Gravity Action Research Articles

String-inspired Teleparallel cosmology

The present paper represents an attempt for a very generic string inspired theory of gravitation, based on a stringy action in the teleparallel gravity which includes a specific functional which depends on the scalar field and its kinetic energy, as well as the torsion and boundary terms, embedding also possible effects from the teleparallel Gauss–Bonnet invariants. We focus our study in FLRW cosmology. After we deduce the cosmological equations for the associated generic theory of gravitation, we focus on string inspired couplings which are studied by considering different analytical techniques. The first analytical technique is based on the linear stability theory, by introducing proper dimensionless variables which enables us to study the structure of the phase space and the associated physical effects. In this case we have obtained different cosmological solutions which corresponds to matter and dark energy dominated solutions, achieving a possible transition between matter and dark energy dominated epochs. For each type of cosmological solutions we have discussed the corresponding physical features, attaining viable constraints for the coupling constants due to dynamical effects. The dynamical study of the physical features included also a numerical analysis by fine–tuning the initial conditions deep into the matter era, obtaining possible trajectories for the effective equation of state for specific coupling functions.

Physical Hamiltonian for mimetic gravity

Starting from a local action for mimetic gravity that includes higher derivatives of a scalar field $\phi$, we derive a gauge-fixed canonical action of the theory in the ADM canonical formalism in the time gauge $\phi=t$. This reduced action reveals (i) a non-vanishing conserved physical Hamiltonian that is a sum of two terms, the expression for the Hamiltonian constraint of general relativity and a function of the expansion scalar, and (ii) a reduced symplectic structure that geometrically provides the Dirac brackets. As applications of our general analysis, we compute the physical Hamiltonians and canonical equations for perturbations around Minkowski spacetime, homogeneous cosmologies, and spherically symmetric spacetimes.

Vilkovisky unique effective action in quantum gravity

The divergent part of the one-loop Vilkovisky unique effective action for quantum Einstein gravity is evaluated in the general parametrization of the quantum field, including the separated conformal factor. The output of this calculation explicitly demonstrates the parametrization and conformal gauge independence of the unique effective action with the configuration space metric chosen following Vilkovisky's prescription.

A cohomological gravity in D = 11

A [Formula: see text] cohomological gravity is introduced. The central piece is a new BRST-like symmetry [Formula: see text] with [Formula: see text] up to a diffeomorphism and gauge symmetry. The only nontrivial observable of this symmetry is a Chern–Simons integral. The construction of the symmetry requires a spin 0 fermion [Formula: see text] and a spin [Formula: see text] boson [Formula: see text] at the minimum, in addition to the standard gravity and gauge fields, to close the algebra off-shell. The gravity action is uniquely determined by requiring a field redefinition type of global symmetry between bosonic and fermionic fields. It is of a simple form [Formula: see text] plus a Chern–Simons term, and appears as the [Formula: see text] supergravity action under a special combination of the fields without independent presence of [Formula: see text] and [Formula: see text]. The [Formula: see text]-invariant path-integral measure, which is also invariant under the global symmetry, is shown to exist for the theory to be valid at quantum level. It achieves only when the fields are redefined under certain combination that is different from the special combination, and the fields in this combination are interpreted as physical fields. At [Formula: see text] limit, where all field combinations converges, the theory reduces to [Formula: see text] supergravity and the global symmetry becomes the local supersymmetry with [Formula: see text] as the local variable. At a nonvanishing [Formula: see text] limit set by residue symmetries upon broken [Formula: see text]-symmetry, the theory admits a cosmological solution that has a wavy term to allow accelerated expansion of the universe while not violate null energy condition.

Implications of Einstein–Maxwell dilaton–axion gravity from the black hole continuum spectrum

ABSTRACT String inspired models can serve as potential candidates to replace general relativity (GR) in the high-energy/high-curvature regime where quantum gravity is expected to play a vital role. Such models not only subsume the ultraviolet nature of gravity but also exhibit promising prospects in resolving issues like dark matter and dark energy, which cannot be adequately addressed within the framework of GR. The Einstein–Maxwell dilaton–axion (EMDA) theory that is central to this work is one such string inspired model arising in the low energy effective action of the heterotic string theory with interesting implications in inflationary cosmology and in the late-time acceleration of the Universe. It is therefore important to survey the role of such a theory in explaining astrophysical observations, e.g. the continuum spectrum of black holes which is expected to hold a wealth of information regarding the background metric. The Kerr–Sen space–time corresponds to the exact, stationary, and axisymmetric black hole solution in EMDA gravity, possessing dilatonic charge and angular momentum originating from the axionic field. In this work, we compute the theoretical spectrum from the accretion disc around quasars in the Kerr–Sen background assuming the thin accretion disc model due to Novikov and Thorne. This is then used to evaluate the theoretical estimates of optical luminosity for a sample of eighty Palomar Green quasars which are subsequently compared with the available observations. Our results based on χ2 analysis indicate that the dilaton parameter r2 ∼ 0.2 is favoured by optical observations of quasars which is further corroborated by other error estimators e.g. the Nash–Sutcliffe efficiency, the index of agreement and their modified versions. We further report that strong dilaton charges (r2 > 1.6) are disfavoured by quasar optical data and the spins associated with the quasars are also estimated.

Carroll versus Galilei from a brane perspective

We show that our previous work on Galilei and Carroll gravity, apt for particles, can be generalized to Galilei and Carroll gravity theories adapted to p-branes (p = 0, 1, 2, ⋯). Within this wider brane perspective, we make use of a formal map, given in the literature, between the corresponding p-brane Carroll and Galilei algebras where the index describing the directions longitudinal (transverse) to the Galilei brane is interchanged with the index covering the directions transverse (longitudinal) to the Carroll brane with the understanding that the time coordinate is always among the longitudinal directions. This leads among other things in 3D to a map between Galilei particles and Carroll strings and in 4D to a similar map between Galilei strings and Carroll strings. We show that this formal map extends to the corresponding Lie algebra expansion of the Poincaré algebra and, therefore, to several extensions of the Carroll and Galilei algebras including central extensions. We use this formal map to construct several new examples of Carroll gravity actions. Furthermore, we discuss the symmetry between Carroll and Galilei at the level of the p-brane sigma model action and apply this formal symmetry to give several examples of 3D and 4D particles and strings in a curved Carroll background.

An action principle for the Einstein–Weyl equations

A longstanding open problem in mathematical physics has been that of finding an action principle for the Einstein–Weyl (EW) equations. In this paper, we present for the first time such an action principle in three dimensions in which the Weyl vector is not exact. More precisely, our model contains, in addition to the Weyl nonmetricity, a traceless part. If the latter is (consistently) set to zero, the equations of motion boil down to the EW equations. In particular, we consider a metric affine f(R) gravity action plus additional terms involving Lagrange multipliers and gravitational Chern–Simons contributions. In our framework, the metric and the connection are considered as independent objects, and no a priori assumptions on the nonmetricity and the torsion of the connection are made. The dynamics of the Weyl vector turns out to be governed by a special case of the generalized monopole equation, which represents a conformal self-duality condition in three dimensions.

A note on the definition of gravitational energy for quadratic curvature gravity via topological regularization* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11865006 and 11847152).

Within the framework of four-dimensional quadratic curvature gravities in the appearance of a negative cosmological constant, a definition for the gravitational energy of solutions with anti-de Sitter (AdS) asymptotics was put forward by Giribet et al. [Phys. Rev. D 98 044046 (2018)]. This was achieved by adding proper topological invariant terms to the gravity action to render the variation problem well-posed. We prove that the definition via the procedure of topological regularization can be covered by our previous work [Int. J. Mod. Phys. A 35 2050102 (2020)] in four dimensions. Motivated by this, we further generalize the results to generic diffeomorphism invariant theories of gravity in arbitrary even dimensions.

Non-relativistic limits and three-dimensional coadjoint Poincaré gravity

We show that a recently proposed action for three-dimensional non-relativistic gravity can be obtained by taking the limit of a relativistic Lagrangian that involves the coadjoint Poincaré algebra. We point out the similarity of our construction with the way that three-dimensional Galilei gravity and extended Bargmann gravity can be obtained by taking the limit of a relativistic Lagrangian that involves the Poincaré algebra. We extend our results to the anti-de Sitter case and we will see that there is a chiral decomposition at both the relativistic and non-relativistic level. We comment on possible further generalizations.

Experimental assessment and numerical study of a pump-assisted loop heat pipe for high capacity thermal systems

In the present work, the results of a study and experimental activities carried out on a pump-assisted heat pipe intended for the use in large-capacity thermal conversion and storage systems are presented. A specific test set-up for the experimental assessment of heat pipes operating in a loop configuration was realised. The key feature of the system is the use of a pump for liquids for the transport of the condensate, thus distinguishing it from the vast majority of commercial or research-grade heat pipes, which employ gravity or capillary action for liquid transport. The set-up is equipped with high-accuracy sensors and allows continuous operation. Results of an experimental campaign realised with R600 as working fluid inside the heat pipes are reported, analysing the effect of temperatures at the evaporator and condenser, flow rate of the working fluid and filling ratio and a correlation was derived to predict the experimental outcomes. The examined system was able to transfer up to 3 kW under the selected boundary conditions with an optimal filling ratio of 46%.

Four-dimensional gravity on a covariant noncommutative space

We formulate a model of noncommutative four-dimensional gravity on a covariant fuzzy space based on SO(1, 4), that is the fuzzy version of the dS4. The latter requires the employment of a wider symmetry group, the SO(1, 5), for reasons of covariance. Addressing along the lines of formulating four-dimensional gravity as a gauge theory of the Poincaré group, spontaneously broken to the Lorentz, we attempt to construct a four-dimensional gravitational model on the fuzzy de Sitter spacetime. In turn, first we consider the SO(1, 4) subgroup of the SO(1, 5) algebra, in which we were led to, as we want to gauge the isometry part of the full symmetry. Then, the construction of a gauge theory on such a noncommutative space directs us to use an extension of the gauge group, the SO(1, 5)×U(1), and fix its representation. Moreover, a 2-form dynamic gauge field is included in the theory for reasons of covariance of the transformation of the field strength tensor. Finally, the gauge theory is considered to be spontaneously broken to the Lorentz group with an extension of a U(1), i.e. SO(1, 3)×U(1). The latter defines the four-dimensional noncommutative gravity action which can lead to equations of motion, whereas the breaking induces the imposition of constraints that will lead to expressions relating the gauge fields. It should be noted that we use the Euclidean signature for the formulation of the above programme.

R\xe9nyi entropies and area operator from gravity with Hayward term

In the context of the holographic duality, the entanglement entropy of ordinary QFT in a subregion in the boundary is given by a quarter of the area of an minimal surface embedded in the bulk spacetime. This rule has been also extended to a suitable one-parameter generalization of the von-Neuman entropy Ŝn that is related to the Rényi entropies Sn, as given by the area of a cosmic brane minimally coupled with gravity, with a tension related to n that vanishes as n → 1, and moreover, this parameter can be analytically extended to arbitrary real values. However, the brane action plays no role in the duality and cannot be considered a part of the theory of gravity, thus it is used as an auxiliary tool to find the correct background geometry.In this work we study the construction of the gravitational (reduced) density matrix from holographic states, whose wave-functionals are described as euclidean path integrals with arbitrary conditions on the asymptotic boundaries, and argue that in general, a non-trivial Hayward term must be haven into account. So we propose that the gravity model with a coupled Nambu-Goto action is not an artificial tool to account for the Rényi entropies, but it is present in the own gravity action through a Hayward term. As a result we show that the computations using replicas simplify considerably and we recover the holographic prescriptions for the measures of entanglement entropy; in particular, derive an area law for the original Rényi entropies (Sn) related to a minimal surface in the n replicated spacetime. Moreover, we show that the gravitational modular flow contains the area operator and can explain the Jafferis-Lewkowycz-Maldacena-Suh proposal.

Hydrophobic-hydrophilic balance of ZSM-5 zeolites on the two-phase ketalization of glycerol with acetone

Herein, ZSM-5 zeolites of several Si/Al mole ratios, and varying hydrophobicity were used as catalysts in the ketalization of glycerol. The water formed in the reaction adsorbs on acidic sites, hampering the catalytic conversion of glycerol. The conversion drops even more by adding water in the reaction mixture, intensifying the adsorption in hydrophilic zeolites. However, the samples with the higher Si/Al ratio and abundant hydrophobic ≡Si-O-Si≡ linkages had the uppermost turnover frequency. The reaction proceeds at the interface of the glycerol and acetone phases. Since the catalyst preferentially stays in the glycerol phase due to gravity action, the full catalytic action of the catalyst is hindered. Contact angle measurements with the glycerol/zeolite systems confirmed that the hydrophobic zeolite had the lowest affinity with glycerol, thus improving the interfacial solubilization of glycerol/acetone in the vicinity of active sites turning it much more efficient. The catalytic action was further enhanced by partially ion-exchanging external acid sites with minor quantities of bulky n-hexylammonium cations, turning the zeolite more hydrophobic and a strong candidate for other acid-catalyzed interfacial liquid-phase reactions.

Effect of the Earth’s Rotation on Wellbore Trajectory and Method of Correction

In this paper the method of mechanical modeling and formula derivation is applied to explains how earth rotation causes the apparent gravity action line to deviate from the ground plumb line. Analyzing the methods of the compass pendulum and the three-accelerometer and three-magnetometer survey, this paper demonstrates the principle that earth rotation causes well inclination error and azimuth error Based on the constructed simulated drilling trajectory, the actual measured inclination angle, azimuth angle, and measured depth errors are calculated. The paper presents methods for correcting the deviation error of the compass pendulum survey method and the deviation and azimuth error of the while-drilling survey method. Finally, the correcting method is applied to the actual measured data. The accuracy and reliability of the proposed error correcting method is verified by comparing the corrected data and actual data.

Extended galactic rotational velocity profiles in f(R) gravity background

An attempt has been made to explore the geometric effects of f(R) action on the galactic dynamics under the weak field approximation. The rotational velocity is calculated beyond the Einstein’s geometric theory of gravity. It is inspired by the cosmological geometric relation obtained in the power-law f(R) gravity model in vacuum. We analyse the action with a small positive deviation from the Einstein–Hilbert gravity action (taking R as f(R)propto R^{1+delta }) at the galactic scales for the explanation of the flatness paradox associated with the clustered galactic dark matter. We obtain the contribution of a dynamical f(R) cosmological background geometry on accelerating the test mass. Furthermore, the integrated effective acceleration of the test mass due to a massive spherically symmetric source in f(R) background is calculated via the study of geodesics for the suitable spacetime metric and an equation for the effective rotational velocity has been developed. We test the viability of the proposed model by tracing the motion of a test mass far from the disk of galactic matter for smaller delta . The possible galactic rotational velocity curves in f(R) background are discussed for the formula obtained with delta<< 1. We also obtain constraints on delta O(10^{-6}) confirmed by observations.

Hamilton–Jacobi analysis of the Freidel–Starodubtsev BF (A)dS gravity action

In this work, we perform the Hamilton–Jacobi analysis of a modified gravity action, the so-called Freidel–Starodubtsev model. The complete set of involutive Hamiltonians that guarantee the system’s integrability is obtained. The generalized Poisson brackets are calculated in the metric phase by means of a suitable constraint matrix inversion. We also present a discussion about the metric and non-metric degrees of freedom. From the fundamental differential we recover the equations of motion and explicitly obtain the generators of local Lorentz transformations and also diffeomorphisms for the tetrad and the spin connection fields.

Brane gravity in 4D from Chern\u2013Simons gravity theory

We evaluate a 5-dimensional Randall Sundrum type metric in the Lagrangian of the Einstein–Chern–Simons gravity, and then we derive an action and its corresponding field equations, for a 4-dimensional brane embedded in the 5-dimensional space-time of the theory, which in the limit lrightarrow 0 leads to the 4-dimensional general relativity with cosmological constant. An interpretation of the h^{a} matter field present in the Einstein–Chern–Simons gravity action is given. As an application, we find some Friedmann–Lemaitre–Robertson–Walker cosmological solutions that exhibit accelerated behavior.

Effect of soil compressibility on the structural response of box culverts using finite element approach

The structural response of box culverts to variable soil compressibility condition was studied in this paper. This was made possible by modelling the soil as springs, and varying the spring stiffness which was represented by the modulus of subgrade reaction of the soil. The results showed that the values of maximum bending moments for gravity actions on box culverts increased linearly with modulus of subgrade reaction, but remained within close values. The results also showed good agreement with results from literature for highly compressible soils. However, for incompressible soil condition, results from standard tables in literature were more conservative with about 10% difference for gravity actions, and 21% difference for lateral actions. The term ‘highly compressible’ that was used in literature for manual analysis was discovered to be more valid for lateral load cases than for gravity load cases. Subsequently, the variations of other action effects such as shear force, axial force, torsion, and soil spring settlement with modulus of subgrade reaction were also studied.&#x0D; Keywords: Box Culvert, Modulus of Subgrade Reaction, Soil Settlement, Staad Pro.

First-order formulation of teleparallel gravity and dual loop gravity

There are at least two ways to encode gravity into geometry: Einstein’s general theory of relativity (GR) for the metric tensor, and teleparallel gravity, where torsion as opposed to curvature encodes the dynamics of the gravitational degrees of freedom. The main purpose of the paper is to explore the relation between loop gravity and teleparallel gravity. We argue that these two formulations of gravity are related to two different discretizations of the Einstein–Cartan action, which were studied recently in the literature. The first discretization leads to the loop gravity kinematical phase space where the zero torsion condition is enforced first and the other is the dual loop gravity kinematical phase space where curvature is imposed to vanish first. Our argument is based on the observation that the GR first-order Einstein–Cartan action can also be seen as a first-order action for teleparallel gravity up to a boundary term. The results of our paper suggest that the dual loop gravity framework is a natural discretization of teleparallel gravity, whereas loop gravity is naturally related to the standard GR metric description.

Topological invariants and the definition of energy in quadratic gravity theory

We provide a simple method to compute the energy in higher curvature gravity in asymptotically AdS spacetimes in even dimensions. It follows from the combined use of topological terms added to the gravity action, and the Wald charges derived from the augmented action functional. No additional boundary terms are needed. As a consistency check, we show that the formula for the conserved quantities derived in this way yields the correct result for the mass of asymptotically AdS black holes.