which means that the condensation is easier to form. [ABSTRACT FROM AUTHOR]

Here we consider the generalized Oppenheimer–Snyder collapse of a star into a four-dimensional Einstein-Gauss–Bonnet black hole as well as a class of regular black holes labeled by the polytropic index of the stellar matter. We then analyze the nature of the horizon and the corresponding surface gravity outside and inside the star. The Hayward and Nielsen–Visser dynamical surface gravity are in agreement with the one resulting from the Killing vector of the outer static metric. However, these two definitions inside the star do not coincide with the Killing surface gravity outside the star when the star crosses the event horizon. This motivates us to study the surface gravity using Fodor's approach to have a unique surface gravity at the mentioned moment. Then the extremality condition and the first law of thermodynamics are discussed at the trapping horizon of the star. [ABSTRACT FROM AUTHOR]

The f(R, T) gravity models, for which R is the Ricci scalar and T is the trace of the energy–momentum tensor, elevate the degrees of freedom of the renowned f(R) theories, by making the Einstein field equations of the theory to also depend on T. While such a dependence can be motivated by quantum effects, the existence of imperfect or extra fluids, or even a cosmological "constant" which effectively depends on T, the formalism can truly surpass some deficiencies of f(R) gravity. As the f(R, T) function is arbitrary, several parametric models have been proposed ad hoc in the literature and posteriorly confronted with observational data. In the present article, we use gaussian process to construct an f (R , T) = R + f (T) model. To apply the gaussian process we use a series of measurements of the Hubble parameter. We then analytically obtain the functional form of the function. By construction, this form, which is novel in the literature, is well-adjusted to cosmological data. In addition, by extrapolating our reconstruction to redshift z = 0 , we were able to constrain the Hubble constant value to H 0 = 69.97 ± 4.13 km s - 1 Mpc - 1 with 5 % precision. Lastly, we encourage the application of the functional form herewith obtained to other current problems of observational cosmology and astrophysics, such as the rotation curves of galaxies. [ABSTRACT FROM AUTHOR]

We study a class of early universe cosmological models based on Einstein–Cartan gravity and including a higher derivative term corresponding to a power of the Holst scalar curvature. The resulting effective action is basically given by General Relativity and an additional neutral pseudoscalar field (the pseudoscalaron), unequivocally related to the corresponding components of the torsion, that necessarily acquire a dynamics. The induced pseudoscalaron potential provides a realistic inflationary phase together with a very rich postinflationary epoch, resulting from the coupling of the pseudoscalaron to ordinary matter. [ABSTRACT FROM AUTHOR]

The higher-curvature gravity with boundary terms i.e the f(Q) theories, grounded on non-metricity as a fundamental geometric quantity, exhibit remarkable efficacy in portraying late-time universe phenomena. The aim is to delineate constraints on two prevalent models within this framework, namely the Log-square-root model and the Hyperbolic tangent-power model, by employing the framework of Big Bang Nucleosynthesis (BBN). The approach involves elucidating deviations induced by higher-curvature gravity with boundary terms in the freeze-out temperature ( T f ) concerning its departure from the standard Λ CDM evolution. Subsequently, constraints on pertinent model parameters are established by imposing limitations on | δ T f T f | derived from observational bounds. This investigation employs dynamical system analysis, scrutinizing both background and perturbed equations. The study systematically explores the phase space of the models, identifying equilibrium points, evaluating their stability, and comprehending the system's trajectory around each critical point. Moreover, the application of "Center Manifold Theory" facilitates the examination of stability properties at non-hyperbolic points. The principal findings of this analysis reveal the presence of a matter-dominated saddle point characterized by the appropriate matter perturbation growth rate. Subsequently, this phase transitions into a stable phase of a dark-energy-dominated, accelerating universe, marked by consistent matter perturbations. Overall, the study substantiates observational confrontations, affirming the potential of higher-curvature gravity with boundary terms as a promising alternative to the Λ CDM concordance model. The methodological approach underscores the significance of dynamical systems as an independent means to validate and comprehend the cosmological implications of these theories. [ABSTRACT FROM AUTHOR]

We have investigated the structural properties of strange quark stars (SQSs) in a modified theory of gravity known as massive gravity. In order to obtain the equation of state (EOS) of strange quark matter, we have employed a modified version of the Nambu–Jona-Lasinio model (MNJL) which includes a combination of NJL Lagrangian and its Fierz transformation by using weighting factors (1 - α) and α. Additionally, we have also calculated dimensionless tidal deformability (Λ) in massive gravity. To constrain the allowed values of the parameters appearing in massive gravity, we have imposed the condition Λ 1.4 M ⊙ ≲ 580. Notably, in the MNJL model, the value of α varies between zero and one. As α increases, the EOS becomes stiffer, and the value of Λ increases accordingly. We have demonstrated that by softening the EOS with increasing the bag constant, one can obtain objects in massive gravity that not only satisfy the constraint Λ 1.4 M ⊙ ≲ 580 , but they also fall within the unknown mass gap region (2.5 M ⊙ - 5 M ⊙). To establish that the obtained objects in this region are not black holes, we have calculated Schwarzschild radius, compactness, and Λ M TOV in massive gravity. [ABSTRACT FROM AUTHOR]

Entanglement islands play an essential role in the recent breakthrough in addressing the black hole information paradox. Inspired by double holography, it is conjectured that the entanglement islands can exist only in massive gravity. There are many pieces of evidence but also debates for this conjecture. This paper recovers the massless entanglement island in wedge holography with negative DGP gravity on the brane. However, the spectrum of negative DGP gravity includes a massive ghost, implying the model is unstable. Our work supports the view that there is no entanglement island in a well-defined braneworld model of massless gravity if one divides the radiation and black hole regions by minimizing entanglement entropy. However, such a partition results in a zero radiation region containing no information. Whether there are other physical non-trivial partitions of the radiation region is an open question and deserves further study. [ABSTRACT FROM AUTHOR]

We consider the Brans–Dicke theory in non-metricity gravity, which belongs to the family of symmetric teleparallel scalar–tensor theories. Our focus lies in exploring the implications of the conformal transformation, as we derive the conformal equivalent theory in the Einstein frame, distinct from the minimally coupled scalar field theory. The fundamental principle of the conformal transformation suggests the mathematical equivalence of the related theories. However, to thoroughly analyze the impact on physical variables, we investigate the spatially flat Friedmann–Lemaître–Robertson–Walker geometry, defining the connection in the non-coincidence gauge. We construct exact solutions for the cosmological model in one frame and compare the physical properties in the conformal related frame. Surprisingly, we find that the general physical properties of the exact solutions remain invariant under the conformal transformation. Finally, we construct, for the first time, an analytic solution for the symmetric teleparallel scalar–tensor cosmology. [ABSTRACT FROM AUTHOR]

In the present work, accretion onto a static spherically symmetric black hole in the hybrid metric-Palatini gravity is considered. The Novikov–Thorne model for a relativistic thin accretion disk is used. The energy flux, temperature distribution, emission spectrum and energy conversion efficiency of accretion disks around such black holes are numerically calculated. A comparison with the results for a Schwarzschild black hole is made and conclusions about the viability of the model are drawn. As a result, it is obtained that the accretion disks around black holes in hybrid metric-Palatini gravity are colder and less luminous than in general relativity. [ABSTRACT FROM AUTHOR]

According to gauge/gravity correspondence, we study the holographic Schwinger effect within an anisotropic background. Firstly, the separate length of the particle–antiparticle pairs is computed within the context of an anisotropic background which is parameterized by dynamical exponent ν . It is found that the maximum separate length x increases with the increase of dynamical exponent ν . By analyzing the potential energy, we find that the potential barrier increases with the dynamical exponent ν at a small separate distance. This observation implies that the Schwinger effect within an anisotropic background is comparatively weaker when contrasted with its manifestation in an isotropic background. Finally, we also find that the Schwinger effect in the transverse direction is weakened compared to the parallel direction in the anisotropic background, which is consistent with the top-down model. [ABSTRACT FROM AUTHOR]

It is believed that gravity can be considered as a quantum coherent mediator. In this study, we propose a plan to test the existence of extra dimensions using the quantum gravity induced entanglement of masses (QGEM) experiment. This experiment involves two freely falling test masses passing through a Stern–Gerlach-like device. We investigate the entanglement witness between these masses within the framework of the Randall–Sundrum II model (RS-II). Our findings indicate that the system reaches entanglement more rapidly in the presence of extra dimensions, particularly when the radius of the extra dimension is large. [ABSTRACT FROM AUTHOR]

We develop a static charged stellar model in f(R, T) gravity where the modification is assumed to be linear in T which is the trace of the energy momentum tensor. The exterior spacetime of the charged object is described by the Reissner–Nordström metric. The interior solution is obtained by invoking the Buchdahl–Vaidya–Tikekar ansatz, for the metric potential g rr , which has a clear geometric interpretation. A detailed physical analysis of the model clearly shows distinct physical features of the resulting stellar configuration under such a modification. We find the maximum compactness bound for such a class of compact stars which is a generalization of the Buchdahl bound for a charged sphere described in f(R, T) gravity. Our result shows physical behaviour that is distinct from general relativity. [ABSTRACT FROM AUTHOR]

This paper investigates charged black holes within the framework of quintic quasi-topological gravity, focusing on their thermodynamics, conserved quantities, and stability. We construct numerical solutions and explore their thermodynamic properties, supplemented by the study of analytically solvable special cases. By verifying the first law of thermodynamics, we validate our approach and compare our findings to those of Einstein gravity. The physical properties of the solutions are examined across anti-de Sitter, de Sitter, and flat spacetime backgrounds. Our analysis reveals that anti-de Sitter solutions demonstrate thermal stability, while de Sitter and flat solutions lack this property. Finally, we discuss the implications of our results and propose potential avenues for future research in this field. [ABSTRACT FROM AUTHOR]

Here, we study the generalized second law (GSL) of thermodynamics in the framework of massive gravity. To do this, we consider a FRW universe filled only with matter and enclosed by the apparent horizon. In addition, we consider two models including generalized massive gravity (GMG) as well as dRGT massive gravity on de Sitter. For both models, we first study the dynamics of background cosmology and then explore the validity of GSL. We conclude that for the selected values of model parameters the GSL is respected. [ABSTRACT FROM AUTHOR]

We show that the transformation of a time-evolving spherically symmetric metric tensor into a Painlevé–Gullstrand–Lemaître form brings forth a few curious consequences. The time evolution describes a non-singular gravitational collapse, leading to a bounce and dispersal of all the clustered matter, or a wormhole geometry for certain initial conditions. The null convergence condition is violated only at the onset of bounce or the wormhole formation. As an example, the requirements to develop a Simpson–Visser wormhole/regular black-hole geometry is discussed. The solution can be regarded as a new time-evolving twin of sonic dumb holes found in analog gravity. [ABSTRACT FROM AUTHOR]

We show that the f(R)-gravity theories with constant Ricci scalar in the Jordan/Einstein frame can be described by Einstein or Einstein–Maxwell gravity with a cosmological term and a modified gravitational constant. To obtain the rotating axisymmetric solutions for the Einstein/Einstein–Maxwell gravity with a cosmological constant, we also propose a modified Newmann–Janis algorithm which involves the non-complexification of the radial coordinate and a complexification of the polar coordinate. Using the duality between the two gravity theories we show that the stationary or static solutions for the Einstein/Einstein–Maxwell gravity with a cosmological constant will also be the solutions for the dual f(R)-gravity with constant Ricci scalar. [ABSTRACT FROM AUTHOR]