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

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

In this paper, we study the critical impact parameter and Lyapunov exponent for a massive charged particle and a photon in the Kerr–Newman–de Sitter (KNdS) spacetime. From the geodesic equations obtained for the particle traveling parallel to the rotation axis (on-axis) of the KNdS spacetime and using the complex angular momentum (CAM) technique, it is found that the total absorption cross-section at a high-frequency limit oscillates with decreasing amplitude. We show that the black hole charge and spin affect the total absorption cross-section in the case of both the massive charged particle and photon. [ABSTRACT FROM AUTHOR]

In this study, the particle motion around the naked singularity and black hole of Kerr–Newman spacetime is investigated with a special attention on the closed timelike orbits. It is found that both in the naked singularity (NS) and in black hole (BH), the singularity is concealed by causality violating regions, and the Cauchy surface consistently resides inside the inner horizon in nonextremal black holes. For neutral particles and particles with an identical charge to the source, only particles with positive angular momentum are permitted to traverse the closed timelike curves. Conversely, for particles with the opposite charge to the source, the strong Coulomb attraction draws all particles inside the Cauchy surface, allowing them to be present in the closed timelike curves irrespective of their angular momentum. However, in both the NS and BH (both extremal and nonextremal), test particles are confined at a considerable distance from the singular point such that there always exists an empty region surrounding the singularity which prevents particles from interacting with it. The radius of the empty surface that depends on the source parameters and the particle characteristics is investigated with an accurate expression. [ABSTRACT FROM AUTHOR]

An explanation of the nature of dark energy has been treated in extra dimensions within the scheme of string theory. One of the most successful models is inspired by the Dvali–Gabadadze–Porrati (DGP) model, in which the universe is a 4-dimensional brane embedded in a 5-dimensional Minkowski spacetime. In this landscape, the study of the evolution of the normal branch has led us to different kinds of dark energy, where the most simple case is the cosmological constant Λ. Moreover, other viable cosmological solutions are related to agegraphic dark energy, which allows a late cosmic acceleration within an interacting mechanism. To explore the viability of these solutions and possible gravitational leakage, in this paper, we present constraints on such models using recent standard sirens forecasting in addition to local observables such as Pantheon (SNIa), H (z) measurements, baryonic acoustic oscillations (BAO). Our results show that the value associated with the species of quantum fields n in these models is strongly restricted for supernovae observations to n = 2 0 , and for GW standard sirens mock data prefers a value of n = 1. [ABSTRACT FROM AUTHOR]

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

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

Colliding or noncolliding plane-fronted electromagnetic or gravitational waves are the asymptotic limit of Robinson–Trautman spherical electromagnetic or gravitational waves. Noncolliding plane-fronted waves contain no information about their sources whereas colliding waves contain information about possibly the motion of their sources. As a first step to investigate the latter phenomenon, we construct an asymptotic limit of Liénard–Wiechert electromagnetic fields in the context of Minkowskian spacetime. This has the advantage that the source is well known and the calculations can be carried out in full detail. The final result is an algebraically general Maxwell field which consists of colliding plane-fronted waves in a subregion of Minkowskian spacetime and an interesting byproduct is a novel perspective on a Maxwell field originally discovered by Bateman. [ABSTRACT FROM AUTHOR]

Based on the work of Chandrasekhar [The Mathematical Theory of Black Holes, Chap. 3, Sec. 20 (Oxford University Press, 1992)], we investigate the null geodesic structure of the emergent Barriola–Vilenkin (BV) spacetime in the context of k-essence theory. For k-essence, the emergent gravity metric is a one-to-one correspondence with the BV metric connected to the Schwarzschild background, where the global monopole charge is replaced by the dark energy density. This equivalence holds specifically for a certain class of k-essence scalar fields that have been constructed by Gangopadhyay and Manna [Eur. Phys. Lett. 100, 49001 (2012)]. We have traced out different trajectories for null geodesic in the presence of dark energy for the k-essence emergent BV spacetime. It is demonstrated that the outcomes deviate from the typical Schwarzschild spacetime owing to the fundamental configuration with a constant dark energy density. [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]

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

This study has established the stability of Reissner–Nordström black holes within a specific parameter range against charged scalar perturbations. Additionally, the near-extremal Reissner–Nordström black holes exhibit a phenomenon known as the Schwinger effect for black holes, involving the production of charged scalar particles. This paper explores the stability and Schwinger effect in the context of static charged black holes within the Einstein–ModMax theory. Our approach closely aligns with the existing literature methods. We aim to investigate how the nonlinearity inherent in the theory may impact both the stability regime and the rate of charged scalar pair production by the black hole. [ABSTRACT FROM AUTHOR]

During the ringdown phase of a gravitational signal emitted by a black hole, the least damped quasinormal frequency dominates. If modifications to Einstein's theory induce noticeable deformations of the black-hole geometry only near the event horizon, the fundamental mode remains largely unaffected. However, even a small change near the event horizon can significantly impact the first few overtones, providing a means to probe the geometry of the event horizon. Overtones are stable against small deformations of spacetime at a distance from the black hole, allowing the event horizon to be distinguished from the surrounding environment. In contrast to echoes, overtones make a much larger energy contribution. These findings open up new avenues for future observations. [ABSTRACT FROM AUTHOR]

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

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

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

This paper advances the state-of-the-art by extending the study of the analogy between the fabric of spacetime and elasticity. As no prior work exists about a potential spacetime thermal expansion coefficient α , we explore the analogy of general relativity with the theory of elasticity by considering the cosmological constant Λ as an additional space curvature of the structure of space due to a thermal gradient coming from the cosmic web and the cold vacuum and we propose ( α S Δ T e) 2 = (1 R 0 ) 2 = Λ with R 0 being the curvature radius of the space fabric. It follows from this analogy and from the supposed space model consisting of thin sheets of Planck thickness l p curved by this thermal gradient Δ T a possible thermal expansion coefficient of the equivalent elastic medium modeling the space α S = l p Λ Δ T of the order of α space-QFT = 1. 1 6 × 1 0 − 6 K − 1 . As spacetime and not only space must be considered in general relativity, this paper also proposes an innovative approach which consists in introducing into the interval ds2 of special relativity a temperature effect T : d s 2 = (1 ± α t T) 2 c 2 d t 2 − (1 ± α s T) 2 [ d x 2 + d y 2 + d z 2 ] (entropy variations correlated with time laps, based on temperature variations affecting always physically the clocks) based on different thermal expansion coefficients for space and time with for the flow of time t : c t c n τ = k B t n h × Δ T = α t Δ T. With T ≈ 106 K, n = 1 , the associate time interval is 4. 8 × 1 0 − 1 7 s and α t = 1. 0 × 1 0 − 6 K − 1 . The consequence of this hypothesis is that dark energy potentially becomes a thermal spacetime curvature ( α f T l p ) 2 with f equal to s or t depending of the temperature, the thermal entropy variation of the universe, the Planck thickness and time, that increases since the Big bang, depending on thermal expansion coefficients for spacetime α s and α t as a function, respectively, of Λ , k B h × t , in opposition to spacetime curvature gravity due to mass/energy density as described in general relativity. [ABSTRACT FROM AUTHOR]

In this paper, we present a study of basic classes of stationary observers orbiting in stationary and axially symmetric Kerr-de Sitter spacetimes. Along with proper definitions of these classes, we focus on their possible matching on particular circular orbits. Especially, we determine a number of such orbits in a given spacetime characterized by its rotational and cosmological parameters that leads to a comprehensive classification of the Kerr-de Sitter spacetimes. Studied profiles of orbital velocities of the observers and of forces keeping them on their orbits directly reflect complex structure of the Kerr-de Sitter spacetimes that is caused by a unique interplay of attraction and rotation of a central object and an ambient cosmic repulsion. [ABSTRACT FROM AUTHOR]

In this paper, we derive and solve the causal heat transport equation, for a collapsing self-similar radiation shell described by the Vaidya geometry, using a semi-tetrad covariant formalism. This enables us to gain a meaningful insight on how the geometrical quantities associated with this spacetime relate to the causal Maxwell–Cattaneo type hyperbolic transport equations obtained in the phenomenological theories of Muller, Stewart and Israel. It is shown that the causal temperature profile in the collapsing shell is distinct from the Eckart temperature, and this temperature profile can be given explicitly. [ABSTRACT FROM AUTHOR]

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

The consideration of a 1 + 3 covariant approach to cold dark matter universe with no shear cosmological dust model with irrotational flows is developed in the context of f (G) gravity theory in this study. This approach reveals the existence of integrability conditions which do not appear in noncovariant treatments. We constructed the integrability conditions in modified Gauss–Bonnet f (G) gravity basing on the constraints and propagation equations. These integrability conditions reveal the linearized silent nature of quasi-Newtonian models in f (G) gravity. Finally, the linear equations for the overdensity and velocity perturbations of the quasi-Newtonian spacetime were constructed in the context of modified f (G) gravity. The application of harmonic decomposition and redshift transformation techniques to explore the behavior of the overdensity and velocity perturbations using f (G) model was made. On the other hand, we applied the quasi-static approximation to study the approximated solutions on small scales which helps to get both analytical and numerical results of the perturbation equations. The analysis of the energy overdensity and velocity perturbations for both short- and long-wavelength modes in a dust-Gauss–Bonnet fluids was done and we see that both energy overdensity and velocity perturbations decay with redshift for both modes. In the limits to Λ CDM, it means f (G) = G the considered f (G) model results coincide with Λ CDM. [ABSTRACT FROM AUTHOR]

We review various aspects of de Sitter spacetime in string theory: its status as an Effective Field Theory spacetime solution, its relation to the vacuum energy problem in string theory, its (global) holographic definition in terms of two entangled and noncanonical conformal field theories as well as a realization of a realistic de Sitter universe endowed with the observed visible matter and the necessary dark sector in order to reproduce the realistic cosmological structure. In particular, based on the new insight regarding the cosmological constant problem in string theory, we argue that in a doubled, T -duality-symmetric, phase-space-like and noncommutative generalized-geometric formulation, string theory can naturally lead to a small and positive cosmological constant that is radiatively stable and technically natural. Such a formulation is fundamentally based on a quantum spacetime, but in an effective spacetime description of this general formulation of string theory, the curvature of the dual spacetime is the cosmological constant of the observed spacetime, while the size of the dual spacetime is the gravitational constant of the same observed spacetime. Also, the three scales associated with intrinsic noncommutativity of string theory, the cosmological constant scale, the Planck scale as well as the Higgs scale, can be arranged to satisfy various seesaw-like formulae. Along the way, we show that these new features of string theory can be implemented in a particular deformation of cosmic-string-like models. [ABSTRACT FROM AUTHOR]

We study a pure connection formulation plus algebraic constraints in four spacetime dimensions where the gauge group G ⊃ S O (1 , 3). We show that the action has, as particular cases, the MacDowell–Mansouri and the Stelle–West formulations for gravity. Also, under adequate specification of the constraint terms, we obtain a subset of Einstein manifolds that are torsionless conformally flat manifolds. [ABSTRACT FROM AUTHOR]

For the electromagnetic field in (D + 1) -dimensional locally de Sitter (dS) spacetime, we analyze the effects of a generalized cosmic string type defect on the vacuum expectation value of the energy-momentum tensor. For the Bunch–Davies vacuum state, the topological contributions are explicitly extracted in both the diagonal and off-diagonal components. The latter describes the presence of radially directed energy flux in the vacuum state. It vanishes for D = 3 because of the conformal invariance of the electromagnetic field and is directed towards the cosmic string for D ≥ 4. The topological contributions in the vacuum stresses are anisotropic and, unlike the geometry of a cosmic string in the Minkowski spacetime, for D > 3 the stresses along the directions parallel to the string core differ from the energy density. Depending on the planar angle deficit and the distance from the cosmic string, the corresponding expectation values can be either positive or negative. Near the cosmic string the effect of the gravitational field on the diagonal components of the topological part is weak and the leading terms in the respective expansions coincide with the expectation values for a cosmic string in the background of Minkowski spacetime. The spacetime curvature essentially modifies the behavior of the topological terms at proper distances from the cosmic string larger than the dS curvature radius. In that region, the topological contributions in the diagonal components of the energy–momentum tensor decay in inverse proportion to the fourth power of the proper distance and the energy flux density behaves as inverse-fifth power for all values of the spatial dimension D. The exception is the energy density in the special case D = 4. For a cosmic string in the Minkowski bulk the energy flux is absent and the diagonal components are proportional to the (D + 1) th power of the inverse distance. [ABSTRACT FROM AUTHOR]

A regular Vaidya-type line-element is proposed in this work. The mass function depends both on the temporal and the spatial coordinates. The curvature invariants and the source stress tensor T b a are finite in the whole space. The energy conditions for T b a are satisfied if k 2 < 2 v r , where k is a positive constant and v, r are coordinates. It is found that the radial pressure has a maximum very close to r = 2 m (r > 2 m) , v = 2 m. The energy crossing a sphere of constant radius is akin to Lundgren–Schmekel–York quasilocal energy. The Newtonian acceleration of the timelike geodesics has an extra term (compared to the result of Piesnack and Kassner) which leads to rejecting effects. [ABSTRACT FROM AUTHOR]

In this paper, we show that a general geometric and thermodynamic but shear-free perturbation of homogeneous and isotropic universes are necessarily silent, without any gravitational waves, in spite of the presence of nonzero electric and magnetic Weyl. We prove this in two steps. First, we establish that a shear-free perturbation of these universes is acceleration-free and the fluid flow geodesics of the background universe map onto themselves in the perturbed universe. This effect then decouples the evolution equations of the electric and magnetic parts of the Weyl tensor in the perturbed spacetimes and the magnetic part no longer contains any tensor modes. Although the electric part that drives the tidal forces does have tensor modes sourced by the anisotropic stress, these modes have homogeneous oscillations at every point on a time slice without any wave propagation. We also show the presence of vorticity vector waves that are sourced by the curl of heat flux. This analysis shows the critical role of the shear tensor in generating cosmological gravitational waves in an expanding universe. [ABSTRACT FROM AUTHOR]

For many years now it has become conventional ([E. Witten, Phys. Today 49 (1996) 24; D. Gross, Einstein and the quest for a unified theory, in Einstein for the 21st Century: His Legacy in Science, Art, and Modern Culture, eds. P. L. Galison, G. Holton and S. S. Schweber (Princeton University Press, 2008), pp. 287–297; N. Arkani-Hamed, Spacetime is doomed (2010)]) for theorists to argue that "spacetime is doomed", with the difficulties in finding a quantum theory of gravity implying the necessity of basing a fundamental theory on something quite different than usual notions of spacetime geometry. But what is this spacetime geometry that is doomed? In this paper, we will explore how our understanding of 4-dimensional geometry has evolved since Einstein, leading to new ideas about such geometry which may not be doomed at all. [ABSTRACT FROM AUTHOR]

In the absence of a fully fledged theory of quantum gravity, we propose a "bottom-up" framework for exploring quantum-gravitational physics by pairing two of the most fundamental concepts of quantum theory and general relativity, namely quantum superposition and spacetime. We show how to describe such "spacetime superpositions" and explore effects they induce upon quantum matter. Our approach capitalizes on standard tools of quantum field theory in curved space, and allows us to calculate physical observables like transition probabilities for a particle detector residing in curvature-superposed de Sitter spacetime, or outside a mass-superposed black hole. Crucially, such scenarios represent genuine quantum superpositions of spacetimes in contrast with superpositions of metrics that only differ by a coordinate transformation and thus are not different according to general relativity. [ABSTRACT FROM AUTHOR]

String tension fundamentally determines the properties of strings; yet its value is often assigned arbitrarily, creating a fine-tuning problem. We describe a mechanism for dynamically generating string tension in a flat or almost flat spacetime, using the 'modified measures formalism', which in turn naturally generates a new type of stringy braneworld scenario. Such a scenario allows strings to achieve near infinite tension confining the strings to two very close expanding surfaces, but the infinite tensions also threaten to distort the near-flat embedding spacetime through large back reactions. We argue that this danger can be neutralized via the creation of a baby universe — a growing region of embedding spacetime that divorces from the ambient embedding spacetime, while our universe is still a brane separating two nearly flat spacetimes. The avoidance of a minimum length and a maximum Hagedorn temperature in the context of dynamical string tension generation is also discussed. [ABSTRACT FROM AUTHOR]

Here, we present a "holographic" reconstruction of bulk spacetime geometry using correlation functions of a massless field living at the "future boundary" of the spacetime, namely future null infinity ℐ + . It is holographic in the sense that there exists a one-to-one correspondence between correlation functions of a massless field in four-dimensional spacetime ℳ and those of another massless field living in three-dimensional null boundary ℐ + . The idea is to first reconstruct the bulk metric g μ ν by "inverting" the bulk correlation functions and re-express the latter in terms of boundary correlators via the correspondence. This effectively allows asymptotic observers close to ℐ + to reconstruct the deep interior of the spacetime using only correlation functions localized near ℐ + . [ABSTRACT FROM AUTHOR]

Inspired by the universality of computation, we advocate for a principle of spacetime complexity, where gravity arises as a consequence of spacetime optimizing the computational cost of its own quantum dynamics. This principle is explicitly realized in the context of the Anti-de Sitter/Conformal Field Theory correspondence, where complexity is naturally understood in terms of state preparation via Euclidean path integrals, and Einstein's equations emerge from the laws of quantum complexity. We visualize spacetime complexity using Lorentzian threads which, conceptually, represent the operations needed to prepare a quantum state in a tensor network discretizing spacetime. Thus, spacetime itself evolves via optimized computation. [ABSTRACT FROM AUTHOR]

In several approaches to evading the information paradox, the semiclassical black hole is replaced by an Exotic Compact Object (ECO). It has been conjectured that gravitational waves emitted by the merger of ECOs can reflect off the ECOs and produce a detectable 'echo'. We argue that while a part of the wave can indeed reflect off the surface of an ECO, this reflected wave will get trapped by a new closed trapped surface produced by its own backreaction. Thus, no detectable signal of the echo will emerge to infinity. The only assumption in this analysis is that causality is maintained to leading order in gently curved spacetime. Thus, if echoes are actually detected, then we would face a profound change in our understanding of physics. [ABSTRACT FROM AUTHOR]

This work analyzes the particle motion in the van Stockum spacetime considering the existence of closed timelike curves (CTCs). Test particles with or without angular momentum are studied in the present geometry. It is found that only nonzero angular momentum test particles exist in the neighborhood of CTCs. The minimum particle energy and the range of backward time jump in closed timelike geodesics (CTGs) are studied within a Cauchy horizon. Finally, a general prescription for CTC and CTG has been presented with appropriate examples. [ABSTRACT FROM AUTHOR]

In this note, a time-dependent spacetime is explored in the background of f (R , T) gravity via the gravitational collapse of a massive star. The star is modeled by the Vaidya spacetime which is time-dependent in nature. The coupling of matter with curvature is the key feature of f (R , T) theory and here we have investigated its effects on a collapsing scenario. Two different types of models, one involving minimal and the other involving nonminimal coupling between matter and curvature are considered for our study. Power law and exponential functionalities are considered as examples to check the outcome of the gravitational collapse. A detailed analysis on the appearance of horizons in Vaidya spacetime is performed and its astrophysical implications are explored. Our prime objective is to explore the nature of singularities (black hole or naked singularity) that form as an end state of the collapse. Existence of outgoing radial null geodesics from the central singularity was probed and such existence implied the formation of naked singularities thus defying the cosmic censorship hypothesis. The absence of such outgoing null geodesics would imply the formation of an event horizon and the singularity formed becomes a black hole. Conditions under which such possibilities occur are derived for all the models and sub-models. Gravitational strength of the singularity is also investigated and the conditions under which we can get a strong or a weak singularity is derived. The results obtained are very interesting and may be attributed to the coupling between curvature and matter. It is seen that for nonminimal coupling there is a possibility of a globally naked singularity, whereas for a minimal coupling scenario local nakedness is the only option. It is also found that the singularity formed can be sufficiently weak in nature, which is cosmologically desirable. [ABSTRACT FROM AUTHOR]

We would first show that how the two fundamental constants, the velocity of light, c , and the so called, cosmological constant, Λ , arise from geometric characterization of forcefree state of spacetime. They are the two most fundamental constants as they have been synthesized in the structure of spacetime, no other constant can claim this degree of fundamentalness. Next when spacetime turns inhomogeneous, Einstein gravity arises most naturally from the spacetime geometry without any prescription from outside; i.e. the inverse square law is mandated by the geometry, and not a prescription. It would then suggest that dynamics of a fundamental force should not be prescribed but should perhaps be determined by the geometry of appropriate spacetime manifold. [ABSTRACT FROM AUTHOR]

In this work, we study quantum metrology protocol in the context of relativistic quantum fields in curved spacetimes. In particular, we restrict our attention to the isotropic and anisotropic models. We investigate how the oscillatory anisotropy affects a quantum metrology protocol. Here, the oscillatory anisotropy is incorporated as a small gravitational disturbance. We calculate the Fisher information for both cosmological parameters (expansion volume) and ρ (expansion rate). We find that, unlike the isotropic case, the Fisher information of both the parameters and ρ are oscillating functions of the momentum k. The oscillatory spectra of Fisher information can theoretically be understood as results of the dependence of the Bogoliubov coefficients on the anisotropic oscillations. In principle, this framework can be used to infer about the underlying spacetime structure and examine the effects of the universe scale factor oscillations. [ABSTRACT FROM AUTHOR]

We continue a study by Adler and Ramazanoǧlu (AR) of "black" holes as modified by a scale invariant dark energy action. For the spherically symmetric Schwarzschild-like case, AR found that there is no event horizon; hence spacetime is not divided by the "black" hole into causally disconnected regions. We review the formalism for locating trapped surfaces and apparent horizons, and show that the modified "black" hole has no trapped surfaces. Thus, one suspects that it is "leaky," and that there will be a "black hole wind" of particles streaming out from the location of the nominal horizon. This will have astrophysical consequences, for example, the wind may feed and stabilize star formation in the vicinity of the "black" hole. We initiate a study of the stationary axially-symmetric rotating "black" hole as modified by a scale invariant dark energy action, i.e. one that is Kerr-like. To set-up the axial case, we note that the conserving completion of the dark energy stress-energy tensor can be calculated algebraically by solving a 2 × 2 matrix equation. Using Mathematica we calculate and simplify the modified system of Einstein equations for the axial case in quasi-isotropic coordinates, and give appropriate boundary conditions. Solution of these equations, which may require developing a special purpose numerical program, is rendered tricky by a residual general coordinate invariance of quasi-isotropic coordinates. This leads to the interesting mathematical question of representing a positive planar function as the gradient squared of a harmonic function. [ABSTRACT FROM AUTHOR]

For distant observers, black holes are trapped spacetime domains bounded by apparent horizons. We review properties of the near-horizon geometry emphasizing the consequences of two common implicit assumptions of semiclassical physics. The first is a consequence of the cosmic censorship conjecture, namely, that curvature scalars are finite at apparent horizons. The second is that horizons form in finite asymptotic time (i.e. according to distant observers), a property implicitly assumed in conventional descriptions of black hole formation and evaporation. Taking these as the only requirements within the semiclassical framework, we find that in spherical symmetry only two classes of dynamic solutions are admissible, both describing evaporating black holes and expanding white holes. We review their properties and present the implications. The null energy condition is violated in the vicinity of the outer horizon and satisfied in the vicinity of the inner apparent/anti-trapping horizon. Apparent and anti-trapping horizons are timelike surfaces of intermediately singular behavior, which manifests itself in negative energy density firewalls. These and other properties are also present in axially symmetric solutions. Different generalizations of surface gravity to dynamic spacetimes are discordant and do not match the semiclassical results. We conclude by discussing signatures of these models and implications for the identification of observed ultra-compact objects. [ABSTRACT FROM AUTHOR]

The latest findings of static and spherically symmetric black hole solution give a potential platform to investigate the novel four-dimensional Einstein Gauss–Bonnet gravity. In order to obtain a rotating black hole solution, we first adopt the Newman Janis algorithm and study the structure of its horizons. To analyze the said black hole shadow, we move forward to compute expressions of the celestial coordinates using the geodesic equations. Furthermore, we provide a detailed analysis of the shadow size and its distortion parameter, adopting the Hioki and Maeda method, together with the applications of a supermassive black hole shadow in the center of nearby galaxy Messier 87 and obtained constraints on the relationships of spin and charge. From the obtained results, we demonstrate that both spin and coupling parameters of the black hole have a substantial influence on shadow structure. The increment in the values of these parameters diminishes the shadow radius. We also study the energy emission rate using the Hawking temperature. Furthermore, it is shown that whenever the collision of two electrically neutral test particles takes place in the vicinity of the black hole horizon, the Gauss–Bonnet black hole may serve as a particle accelerator with an infinitely large center of mass energy. [ABSTRACT FROM AUTHOR]

In this paper, we study dynamics of massless (photon and neutrino-like) particles in the spacetime of regular Bardeen black hole (BH) in novel four-dimensional Einstein–Gauss–Bonnet (4D EGB) theory. First, we have analyzed the hor properties of the spacetime of the Bardeen BH with respect to the Gauss–Bonnet (GB) coupling parameter α. Our detailed analysis has shown that the GB coupling parameter is limited by maximum and minimum values at, α ∈ (− 0. 1 5 ÷ 1) and the extreme value of magnetic charge of the BH depends on the parameter α. We have explored motion of neutrino-like particles and photons around the Bardeen BH. We have also explored the possibility of mimicking cases of the spin of Kerr BH and charge of Reissner–Nordström BH by the Bardeen charge, providing the same values of impact parameter for photons corresponding to the BH shadow size. Finally, we have investigated spherically infalling accretion and shadow of the Bardeen BHs in 4D EGB theory. [ABSTRACT FROM AUTHOR]

The main goal of this paper is to present the Lemaître–Tolman–Bondi (LTB) spacetime for the radiating case by extending the study of [L. Herrera, A. Di Prisco, J. Ospino and J. Carot, Phys. Rev. D 82 (2010) 024021] in f (R , T) theory, where R is the Ricci scalar and T is the energy–momentum tensor trace. We will start with some fundamental equations, such as the kinematical variables, mass function introduced by Misner and Sharp, the Weyl tensor under the influence of the f (R , T) modified Bianchi identities. We have also considered the evolution of the modified constraint equation. Later on, we have analyzed LTB in terms of modified structure scalars emerging from the orthogonal splitting of the Riemann tensor, and the transport equation in f (R , T) gravity theory. We will also discuss a case in order to generalize LTB metric. [ABSTRACT FROM AUTHOR]

In this paper, we present a generalization of Rastall's gravity in which the conservation law of the energy–momentum tensor is altered, and as a result, the trace of the energy–momentum tensor is taken into account together with the Ricci scalar in the expression for the covariant derivative. Afterwards, we obtain the field equations in this theory and solve them by considering a spherically symmetric spacetime. We show that the external solution has two possible classes of solutions with spherical symmetry in the vacuum in generalized Rastall's gravity, and we analyze one of them explicitly. The generalization, in contrast to constant value k = 8 π G in general relativity, has a gravitational parameter k that depends on the Rastall constant α. As an application, we perform a careful analysis of the effects of the theory on neutron stars using realistic equations of state (EoS) as input. Our results show that important differences on the profile of neutron stars are obtained within two representatives EoS. [ABSTRACT FROM AUTHOR]

This paper focuses on the new approach to the nature of a nut parameter n , usually treated as a dual charge to the mass. Instead of thinking about it as a gravitational analog of the magnetic monopole, we will show that the Taub–NUT spacetime conceals a peculiar rotation coming from the Misner strings depending on the nut value. Besides reconciling several confusing results present in the literature, this allows us to establish more the less a consistent description of the black hole thermodynamics for the Lorentzian Taub–NUT spacetime with essential contribution from the Misner strings to the angular momentum and total entropy. [ABSTRACT FROM AUTHOR]

In this paper, we consider a specific model, implementing the existence of a fundamental limit distance L 0 between (space or time separated) points in spacetime, which in the recent past has exhibited the intriguing feature of having a minimum-length Ricci scalar R (q) that does not approach the ordinary Ricci scalar R in the limit of vanishing L 0 . R (q) at a point has been found to depend on the direction along which the existence of minimum distance is implemented. Here, we point out that the convergence R (q) → R in the L 0 → 0 limit is anyway recovered in a relaxed or generalized sense, which is when we average over directions, this suggesting we might be taking the expectation value of R (q) promoted to be a quantum variable. It remains as intriguing as before the fact that we cannot identify (meaning this is much more than simply equating in the generalized sense above) R (q) with R in the L 0 → 0 limit, namely, when we get ordinary spacetime. Thing is like if, even when L 0 (read here the Planck length) is far too small to have any direct detection of it feasible, the intrinsic quantum nature of spacetime might anyway be experimentally at reach, witnessed by the mentioned special feature of Ricci, not fading away with L 0 (i.e. persisting when taking the ℏ → 0 limit). [ABSTRACT FROM AUTHOR]

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

In this work, we present all the possible solutions for a static cylindrical symmetric spacetime in the Einstein-Aether (EA) theory. As far as we know, this is the first work in the literature that considers cylindrically symmetric solutions in the theory of EA. One of these solutions is the generalization in EA theory of the Levi-Civita (LC) spacetime in General Relativity (GR) theory. We have shown that this generalized LC solution has unusual geodesic properties, depending on the parameter c 1 4 of the aether field. The circular geodesics are the same of the GR theory, no matter the values of c 1 4 . However, the radial and z -direction geodesics are allowed only for certain values of σ and c 1 4 . The z -direction geodesics are restricted to an interval of σ different from those predicted by the GR and the radial geodesics show that the motion is confined between the origin and a maximum radius. The latter is not affected by the aether field but the velocity and acceleration of the test particles are besides, for 0 ≤ σ < 1 / 2 , when the cylindrical symmetry is preserved, this spacetime is singular at the axis r = 0 , although for σ > 1 / 2 exists interval of c 1 4 where the spacetime is not singular, which is completely different from that one obtained with the GR theory, where the axis r = 0 is always singular. [ABSTRACT FROM AUTHOR]

A sufficiently massive star in the end of its life will inevitably collapse into a black hole as more deconfined degrees of freedom make the core ever softer. One possible way to avoid the singularity in the end is by dimensional phase transition of spacetime. Indeed, the black hole interior, two-dimensional (2D) in nature, can be described well as a perfect fluid of free massless Majorana fermions and gauge bosons under a 2D supersymmetric mirror model with new understanding of emergent gravity from dimensional evolution of spacetime. In particular, the 2D conformal invariance of the black hole gives rise to desired consistent results for the interior microphysics and structures including its temperature, density and entropy. [ABSTRACT FROM AUTHOR]

It is possible that both the classical description of spacetime and the rules of quantum field theory emerge from a more-fundamental structure of physical law. Pregeometric frameworks transfer some of the puzzles of quantum gravity to a semiclassical arena where those puzzles pose less of a challenge. However, in order to provide a satisfactory description of quantum gravity, a semiclassical description must emerge and contain in its description a macroscopic spacetime geometry, dynamical matter, and a gravitational interaction consistent with general relativity at long distances. In this essay, we argue that a framework that includes a stochastic origin for quantum field theory can provide both the emergence of classical spacetime and a quantized gravitational interaction. [ABSTRACT FROM AUTHOR]

The gravitational dynamics of the coarse-grained spacetime geometry should emerge from extremizing the number of microscopic configurations, Ω , of the pre-geometric variables corresponding to a given geometry. This Ω will be the product over all events of the density, ρ () , of microscopic configurations associated with each event . I show how ρ can be computed, in terms of the van Vleck determinant, and thus obtain directly the gravitational effective action ℒ E at mesoscopic scales. The leading term of this, nonperturbative, effective action gives the Einstein–Hilbert action, thereby providing its microscopic derivation. The higher-order corrections are finite without any need for regularization and I demonstrate how they can be computed in a systematic manner. [ABSTRACT FROM AUTHOR]

Quantum gravity effects are traditionally tied to short distances and high energies. In this essay, we argue that, perhaps surprisingly, quantum gravity may have important consequences for the phenomenology of the infrared. We center our discussion around a conception of quantum gravity involving a notion of quantum spacetime that arises in metastring theory. This theory allows for an evolution of a cosmological Universe in which string-dual degrees of freedom decouple as the Universe ages. Importantly, such an implementation of quantum gravity allows for the inclusion of a fundamental length scale without introducing the fundamental breaking of Lorentz symmetry. The mechanism seems to have potential for an entirely novel source for dark matter/energy. The simplest observational consequences of this scenario may very well be residual infrared modifications that emerge through the evolution of the Universe. [ABSTRACT FROM AUTHOR]