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A new definition of entropy is introduced using a model that simulates an expanding space-time compatible with the fundamental principle of cosmology. The entropy is obtained by mean of a state function that measures the variation of the space-time normal curvature, from a highly compressed space to a lower compressed space. The defined entropy leads to work out a new understanding of the earliest conditions that last for a period estimated to 380,000 years after the Big Bang. It leads to understand via a short period of inflation the process that generates the uniform distribution of matter and energy at the surface of the last scattering. It involves gravitational singularities in a process of gradual decompression propitious to the incubation of matter recombination, and it allows to trace back gravity origin. [ABSTRACT FROM AUTHOR]

The present communication explores n-dimensional cosmological models within the framework of the f(R,T) theory. The exploration is motivated by Kaluza–Klein theory, where the existence of extra dimensions becomes a focal point in understanding the fundamental forces of the universe. A cosmological model has been constructed by assuming Fractional quadratic deceleration parameter. H(t) and q(t) provide a crucial link between theoretical predictions and empirical observations. Through rigorous mathematical analysis and theoretical modeling, we investigate the implications of this extended gravitational theory on cosmic evolution and the structure of spacetime. Our findings shed light on the interplay between geometry, matter and gravitational dynamics in multidimensional space, offering new insights into the nature of gravity and cosmological phenomenon. [ABSTRACT FROM AUTHOR]

We review Holst formalism and dynamical equivalence with standard GR (in dimension 4). Holst formalism is written for a spin coframe field e μ I and a Spin (3 , 1) -connection ω μ I J on spacetime M and it depends on the Holst parameter γ ∈ ℝ − { 0 }. We show the model is dynamically equivalent to standard GR, in the sense that up to a pointwise Spin (3 , 1) -gauge transformation acting on (uppercase Latin) frame indices, solutions of the two models are in one-to-one correspondence. Hence, the two models are classically equivalent. One can also introduce new variables by splitting the spin connection into a pair of a Spin (3) -connection A μ i and a Spin (3) -valued 1-form k μ i . The construction of these new variables relies on a particular algebraic structure, called a reductive splitting. A weaker structure than requiring that the gauge group splits as the products of two sub-groups, as it happens in Euclidean signature in the selfdual formulation originally introduced in this context by Ashtekar, and it still allows to deal with the Lorentzian signature without resorting to complexifications. The reductive splitting of SL (2 , ℂ) is not unique and it is parameterized by a real parameter β which is called the Immirzi parameter. The splitting is here done on spacetime, not on space as it is usually done in the literature, to obtain a Spin (3) -connection A μ i , which is called the Barbero–Immirzi connection on spacetime. One obtains a covariant model depending on the fields (e μ I , A μ i , k μ i) which is again dynamically equivalent to standard GR. Usually in the literature one sets β = γ for the sake of simplicity. Here, we keep the Holst and Immirzi parameters distinct to show that eventually only β will survive in boundary field equations. [ABSTRACT FROM AUTHOR]

A new mechanism based on space-time noncommutativity has shown to produce anisotropy and an axial-like interaction, yielding leptonic asymmetry for fermionic particles propagating in a noncommutative curved FRW universe. As a byproduct, an analytical expression of this asymmetry for ultra-relativistic particles, such as neutrinos, is derived explicitly. Constraints and bounds from the cosmological parameters are also discussed. [ABSTRACT FROM AUTHOR]

In this study, we explore a topologically charged higher-dimensional traversable defect wormhole, with a specific emphasis on five dimensions. Particularly noteworthy is the fact that the matter–energy distribution attributed to this charged wormhole configuration adheres to the weak energy condition, thus presenting an instance of a five-dimensional wormhole supported by non-exotic matter. Furthermore, our analysis shows that scalar quantities related to space-time curvature and parameters associated with the matter–energy distribution remain finite at the wormhole throat. Moreover, they diminish as distance extends toward infinity, indicating the asymptotic flatness inherent in our model. [ABSTRACT FROM AUTHOR]

This paper delivers the characterization of a pseudo B-symmetric spacetimes and we illustrate that a pseudo B-symmetric spacetime admitting Codazzi type of B-tensor represents a perfect fluid spacetime and if this spacetime admits the time-like convergence criterion, then the pseudo B-symmetric spacetime fulfills cosmic strong energy criterion and contains pure matter. Besides, we find in a pseudo B-symmetric spacetime with Codazzi type of B-tensor the electric part of the Weyl tensor vanishes and has Riemann and Weyl compatible vector fields. Furthermore, it is established that the chosen spacetime with Codazzi type of B-tensor is conformally flat and represents a Robertson–Walker spacetime. Also, we calculate the scale factor Ψ(t) for these spacetimes in a spatially flat Robertson–Walker spacetime. Finally, we study the impact of this spacetime under f(R) gravity scenario and deduce several energy conditions by considering a new model f(ℛ) = e(αℛ) −ln(βℛ) in which α and β are positive constants. [ABSTRACT FROM AUTHOR]

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

This article deals with the characterization of an almost pseudo symmetric spacetime and we illustrate that a conformally flat almost pseudo symmetric spacetime with nonzero constant scalar curvature represents a perfect fluid spacetime. Also, it is established that the chosen spacetime represents a spacetime of quasi constant sectional curvature and generalized Robertson-Walker spacetime. Besides, we find under what condition the spacetime represents radiation era and dust matter fluid. Further it is shown that under certain restriction in an almost pseudo symmetric spacetime, if the Ricci tensor is Killing, then it represents a static spacetime and it is vacuum. Lastly, we study the impact of this spacetime under f (R) gravity scenario and deduce several energy conditions. [ABSTRACT FROM AUTHOR]

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

This paper is devoted to investigating the behavior of charged compact stars in the f(R,ϕ,X) theory of gravity, where R denotes the Ricci Scalar, ϕ is a scalar potential, and X is a kinetic term. For this purpose, we consider spherically symmetric spacetime with Bardeen geometry as exterior spacetime to investigate various properties of compact stars, including energy density and pressure components. Matching conditions are used to find model parameters. We present a detailed analysis, including a discussion of equations of state parameters, and stability criteria. In this analysis, we discuss the energy conditions to check the viability of our model. Furthermore, an equilibrium condition can be visualized through the modified Tolman–Oppenheimer–Volkov equation. It is concluded that Bardeen’s geometry supports the existence of compact stars in the background of the f(R,ϕ,X) theory of gravity. [ABSTRACT FROM AUTHOR]

In [Phys. Rev. D 107, 104008 (2023)], we reported a novel exact closed-form solution which describes asymptotically flat spacetimes in pure ℛ 2 gravity. The solution is Ricci scalar flat, viz. ℛ ≡ 0 everywhere. Whereas any metric with a null Ricci scalar would trivially satisfy the ℛ 2 vacuo field equation, ℛ (ℛ μ ν − 1 4 g μ ν ℛ) + g μ ν □ ℛ − ∇ μ ∇ ν ℛ = 0 , in this paper, we shall show that our solution satisfies a "stronger" version of the ℛ 2 vacuo field equation, viz. ℛ μ ν − 1 4 g μ ν ℛ + ℛ − 1 (g μ ν □ ℛ − ∇ μ ∇ ν ℛ) = 0 , despite the term ℛ − 1 being singular. Even though ℛ identically vanishes, for our solution, the combinations ℛ − 1 ∇ μ ∇ ν ℛ and ℛ − 1 □ ℛ are free of singularity. This exceptional property sets our solution apart from the set of null-Ricci-scalar metrics and makes it a genuinely nontrivial solution. We further demonstrate that, as a member of a larger class of asymptotically de Sitter metrics, our solution is resilient against perturbations in the scalar curvature at largest distances, making it relevant for physical situations where the background deviates from asymptotic flatness. [ABSTRACT FROM AUTHOR]

Wormhole solutions, bridges that connect different parts of spacetime, were proposed early in the history of General Relativity. Soon after, it was shown that all wormholes violate classical energy conditions, which are non-negativity constraints on contractions of the stress–energy tensor. Since these conditions are violated by quantum fields, it was believed that wormholes can be constructed in the context of semiclassical gravity. But negative energies in quantum field theory are not without restriction: quantum energy inequalities (QEIs) control renormalized negative energies averaged over a geodesic. Thus, QEIs provide restrictions on the construction of wormholes. This work is a review of the relevant literature, thus focusing on results where QEIs restrict traversable wormholes. Both 'short' and 'long' (without causality violations) wormhole solutions in the context of semiclassical gravity are examined. A new result is presented on constraints on the Maldacena, Milekhin, and Popov 'long' wormhole from the recently derived doubled smeared null energy condition. [ABSTRACT FROM AUTHOR]

The concept of wormholes is still hypothetical in physics but attractive among researchers. In this work, motivated by the important effect of f (R , T) gravity theory, we seek the existence of wormhole solutions. We consider the field equations that can be derived from two different choices of matter Lagrangian density and utilize them to find the possibility of having some wormhole geometry. This procedure is based on the specific choice of an equation-of-state that obeys a certain physical requirement on the matter stress-energy [S. Kar and D. Sahdev, Restricted class of traversable wormholes with traceless matter, Phys. Rev. D 52 (1995) 2030]. For this purpose, the functional form of the redshift function is taken into consideration, and the possible existence of wormhole solutions is proven. Finally, the work examines the energy conditions and shows that violations of null energy conditions hold throughout spacetime. [ABSTRACT FROM AUTHOR]

This research work discusses the energy bounds in f(R,T) gravity with an anisotropic background, where R is the Ricci scalar and T is the trace of the energy–momentum tensor. For our study, we consider the locally rotationally symmetric Bianchi Type-I space-time. Moreover, we calculate the field equations and find exact solutions by considering different f(R,T) gravity models. We investigate the graphical behavior of energy density and pressure components by considering these distinct gravity models. We further use Hubble, deceleration, jerk, and snap parameters to evaluate the viability of our models. We also perform the graphical analysis to observe the behavior of energy conditions especially null energy conditions, weak energy conditions, strong energy conditions, and dominant energy conditions. It can be noticed that all considered f(R,T) gravity models support the idea of the presence of exotic matter, which justifies the phenomenon of expansion of the universe. [ABSTRACT FROM AUTHOR]

The question of where the inertial properties of matter come from has been open for a long time. Isaac Newton considered inertia an intrinsic property of matter. Ernst Mach held a different view whereby the inertia of a body comes from its interaction with the rest of the universe. This idea is known today as Mach's principle. We discuss Mach's principle based on transactional gravity, the recently developed connection of entropic gravity to the physics of quantum events, induced by transactions. It is shown that Mach's principle holds and that there is a fundamental relation between the gravitational constant G and the total mass in the causal universe. This relationship, derived by means of entropic principles, is rigorously proven. [ABSTRACT FROM AUTHOR]

General theory of relativity can be equivalently formulated on a flat spacetime associating a torsion-free affine connection of non-vanishing non-metricity scalar Q. In this paper, we present an extension of this, viz., the f (Q) theory of gravity, and explore the early evolution of the universe in the background of anisotropic Bianchi-I model. The f (Q) theory in the current setting through its geometric modification is quite successful in explaining the late time accelerated expansion. Here, we note that it accommodates latest released constraints on the inflationary parameters by Planck's collaboration group with excellent precession, but fails to produce a viable decelerated expansion in the radiation dominated era. [ABSTRACT FROM AUTHOR]

In this paper, we introduce and characterize a pseudo-generalized Ricci-recurrent spacetimes and produce an example to verify the existence of such a spacetime. Then we demonstrate that a conformally flat generalized Ricci-recurrent spacetime with certain condition is a pseudo quasi-Einstein spacetime. Besides, it is proved that a pseudo-generalized Ricci-recurrent generalized Robertson–Walker spacetime represents a perfect fluid spacetime. Lastly, we study the impact of this spacetime under f (ℛ , T 2) and f (ℛ ∗) gravity scenario and deduce several energy conditions. [ABSTRACT FROM AUTHOR]

We investigate the turnaround radius of the Reissner–Nordström deSitter Spacetime and how the turnaround radius changes if a test particle carries charge. We also consider the Martínez–Troncoso–Zanelli (MTZ) solution of conformally coupled gravity and investigate how the turnaround radius changes for a scalar test charge. In both scalar and electric interaction cases we find that the Turnaround Radius depends on the particle's energy. [ABSTRACT FROM AUTHOR]

A complete canonical formulation of general covariance makes it possible to construct new modified theories of gravity that are not of higher-curvature form, as shown here in a spherically symmetric setting. The usual uniqueness theorems are evaded by using a crucial and novel ingredient, allowing for fundamental fields of gravity distinct from an emergent space-time metric that provides a geometrical structure to all solutions. As specific examples, there are new expansion-shear couplings in cosmological models, a form of modified Newtonian dynamics can appear in a space-time covariant theory without introducing extra fields, and related effects help to make effective models of canonical quantum gravity fully consistent with general covariance. [ABSTRACT FROM AUTHOR]

In this study, the energy–momentum localization problem is addressed within the framework of rainbow gravity. Considering the black string black hole, one of the cylindrical black hole models, energy–momentum distributions are obtained for Einstein, Bergmann–Thomson, and Landau–Liftshitz prescriptions in rainbow gravity. All prescriptions' energy densities are non-zero and depend on the rainbow functions, while the momentum distributions are zero. It has been observed that space-time energy does not depend on the energy of the test particle for some particular choices of rainbow functions. The results obtained for all special cases are given in a table. [ABSTRACT FROM AUTHOR]

In this paper, we explore the existence of wormholes in the context of f (R , T) gravity. The f (R , T) theory is a curvature-matter coupled modified gravity that depends on an arbitrary function of the Ricci scalar R and the trace of the stress-energy tensor T. In this work, we adopt two different choices for the matter Lagrangian density ( ℒ m = and ℒ m = p r ) and investigate the impact of each one on wormhole structure. By adequately specifying the redshift function and the shape function, we found a variety of exact wormhole solutions in the theory. Our finding indicates that for both classes of wormholes, the energy density is always positive throughout the spacetime, while the radial pressure is negative. This means exotic matter is necessary for the existence of wormholes in f (R , T) gravity. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to obtain physically stable stellar models coupled with anisotropic matter distribution in the context of f (ℛ , T 2) theory. For this, we consider a static spherical geometry and formulate modified field equations containing various unknowns such as matter determinants and metric potentials. We then obtain a unique solution to these equations by employing Durgapal–Fuloria ansatz possessing a constant doublet. We also use matching criteria to calculate the values of these constants by considering the Schwarzschild exterior spacetime. Two different viable models of this modified theory are adopted to analyze the behavior of effective matter variables, anisotropy, energy conditions, compactness and redshift in the interiors of Her X-1, PSR J0348-0432, LMC X-4, SMC X-1, Cen X-3, and SAX J 1808.4-3658 star candidates. We also check the stability of these models by using three different physical tests. It is concluded that our considered stars satisfy all the physical requirements and are stable in this modified gravity for the considered parametric values. [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]

Nowadays, alternative gravity is a crucial tool for addressing some enduring observational problems, such as the dark universe. Additionally, they can be used to advance the results of general relativity in astrophysics. Wormholes, or imagined passageways through spacetime, have the potential to revolutionize interstellar transport and cosmic interconnection. In the present work, the existence and characteristics of wormhole solutions in the context of the non‐linear function of f(Q)$f(\mathcal {Q})$ gravity are investigated. A typical characteristic of maintaining wormholes is the presence of exotic stuff that defines the energy conditions (ECs). The authors looked at the ECs that must be met and analyzed how the f(Q)$f(\mathcal {Q})$ function affects the energy‐momentum requirements for maintaining the wormholes open. For a comprehensive understanding of wormhole geometries, we also studied the embedding diagram and volume integral quantifier. Nowadays, alternative gravity is a crucial tool for addressing some enduring observational problems, such as the dark universe. Additionally, they can be used to advance the results of general relativity in astrophysics. Wormholes, or imagined passageways through spacetime, have the potential to revolutionize interstellar transport and cosmic interconnection. In the present work, the existence and characteristics of wormhole solutions in the context of the non‐linear function of f(Q)$f\!(\mathcal{Q})$ gravity are investigated. A typical characteristic of maintaining wormholes is the presence of exotic stuff that defines the energy conditions (ECs). The authors looked at the ECs that must be met and analyzed how the f(Q)$f\!(\mathcal{Q})$ function affects the energy‐momentum requirements for maintaining the wormholes open. For a comprehensive understanding of wormhole geometries, we also studied the embedding diagram and volume integral quantifier. [ABSTRACT FROM AUTHOR]

In this work, we investigate the quantum dynamics of non-relativistic systems in a Born–Infeld spacetime. We analyze some general scenarios, which concerns the effects of a harmonic oscillator and an inverse square-type potential as well as an Aharonov–Bohm geometric phase. We obtain an analytical solution for the most general case in terms of a confluent Heun function and define the energy spectrum by referring to the ground state, together with its respective eigenfunction. A graphical representation was built so that it was possible to visualize some of the possible energy configurations. For negative values of the Born–Infeld parameter < 0 , we perform a connection with results of quantum dynamics already investigated in the literature for the Ellis–Bronnikov spacetime. [ABSTRACT FROM AUTHOR]

In this paper, the investigation explores an anisotropic cosmological model based on Marder's space-time Tsallis holographic dark energy (THDE) within the framework of f(R, T) theory of gravity, where R represents the Ricci scalar and T signifies the trace of the stress energy-momentum tensor. field equation have solved for class of f(R, T) gravity i.e. f(R, T) = R + f(T). To obtain the precise solution, we employed the density of the THDE model along with the volumetric expansion laws, namely the power law and exponential law. Also explores the physical and geometrical aspects of the model. [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]

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]

The construction of compact star models with quintessence and cloud of strings anisotropic fluid lays the foundation for the research presented in this paper, which focuses on the ansatz of f(Q) gravity. We utilized a spherically symmetric spacetime to derive the field equations incorporating quintessence and cloud of strings anisotropic matter fields. The well-known Karmarkar condition was employed to obtain the gravitational components of the spherically symmetric spacetime. The constant parameters used in the study were obtained by applying junction conditions on the interior and exterior spacetimes. We used the mass and radius of P S R J 1 6 1 4 − 2 2 3 0 & S M C X − 1 , which are already present in the literature. Afterward, we calculated the important properties of quintessence stars in the presence of cloud of strings to ensure the stability and physical presence of compact models. To achieve this goal, we studied features such as energy density, quintessence density, pressure profiles, gradients, anisotropic factor, energy conditions, sound speeds, TOV forces, EoS components, mass function, compactification and redshift. [ABSTRACT FROM AUTHOR]

In this article, we present a heuristic derivation of the on-shell equation of the Lorentzian classicalized holographic tensor network in the presence of a non-zero mass in the bulk spacetime. This derivation of the on-shell equation is based on two physical assumptions. First, the Lorentzian bulk theory is in the ground state. Second, the law of Lorentzian holographic gravity is identified with the time-energy uncertainty principle. The arguments in this derivation could lead to a novel picture of Lorentzian gravity as a quantum mechanical time uncertainty based on the holographic principle and classicalization. [ABSTRACT FROM AUTHOR]

In this paper, we consider the framework of the newly proposed modified theory of gravity named f (R , A) gravity, wherein R and A signify the Ricci and anticurvature scalars, respectively. For our cosmic discussions, we evaluate a renowned f (R , A) model, incorporating various forms of the scale factor within the context of Friedmann–Robertson–Walker (FRW) spacetime. Our exploration delves into energy conditions, the effective equation of state parameter and the stability of models within the f (R , A) theory. These models are further elucidated through graphical representations plotted against time and their respective model constants. Notably, it is observed that scale factors with exponential terms yield more consistent results in comparison to those of power-law forms, given specific values of model constants. [ABSTRACT FROM AUTHOR]

We explore the complexity equals volume proposal for planar black holes in anti-de Sitter (AdS) spacetime in 2+1 dimensions, with an end of the world (ETW) brane behind the horizon. We allow for the possibility of intrinsic gravitational dynamics in the form of Jackiw-Teitelboim (JT) gravity to be localized on the brane. We compute the asymptotic rate of change of volume complexity analytically and obtain the full time dependence using numerical techniques. We find that the inclusion of JT gravity on the brane leads to interesting effects on time dependence of holographic complexity. We identify the region in parameter space (the brane location and the JT coupling) for which the rate of change of complexity violates the Lloyd bound. In an equivalent description of the model in terms of an asymptotically AdS wormhole, we connect the violation of the Lloyd bound to the violation of a suitable energy condition in the bulk that we introduce. We also compare the Lloyd bound constraints to previously derived constraints on the bulk parameters in this model that are based on bounds on entanglement growth in the dual CFT state. [ABSTRACT FROM AUTHOR]

Aim of this paper is to investigate an anisotropic locally rotationally symmetric (LRS) Bianchi type-I space–time in the context of the recently proposed f(Q, T) gravity, where Q is the non-metricity scalar and T is energy–momentum tensor. We have considered f(Q, T) = αQ + βT a linear form, where α and β are model parameters. We have analyzed the exact solution of LRS Bianchi type-I space–time by assuming relation between metric potential A = Bn, where n is arbitrary non-zero real number. To study the anisotropic nature of the dynamical dark energy, we assume that the skewness parameters are time dependent and n ≠ 1. We have constrained to our model by using observational Hubble dataset. Onwards, discussed the physical behavior of cosmological parameters such as energy density, pressure, EoS parameter, deceleration parameter and, Energy conditions. [ABSTRACT FROM AUTHOR]

By restricting ω eff = − 1 in this study, we rebuild a f (T) gravity function from a Λ CDM model. The equation for obtaining the f (T) function is f (T) = 1 2 T − 2 α (− T) 1 2 − β , where α and β are positive constants. A flat FLRW space-time metric has been taken into account, and modified field equations for the Hubble function and scaling factor have been solved. With two observational datasets, H (z) and SNe Ia data, we placed observational constraints on the Hubble function and apparent magnitude m (z) and we were able to determine the model parameters with the best match. These numbers have been used to analyze a number of cosmic phenomena occurring in the expanding cosmos. We have discovered a Λ model-closed transit phase decelerating to accelerating universe model. [ABSTRACT FROM AUTHOR]

We study the degrees of freedom of R2 gravity in flat spacetime with two approaches. By rewriting the theory a la Stueckelberg, and implementing Lorentz-like gauges to the metric perturbations, we confirm that the pure theory propagates one scalar degree of freedom, while the full theory contains two tensor modes in addition. We then consider the degrees of freedom by directly examining the metric perturbations. We show that the degrees of freedom of the full theory match with those obtained with the manifestly covariant approach. In contrast, we find that the pure R2 gravity has no degrees of freedom. We show that a similar discrepancy between the two approaches appears also in a theory dual to the three-form, and appears due to the Lorentz-like gauges, which lead to the fictitious modes even after the residual gauge redundancy has been taken into account. At first sight, this implies a discontinuity between the full theory and the pure case. By studying the first-order corrections of the full R2 gravity beyond the linear regime, we show that at high-energies, both scalar and tensor degrees of freedom become strongly coupled. This implies that the apparent discontinuity of pure and full R2 gravity is just an artefact of the perturbation theory, and further supports the absence of degrees of freedom in the pure R2 gravity. [ABSTRACT FROM AUTHOR]

Considering the condition on conservation of energy–momentum tensor (EMT), we study late time cosmological solutions in the context of f (R , T) = R + α T n gravity (where α and n are constants) in a flat Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime. The present model discusses the case of a barotropic perfect fluid as the matter content of the Universe, along with the case when dissipative effects are taken into account. Briefly, assuming a single perfect fluid, we find that the mentioned model is not capable of presenting an observationally consistent picture of the late time accelerated expansion of the Universe; nevertheless, the model leads to admissible solutions when the bulk viscosity is included. In this regard, a consistent setting is found considering the Eckart, Truncated and Full Israel–Stewart theories which determine the behavior of bulk viscosity. In the presence of bulk viscosity, the behavior of the deceleration parameter (DP) shows that the underlying model can describe an acceptable evolution even if the isotropic pressure of matter content of the Universe is negligible. [ABSTRACT FROM AUTHOR]

The topic of gravitational lensing in the Mannheim–Kazanas solution of Weyl conformal gravity and the Schwarzschild–de Sitter solution in general relativity has featured in numerous publications. These two solutions represent a spherical massive object (lens) embedded in a cosmological background. In both cases, the interest lies in the possible effect of the background non-asymptotically flat spacetime on the geometry of the local light curves, particularly the observed deflection angle of light near the massive object. The main discussion involves possible contributions to the bending angle formula from the cosmological constant Λ in the Schwarzschild–de Sitter solution and the linear term γ r in the Mannheim–Kazanas metric. These effects from the background geometry, and whether they are significant enough to be important for gravitational lensing, seem to depend on the methodology used to calculate the bending angle. In this paper, we review these techniques and comment on some of the obtained results, particularly those cases that contain unphysical terms in the bending angle formula. [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]

First, we illustrate that a conformally flat generalized Ricci recurrent spacetime is a perfect fluid spacetime. As a consequence, we prove that such a spacetime represents a dust matter fluid, stiff matter and dark energy era under certain restrictions on the Ricci scalar. Then we establish that such a spacetime is a Robertson–Walker spacetime. Also, we note that in a conformally flat generalized Ricci recurrent spacetime, the fluid is shear-free, vorticity-free and its flow vector is hypersurface orthogonal. We investigate a conformally flat generalized Ricci recurrent spacetime as a solution of curvature-squared gravity theory. In this study, various energy conditions in terms of the Ricci scalar are examined and state that the Universe is in an accelerating phase and satisfies the weak, null, dominant, and strong energy conditions. [ABSTRACT FROM AUTHOR]

This paper argues for the appropriateness of Maxwell spacetime as the minimal spacetime structure in which one may formulate a theory of Newtonian gravity. I begin by presenting an intrinsic characterization of Maxwell gravitation that, eschewing covariant derivative operators, makes use only of a standard of rotation and other more primitive structures. I then revisit the question of whether Maxwell gravitation and Newton-Cartan theory are equivalent, demonstrating that previous results may be extended to all but the vacuum case since candidate geometrizations are not free to vary through purely gravitational degrees of freedom. Lastly, I consider the space of possible geometrizations of Maxwell gravitation more broadly and argue for a sense in which curvature is not entirely a matter of convention in classical spacetimes. [ABSTRACT FROM AUTHOR]

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

We show that in Jackiw-Teitelboim (JT) gravity, late-time two-point functions can get a leading non-decaying contribution from a spacetime with the topology of a Möbius strip (a disk with one crosscap). There is an interesting interplay between this contribution and the standard "plateau". The two can add together or cancel, depending on topological weighting factors. We match this behavior to Random Matrix Theory (RMT) and the N mod 8 periodicity of Sachdev-Kitaev-Ye (SYK) results. [ABSTRACT FROM AUTHOR]

In this paper we investigate the localization of spinor fields in braneworld models by reducing a Dirac spinor in 2n + 2-dimensional spacetime to spinors in 2n dimensions. The high-dimensional Dirac can be reduced to low-dimensional spinors including Weyl or Dirac. In conformally flat extra-dimensional spacetime, fermions cannot be localized through minimal coupling with gravity. To achieve the localization of spinor fields, we introduce a tensor coupling term given by Ψ ¯ Γ M Γ N Γ P ⋯ T MNP ⋯ Ψ , which ensures SO(n, 1) symmetry. For a tensor TMNP⋯ of odd order, the left and right chiralities of high-dimensional spinors are decoupled. We find that a special form of tensor coupling Ψ ¯ Γ M ∂ M F ϕ R R μν R μν ⋯ Ψ may facilitate the localization of the spinor field when F(ϕ) = ϕn. [ABSTRACT FROM AUTHOR]

It has been shown that the theory of relativity can be applied physically to the functioning brain, so that the brain connectome should be considered as a four-dimensional spacetime entity curved by brain activity, just as gravity curves the four-dimensional spacetime of the physical world. Following the most recent developments in modern theoretical physics (black hole entropy, holographic principle, AdS/CFT duality), we conjecture that consciousness can naturally emerge from this four-dimensional brain connectome when a fifth dimension is considered, in the same way that gravity emerges from a 'flat' four-dimensional quantum world, without gravitation, present at the boundaries of a five-dimensional spacetime. This vision makes it possible to envisage quantitative signatures of consciousness based on the entropy of the connectome and the curvature of spacetime estimated from data obtained by fMRI in the resting state (nodal activity and functional connectivity) and constrained by the anatomical connectivity derived from diffusion tensor imaging. [ABSTRACT FROM AUTHOR]

We consider holomorphic Poisson-BF theory on twistor space. Classically, this describes self-dual Einstein gravity on space-time, but at the quantum level it is plagued by an anomaly. The anomaly corresponds to the fact that integrability of the self-dual vacuum Einstein equations does not survive in self-dual quantum gravity. We compute the anomaly polynomials in the Poisson-BF theory, as well as in this theory coupled to a holomorphic BF theory on twistor space describing self-dual Yang–Mills. We show that all anomalies may be cancelled by further coupling to a twistor field representing a type of axion on space-time. When the twistor anomalies are cancelled, all n ≥ 4 -pt amplitudes vanish and integrability is restored. [ABSTRACT FROM AUTHOR]

After finding a solution for the Hayward regular black hole (HRBH) in massive gravity, we embed the (3+1)-dimensional HRBHs both in massless and in massive gravities into (5+2)- and (6+3)-dimensional Minkowski spacetimes, respectively. Here, massive gravity denotes that a graviton acquires a mass holographically by broken momentum conservation in the HRBH. The original HRBH has no holographically added gravitons, which we call 'massless'. Making use of newly found embedding coordinates, we obtain desired Unruh temperatures and compare them with the Hawking and local fiducial temperatures, showing that the Unruh effect for a uniformly accelerated observer in a higher-dimensional flat spacetime is equal to the Hawking effect for a fiducial observer in a black hole spacetime. We also obtain freely falling temperatures of the HRBHs in massless and massive gravities seen by freely falling observers, which remain finite even at the event horizons while becoming the Hawking temperatures in asymptotic infinity. [ABSTRACT FROM AUTHOR]

The recently obtained special Buchdahl-inspired metric Phys. Rev. D 107, 104008 (2023) describes asymptotically flat spacetimes in pure Ricci-squared gravity. The metric depends on a new (Buchdahl) parameter k ˜ of higher-derivative characteristic, and reduces to the Schwarzschild metric, for k ˜ = 0 . For the case k ˜ ∈ (− 1 , 0) , it was shown that it describes a traversable Morris–Thorne–Buchdahl (MTB) wormhole Eur. Phys. J. C 83, 626 (2023), where the weak energy condition is formally violated. In this paper, we briefly review the special Buchdahl-inspired metric, with focuses on the construction of the Kruskal–Szekeres (KS) diagram and the situation for a wormhole to emerge. Interestingly, the MTB wormhole structure appears to permit the formation of closed timelike curves (CTCs). More specifically, a CTC straddles the throat, comprising of two segments positioned in opposite quadrants of the KS diagram. The closed timelike loop thus passes through the wormhole throat twice, causing two reversals in the time direction experienced by the (timelike) traveller on the CTC. The key to constructing a CTC lies in identifying any given pair of antipodal points (T , X) and (− T , − X)  on the wormhole throat in the KS diagram as corresponding to the same spacetime event. It is interesting to note that the Campanelli–Lousto metric in Brans–Dicke gravity is known to support two-way traversable wormholes, and the formation of the CTCs presented herein is equally applicable to the Campanelli–Lousto solution. [ABSTRACT FROM AUTHOR]

A set of exact spherically symmetric solutions for describing the interior of a relativistic star in the context of f(R,T)$f(\mathcal {R}, T)$ modified theory of gravity is presented. The existence of a Conformal Killing Vectors (CKV) and Bardeen's model to find the boundary conditions as an exterior space‐time geometry to achieve the metric potentials for current study is used. Moreover, matching conditions have been employed to obtain a set of free parameters for the physical analysis. Furthermore, some physically valid attributes like the behavior of energy density, pressure, equilibrium conditions, the adiabatic index, mass‐radius relation, surface red‐shifts, and compactness factor for the realistic stars such as PSRJ1614−2230$PSR\nobreakspace J1614-2230$, PSRJ1903+327$PSR\nobreakspace J1903+327$, SMCX−1$SMC\nobreakspace X-1$, CenX−3$Cen\nobreakspace X-3$, LMCX−4$LMC\nobreakspace X-4$ and EXO1785−248$EXO\nobreakspace 1785-248$ are investigated. It is observed that for the current investigation, the derived compact star solutions are physically acceptable and authentic in the presence of charge with conformal motion. A set of exact spherically symmetric solutions for describing the interior of a relativistic star in the context of f(R,T)$f(\mathcal {R}, T)$ modified theory of gravity is presented. The existence of a Conformal Killing Vectors (CKV) and Bardeen's model to find the boundary conditions as an exterior space‐time geometry to achieve the metric potentials for current study is used. Moreover, matching conditions have been employed to obtain a set of free parameters for the physical analysis. Furthermore, some physically valid attributes like the behavior of energy density, pressure, equilibrium conditions, the adiabatic index, mass‐radius relation, surface red‐shifts, and compactness factor for the realistic stars such as PSRJ1614−2230$\textit{PSR}\nobreakspace J1614- 2230$, PSRJ1903+327$\textit{PSR}\nobreakspace J1903+327$, SMCX−1$\textit{SMC}\nobreakspace X-1$, CenX−3$\textit{Cen}\nobreakspace X-3$, LMCX−4$\textit{LMC}\nobreakspace X-4$ and EXO1785−248$\textit{EXO}\nobreakspace 1785- 248$ are investigated. It is observed that for the current investigation, the derived compact star solutions are physically acceptable and authentic in the presence of charge with conformal motion. [ABSTRACT FROM AUTHOR]

We prove that any smooth vacuum spacetime containing a compact Cauchy horizon with surface gravity that can be normalised to a nonzero constant admits a Killing vector field. This proves a conjecture by Moncrief and Isenberg from 1983 under the assumption on the surface gravity and generalises previous results due to Moncrief–Isenberg and Friedrich–Rácz–Wald, where the generators of the Cauchy horizon were closed or densely filled a 2-torus. Consequently, the maximal globally hyperbolic vacuum development of generic initial data cannot be extended across a compact Cauchy horizon with surface gravity that can be normalised to a nonzero constant. Our result supports, thereby, the validity of the strong cosmic censorship conjecture in the considered special case. The proof consists of two main steps. First, we show that the Killing equation can be solved up to infinite order at the Cauchy horizon. Second, by applying a recent result of the first author on wave equations with initial data on a compact Cauchy horizon, we show that this Killing vector field extends to the globally hyperbolic region. [ABSTRACT FROM AUTHOR]

Classical gravity coupled to a CFT4 (matter) is considered. The effect of the quantum dynamics of matter on gravity is studied around maximally symmetric spaces (flat, de Sitter and Anti de Sitter). The structure of the graviton propagator is modified and non-trivial poles appear due to matter quantum effects. The position and residues of such poles are mapped as a function of the relevant parameters, the central charge of the CFT4, the two R2 couplings of gravity as well as the curvature of the background space-time. The instabilities induced are determined. Such instabilities can be important in cosmology as they trigger the departure from de Sitter space and in some regions of parameters are more important than the well-known scalar instabilities. It is also determined when the presence of such instabilities is unreliable if the associated scales are larger than the "species" cutoff of the gravitational theory. [ABSTRACT FROM AUTHOR]