Theory Of Gravity Research Articles

Gravity and [formula omitted] flows in higher dimensions

We study systems in arbitrary space-time dimensions where matter, deformed by TT‾-like irrelevant operators, is coupled to gravity in the Palatini formalism. The dynamically equivalent perspective is investigated, wherein the deformation transitions from the matter action to the gravitational one or vice versa. This alternative viewpoint leads to the emergence of Ricci-based gravity theories, thus providing a high-dimensional generalisation of the well-known equivalence between two-dimensional TT‾ deformations and coupling to Jackiw-Teitelboim gravity. This dynamical equivalence is examined within the framework of the recently introduced Lagrangian flow equation, which notably led to the discovery of a direct link between Nambu-Goto theory and TT‾ in d=2, as well as significant insights into nonlinear electrodynamics models in d=4. The investigation involves explicit examples in d=4 dimensions; it builds upon earlier research concerning the metric interpretation of TT‾-like perturbations, incorporates and extends recent findings in the cosmology-related literature associated to the concept of reframing. We focus on scenarios where the resulting modified gravity theories manifest as Born-Infeld and Starobinsky types.

Topology change and non-geometry at infinite distance

The distance conjecture diagnoses viable low-energy effective realisations of consistent theories of quantum gravity by examining their breakdown at infinite distance in their parameter space. At the same time, infinite distance points in parameter space are naturally intertwined with string dualities. We explore the implications of the distance conjecture when T-duality is applied to curved compact manifolds and in presence of (non-)geometric fluxes. We provide evidence of how divergent potentials signal pathological infinite distance points in the scalar field space where towers of light states cannot be sustained by the curved background. This leads us to suggest an extension to the current statement of the Swampland distance conjecture in curved spaces or in presence of non-trivial fluxes supporting the background.

Thermodynamic features of AdS black holes within the rastall gravity and perfect fluid matter framework

In this study, we analyzed the impact of a perfect fluid on the phase transition of Anti-de Sitter (AdS) black holes within the Rastall gravitational background. Compared to similar studies in the literature, the findings of this work are highlighted by the determination of analytical expressions for critical points for charged and Kerr-Newman AdS black holes using approximated formulas for the horizon radius. An accurate analysis of these new analytical expressions allowed us to discover a new viable condition that relates the Rastall parameter κ λ to the equation of state parameter ω, expressed as: κλ=ω1+ω . Thanks to this new condition, we were able to reproduce all the analytical expressions of critical points calculated within the framework of Einsteinâs general relativity for two cases: a charged AdS black hole and a rotating AdS black hole. These findings suggest that Rastall gravity, considering this new condition, could serve as an alternative theory of gravitation to general relativity. The approximate expression of the horizon radius also enabled the exploration of the distinctiveness of fractional-order phase transitions in these AdS black holes. Furthermore, we calculated the critical exponents, offering insights into the behavior of crucial thermodynamic quantities near the inflection point. Examining how a perfect fluid influences phase transition reveals various critical behaviors, demonstrating that the variation in the phase transition depends on the intensity of the perfect fluid. Notably, this variation is portrayed by a linearly increasing trajectory with the escalation of this intensity.

Exploring wormholes in f(R,T) gravity

The concept of wormholes is still hypothetical in physics but attractive among researchers. In this work, motivated by the important effect of [Formula: see text] 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.

Dynamical system analysis of LRS-BI Universe with [formula omitted] gravity theory

Considering the Universe as a dynamic system, the understanding of its evolution is an interesting aspect of study in cosmology. Here, we investigate the anisotropic locally rotationally symmetric (LRS) Bianchi type-I (LRS-BI) spacetime under the f(Q) gravity of symmetric teleparallel theory equivalent to the GR (STEGR) as a dynamical system and try to understand the role of anisotropy in the evolution of its various phases. For this work, we consider two models, viz., f(Q)=−(Q+2Λ) and f(Q)=−βQn to study the critical points and stability of the LRS-BI Universe. In both the models, it is found that the various phases like radiation-dominated, matter-dominated, and dark energy-dominated phases are heteroclinically connected, and there is some role of anisotropy in the evolution of these phases of the Universe. However, for both the models, there are some unphysical solutions too depending upon the values of model parameters β and n along with the anisotropic parameter α.

Consistency of eight-dimensional supergravities: anomalies, lattices and counterterms

We reexamine the question of quantum consistency of supergravities in eight dimensions. Theories with 16 supercharges suffer from the anomalies under the action of its discrete modular groups. In minimally supersymmetric theory coupled to Yang-Mills multiples of rank l with the moduli space given by SO(2, l)/(U(1) × SO(l)), the existence of a counterterm together with the requirement that its poles and zeros correspond to the gauge symmetry enhancement imposes nontrivial constraints on the lattice. The counterterms needed for anomaly cancellation for all cases, that are believed to lead to consistent theories of quantum gravity (l = 2, 10, 18), are discussed.

Modification entropy of Kerr–Sen-like black hole in Lorentz-breaking bumblebee gravity

The Lorentz symmetry breaking theory not only affects the space–time background but also the dynamic behavior of bosons and fermions in curved space–time. Therefore, the Lorentz symmetry breaking theory will affect the quantum tunneling rate, Hawking temperature, black hole entropy, and other physical quantities of black holes. According to the modification of the space–time background and the modification of the particle dynamic equations, the quantum tunneling radiation of the Kerr–Sen-like black hole in bumblebee gravitational theory and its related contents are deeply studied. The research methods and a series of new results obtained in this paper are discussed. This makes the research methods and conclusions in this paper of more astrophysical significance and reference value.

New infinite class of 6d , N=(1,0) supergravities

We present a new infinite class of non-Abelian, 6d supergravities with eight supercharges. These theories not only satisfy all known low-energy consistency conditions, such as being free of anomalies, but also evade the constraints arising from the consistency of string probes, even after assuming completeness of the Bogomol’nyi–Prasad–Sommerfield (BPS) spectrum. This demonstrates that some additional UV input or hitherto unknown IR condition is needed in order to be left with a finite landscape, as is generally anticipated from a theory of quantum gravity. Published by the American Physical Society 2024

Global flows of foliated gravity-matter systems

Asymptotic safety is a promising mechanism for obtaining a consistent and predictive quantum theory for gravity. The ADM formalism allows to introduce a (Euclidean) time-direction in this framework. It equips spacetime with a foliation structure by encoding the gravitational degrees of freedom in a lapse function, shift vector, and a metric measuring distances on the spatial slices. We use the Wetterich equation to study the renormalization group flow of the graviton 2-point function extracted from the spatial metric. The flow is driven by the 3- and 4-point vertices generated by the foliated Einstein-Hilbert action supplemented by minimally coupled scalar and vector fields. We derive bounds on the number of matter fields cast by asymptotic safety. Moreover, we show that the phase diagram obtained in the pure gravity case is qualitatively stable within these bounds. An intriguing feature is the presence of an IR-fixed point for the graviton mass which prevents the squared mass taking negative values. This feature persists for any number of matter fields and, in particular, also in situations where there is no suitable interacting fixed point rendering the theory asymptotically safe. Our work complements earlier studies of the subject by taking contributions from the matter fields into account.

Aspects of non-topological soliton stars in a class of induced gravity theories

We explore some interesting properties of soliton stars arising in a class of effective gravity theories having a Higgs scalar field that serves a dual role of generating an effective mass of a fermion field as well as generating an effective gravitational constant. With a suitable choice of non-minimal coupling, such solitons have the canonical effective gravitational constant in their exterior while in the interior the effective gravitational constant could be arbitrarily smaller. Such a choice enables an exact determination of gravitational effects on the total conserved energy of the soliton. While the external metric of such a soliton is the standard Schwarzschild metric, the interior entropy is much larger than that of a black hole of the same equilibrium temperature.

Emergent cosmology in 4D Einstein Gauss Bonnet theory of gravity

In this paper, in an FLRW background and a perfect fluid equation of state, we explore the possibility of the realization of an emergent scenario in a 4D regularized extension of Einstein-Gauss-Bonnet gravity, with the field equations particularly expressed in terms of scalar-tensor degrees of freedom. By assuming non-zero spatial curvature (k = ± 1), the stability of the Einstein static universe (ESU) and its subsequent exit into the standard inflationary scenario is tested through different approaches. In terms of dynamical systems, a spatially closed universe rather than an open universe shows appealing behaviour to exhibit a graceful transition from the ESU to standard cosmological history. We found that under linear homogeneous perturbations, for some constraints imposed on the model parameters, the ESU is stable under those perturbations. Moreover, it is noted that for a successful graceful transition, the equation of state ω must satisfy the conditions −1 < ω < 0 and ω < − 1 for closed and open universes, respectively. Furthermore, the ESU is seen to be neutrally stable under matter perturbation in the Newtonian gauge.

Possible dark energy stars in Rastall gravity

Observational evidence revealed that in the composition of the Universe, the dark energy is in heap proportion which is mainly responsible for the accelerated expansion of the Universe. The astrophysicists hypothesized that every astrophysical object is likely interact with the dark energy Sakti and Sulaksono [Phys. Rev. D 103 (2021) 084042]. Therefore, researchers are intended to study the astrophysical objects with dark energy interaction. In this paper, we proposed a dark energy stars (composed of with dark matter and ordinary matter) model in the Rastall theory of gravity. The Rastall field equations are obtained in the Tolman–Kuchowicz-type metric. To solve the field equations, the arbitrary constants involved in the field equations (due to Tolman–Kuchowicz ansatz) are obtained by matching the interior geometry with the exterior Schwarzschild geometry. The obtained solution is tested physically by computing some analytical results and plotting graphs of different physical parameters like pressure, density for ordinary matter, density for dark matter, mass function, compactness and surface redshift. Stability of the model is also analyzed by different tests like velocity of sound, adiabatic index and TOV equations. The influence of the Rastall parameter on the model has been studied by plotting graphs and computing numerical values of model’s physical parameters for different values of the Rastall parameter. We infer that our propounded model fulfills all the necessary requirements for a physically plausible model.

Gravitational Wormholes

Spacetime wormholes are evidently an essential component of the construction of a time machine. Within the context of general relativity, such objects require, for their formation, exotic matter—matter that violates at least one of the standard energy conditions. Here, we explore the possibility that higher-curvature gravity theories might permit the construction of a wormhole without any matter at all. In particular, we consider the simplest form of a generalized quasi topological theory in four spacetime dimensions, known as Einsteinian Cubic Gravity. This theory has a number of promising features that make it an interesting phenomenological competitor to general relativity, including having non-hairy generalizations of the Schwarzschild black hole and linearized equations of second order around maximally symmetric backgrounds. By matching series solutions near the horizon and at large distances, we find evidence that strong asymptotically AdS wormhole solutions can be constructed, with strong curvature effects ensuring that the wormhole throat can exist.

Accelerating universe with wet dark fluid in modified theory of gravity

In the present work we have focused on the investigation of LRS Bianchi type –I metric within the framework of f(R) theory of gravity filled with wet dark fluid as the candidate of dark energy. The solution of the metric is an accelerating universe, derived by assuming the negative constant deceleration parameter and utilizing a power law relation. Additionally, we have visually analyzed the metric's dynamical and geometrical properties through graphical representations.

A study of layered holographic superconductor

We have investigated a holographic model of a multi-layered superconductor in (2+1)-dimensions using the AdS/CFT correspondence. This correspondence allows us to study strongly interacting condensed matter systems through a weakly interacting gravitational theory. Our study focused on the effects of a finite system size on the superconductor’s properties. We observed significant variations in the critical temperature and critical magnetic field depending on the number of layers and the spacing between them. Notably, our results capture the transition from a two-dimensional (2D) to a three-dimensional (3D) behavior in the limit of a large number of closely spaced layers. This transition signifies a change in how the superconductivity operates within real material.

Cosmology in non-coincident gauge formulation of f(Q,C) gravity theory

This paper is an investigation of dark energy cosmological models in non-coincident gauge formulation of non-metricity gravity with boundary term. We have considered an arbitrary function [Formula: see text], where [Formula: see text] is the non-metricity scalar, [Formula: see text] is the boundary term derived by [Formula: see text], and [Formula: see text] are the model parameters, to obtain the modified field equations from action. We have parameterized the Hubble function as [Formula: see text] with [Formula: see text] are arbitrary constants and [Formula: see text] is the scale-factor depending upon cosmic time. We have compared this Hubble function with [Formula: see text] datasets and obtained best fit values of model parameters using likelihood analysis. Using these best fit values of model parameters, we have done our result analysis and discussion with cosmological parameters viz effective equation of state parameter, energy density, energy conditions, deceleration parameter, om diagnostic analysis and age of the universe. We have found a transit phase universe model with effective EoS parameter [Formula: see text] over redshift [Formula: see text].

Unification of conformal gravity and internal interactions

Based on the observation that the dimension of the tangent space is not necessarily equal to the dimension of the corresponding curved manifold and on the known fact that gravitational theories can be formulated in a gauge theoretic way, we discuss how to describe all known interactions in a unified manner. This is achieved by enlarging the tangent group of the four-dimensional manifold to SO(2, 16), which permits the inclusion of both gauge groups, the one that describes gravity as a gauge theory as well as the SO(10) describing the internal interactions. Moreover it permits the use of both Weyl and Majorana conditions imposed on the fermions, as to avoid the duplication of fermion multiplets of SO(10) appearing in previous attempts. The gravity theory discussed in the present work is the Conformal Gravity which, after a spontaneous symmetry breaking, can lead either to Weyl Gravity or to the usual Einstein Gravity.

Extending Finch-Skea isotropic model to anisotropic domain in modified f(, T ) gravity

This paper considers the Finch-Skea isotropic solution and extends its domain to three different anisotropic interiors by using the gravitational decoupling strategy in the context of f(R,T) gravitational theory. For this, we consider that a static spherical spacetime is initially coupled with the perfect matter distribution. We then introduce a Lagrangian corresponding to a new gravitating source by keeping in mind that this new source produces the effect of pressure anisotropy in the parent fluid source. After calculating the field equations for the total matter setup, we apply a transformation on the radial component, ultimately providing two different systems of equations. These two sets are solved independently through different constraints that lead to some new solutions. Further, we consider an exterior spacetime to calculate three constants engaged in the seed Finch-Skea solution at the spherical interface. The estimated radius and mass of a star candidate LMC X-4 are utilized to perform the graphical analysis of the developed models. It is concluded that only the first two resulting models are physically relevant in this modified theory for all the considered parametric choices.

Stringy evidence for a universal pattern at infinite distance

Infinite distance limits in the moduli space of a quantum gravity theory are characterized by having infinite towers of states becoming light, as dictated by the Distance Conjecture in the Swampland program. These towers imply a drastic breakdown in the perturbative regimes of the effective field theory at a quantum gravity cut-off scale known as the species scale. In this work, we find a universal pattern satisfied in all known infinite distance limits of string theory compactifications, which relates the variation in field space of the mass of the tower and the species scale: ∇→mm·∇→ΛspΛsp=1d−2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\frac{\\overrightarrow{\ abla}m}{m}\\cdotp \\frac{\\overrightarrow{\ abla}{\\Lambda}_{\ extrm{sp}}}{\\Lambda_{\ extrm{sp}}}=\\frac{1}{d-2} $$\\end{document} in d spacetime dimensions. This implies a more precise definition of the Distance conjecture and sharp bounds for the exponential decay rates. We provide plethora of evidence in string theory in diverse dimensions and with different levels of supersymmetry, and identify some sufficient conditions that allow the pattern to hold from a bottom-up perspective.

P -brane Galilean and Carrollian geometries and gravities

We study D-dimensional p-brane Galilean geometries via the intrinsic torsion of the adapted connections of their degenerate metric structure. These non-Lorentzian geometries are examples of G-structures whose characteristic tensors consist of two degenerate ‘metrics’ of ranks (p+1) and (D−p−1) . We carry out the analysis in two different ways. In one way, inspired by Cartan geometry, we analyse in detail the space of intrinsic torsions (technically, the cokernel of a Spencer differential) as a representation of G, exhibiting for generic (p, D) five classes of such geometries, which we then proceed to interpret geometrically. We show how to re-interpret this classification in terms of ( D−p−2 )-brane Carrollian geometries. The same result is recovered by methods inspired by similar results in the physics literature: namely by studying how far an adapted connection can be determined by the characteristic tensors and by studying which components of the torsion tensor do not depend on the connection. As an application, we derive a gravity theory with underlying p-brane Galilean geometry as a non-relativistic limit of Einstein–Hilbert gravity and discuss how it gives a gravitational realisation of some of the intrinsic torsion constraints found in this paper. Our results also have implications for gravity theories with an underlying ( D−p−2 )-brane Carrollian geometry.