Gravity Action Research Articles

Nonsingular black hole chemistry in 4D Einstein-Gauss-Bonnet gravity

The EGB is an outcome of quadratic curvature corrections to the Einstein-Hilbert gravity action in the form of a Gauss-Bonnet (GB) term in D>4 dimensions, and EGB gravity is topologically invariant in 4D. Several ways have been proposed for regularizing the D→4 limit of EGB for non-trivial gravitational dynamics in 4D. Motivated by the importance of AdS/CFT, we obtain an exact static spherically symmetric nonsingular black hole in 4D EGB gravity coupled to the nonlinear electrodynamics (NED) in an AdS spacetime. We interpret the negative cosmological constant Λ as the positive pressure, via P=−Λ/8π, of the system's thermodynamic properties of the nonsingular black hole with an AdS background. We find that for P<Pc, the black holes with CP>0 are stable to thermal fluctuations and unstable otherwise. We also analyzed the Gibbs free energy to find that the small globally unstable black holes undergo a phase transition to the large globally stable black holes. Further, we study the P−V criticality of the system and then calculate the critical exponents to find that our system behaves like Van der Walls fluid.

Problems on Controlling the Stress State of Large-Sized Structures Being Consecutively Constructed of Materials with Rheological Properties

The presented study uses the mathematical theory of growing bodies to describe from the view point of continuum mechanics the processes of consecutive construction of large-sized arched structures based on flat foundations with taking into account the synchronous gravity action on the structure and to detect some ways of possible technological controlling these structures stress-strain state by a proper choice of the pace of structural material elements layerwise adding and of the values of pre-stretching stresses being set in the material elements. The structure material is implied to have rheological properties of creeping and aging what is decisively for the adoption of mentioned influence factors in control problems under consideration. The results of numeral calculations are given for specific material and process characteristics to illustrate the examples of successful stress controlling in thin-walled structures being constructed.

Planar black holes in holographic axion gravity: Islands, Page times, and scrambling times

The present work investigates the entanglement entropies of the Hawking radiations, the Page times, and the scrambling times, for the eternal planar black holes in the holographic axion gravity. The solutions correspond to a new class of charged black holes, because the boundary diffeomorphism is broken due to the graviton mass induced by the axion fields in the bulk. The information theoretical aspects of these black hole solutions is determined upon applying the island rule for the entanglement entropy. Like non-extremal charged black holes, the radiation entropy grows linearly in the no-island configurations, while is saturated at late times by asymptotic values set by the Bekenstein-Hawking entropy in the island configurations, with the boundary being located slightly outside the outer horizon. In particular, for the extremal black planes of holographic axion gravity, we find that: (a) the entanglement entropy of the Hawking radiation is ill-defined at the early times when the island is absent; (b) it tends to a distinctive constant at the late times; (c) the late-time location of the island is indeed universal. Moreover, we investigate how the Page time is affected by the holographic massive gravity deformation. For neutral solutions at the small deformation parameter, and for charged solutions with almost-extremal deformation parameter, we find that the Page transition happens at earlier times.

The effective theory of gravity and dynamical vacuum energy

Gravity and general relativity are considered as an Effective Field Theory (EFT) at low energies and macroscopic distances. The effective action of the conformal anomaly of light or massless quantum fields has significant effects on macroscopic scales, due to associated light cone singularities that are not captured by an expansion in local curvature invariants. A compact local form for the Wess-Zumino effective action of the conformal anomaly and stress tensor is given, requiring the introduction of a new light scalar field, which it is argued should be included in the low energy effective action for gravity. This scalar conformalon couples to the conformal part of the spacetime metric and allows the effective value of the vacuum energy, described as a condensate of an exact 4-form abelian gauge field strength F = dA, to change in space and time. This is achieved by the identification of the torsion dependent part of the Chern-Simons 3-form of the Euler class with the gauge potential A, which enters the effective action of the conformal anomaly as a J · A interaction analogous to electromagnetism. The conserved 3-current J describes the worldtube of 2-surfaces that separate regions of differing vacuum energy. The resulting EFT thus replaces the fixed constant Λ of classical gravity, and its apparently unnaturally large sensitivity to UV physics, with a dynamical condensate whose ground state value in empty flat space is Λeff = 0 identically. By allowing Λeff to vary rapidly near the 2-surface of a black hole horizon, the proposed EFT of dynamical vacuum energy provides an effective Lagrangian framework for gravitational condensate stars, as the final state of complete gravitational collapse consistent with quantum theory. The possible consequences of dynamical vacuum dark energy for cosmology, the cosmic coincidence problem, and the role of conformal invariance for other fine tuning issues in the Standard Model are discussed.

Higher-curvature generalization of Eguchi-Hanson spaces

We construct higher-dimensional generalizations of the Eguchi-Hanson gravitational instanton in the presence of higher-curvature deformations of general relativity. These spaces are solutions to Einstein gravity supplemented with the dimensional extension of the quadratic Chern-Gauss-Bonnet invariant in arbitrary even dimension $D=2m\geq 4$, and they are constructed out of non-trivial fibrations over $(2m-2)$-dimensional K\"ahler-Einstein manifolds. Different aspects of these solutions are analyzed; among them, the regularization of the on-shell Euclidean action by means of the addition of topological invariants. We also consider higher-curvature corrections to the gravity action that are cubic in the Riemann tensor and explicitly construct Eguchi-Hanson type solutions for such.

Non-relativistic and ultra-relativistic expansions of three-dimensional spin-3 gravity theories

In this paper, we present novel and known non-relativistic and ultra-relativistic spin-3 algebras, by considering the Lie algebra expansion method. We start by applying the expansion procedure using different semigroups to the spin-3 extension of the AdS algebra, leading to spin-3 extensions of known non-relativistic and ultra-relativistic algebras. We then generalize the procedure considering an infinite-dimensional semigroup, which allows to obtain a spin-3 extension of two new infinite families of the Newton-Hooke type and AdS Carroll type. We also present the construction of the gravity theories based on the aforementioned algebras. In particular, the expansion method based on semigroups also allows to derive the (non-degenerate) invariant bilinear forms, ensuring the proper construction of the Chern-Simons gravity actions. Interestingly, in the vanishing cosmological constant limit we recover the spin-3 extensions of the infinite-dimensional Galilean and infinite-dimensional Carroll gravity theories.

Energy functionals from Conformal Gravity

We provide a new derivation of the Hawking mass and Willmore energy functionals for asymptotically AdS spacetimes, by embedding Einstein-AdS gravity in Conformal Gravity. By construction, the evaluation of the four-dimensional Conformal Gravity action in a manifold with a conical defect produces a codimension-2 conformal invariant functional LΣ. The energy functionals are then particular cases of LΣ for Einstein-AdS and pure AdS ambient spaces, respectively. The bulk action is finite for AdS asymptotics and both Hawking mass and Willmore energy are finite as well. The result suggests a generic relation between conformal invariance and renormalization, where the codimension-2 properties are inherited from the bulk gravity action.

Cosmology in 5D and 4D Einstein–Gauss–Bonnet gravity

We consider the five-dimensional Einstein–Gauss–Bonnet gravity, which can be obtained by means of an appropriate choice of coefficients in the five-dimensional Lanczos–Lovelock gravity theory. The Einstein–Gauss–Bonnet field equations for the Friedmann–Lemaître–Robertson–Walker metric are found as well as some of their solutions. The hyperbolicity of the corresponding equations of motion is discussed. A four-dimensional gravity action is obtained from the Gauss–Bonnet gravity using the Randall–Sundrum compactification procedure and then it is studied the implications of the compactification procedure in the cosmological solutions. The same procedure is used to obtain gravity in four dimensions from the five-dimensional AdS–Chern–Simons gravity to then study some cosmological solutions. Some aspects of the construction of the four-dimensional action gravity, as well as a brief review of Lovelock gravity in 5D are considered in an Appendix.

Cosmology with modified continuity equation in asymptotically safe gravity

We study FLRW cosmology, taking into account quantum gravitational corrections in the formalism of the exact renormalization group flow of the effective action for gravity. We calculate the quantum corrected scale factor, energy density, and entropy production at late times, taking different choices of cut-off functions. Our approach is in line with previous studies which involve the running Newton constant G(k) in the definition of energy–momentum tensor, and then imposing the covariant conservation identity of the Einstein tensor. The quantum corrections obtained in this approach are different from what are found by letting the conservation equation remain the same as for a scale-independent Newton constant. The main observation made here is that different cut-off identifications results in different late time cosmologies. Further, the choices made also led to exact analytical solutions for the Hubble parameter in terms of the cosmological time and scale factor.

Numerical modelling of vibration response in loess hills due to a high-speed train on railway viaduct

In order to accurately analyze vibration characteristics and site effects of loess hills under moving load of a high-speed train, four types of loess hill models under railway viaduct was established by ABAQUS of finite element analysis software by field test. The dynamic response and stability of loess hills under two different vibration sources under high-speed train load were studied by using two-dimensional equivalent linear response time-history analysis, and the influence of the mechanical parameters of loess on the vibration of different types of loess hill was analyzed. Results show that there are obvious differences between peak displacement cloud maps of loess hills under the railway viaduct under gravity and train load action. We analyzed the influence of the change of elastic modulus on vibration propagation of soil of foundation and loess knoll, and found that the change of elastic modulus of soil in different position of foundation has more effect on vibration propagation than that of loess knoll soil. At the same time, the vertical acceleration cloud maps of the four types of loess hills are obviously different.

Axial Symmetric Granular Flow Due to Gravity in a Circular Pipe

Axisymmetric granular flows in vertical cylindrical pipes under action of gravity are studied using mathematical particle–particle models based on the Hertz–Mindlin theory. By and large, in granular flows, the density field and the pressure are unknown scalar functions. A well-known relationship between these fields gives the pressure field a power law of the density. The aim of this paper was to study unsteady, axisymmetric, fully developed granular flow under gravity action in a vertical cylindrical pipe, under the assumptions that the density field is constant and the velocity on the pipe’s wall is time-dependent. Using integral transforms method and appropriate initial-boundary conditions, the analytical solution for axial velocity is determined. The obtained analytical solution is used to determine the steady-state solution (the solution for large values of the time). The properties of the flow in some particular cases of the velocity on the pipe’s surface are analyzed and the transient flow is compared with the stationary one.

Metric-Affine Vector–Tensor correspondence and implications in F(R,T,Q,T,D) gravity

We extend the results of antecedent literature on quadratic Metric-Affine Gravity by studying a new quadratic gravity action in vacuum which, besides the usual (non-Riemannian) Einstein-Hilbert contribution, involves all the parity even quadratic terms in torsion and non-metricity plus a Lagrangian that is quadratic in the field-strengths of the torsion and non-metricity vector traces. The theory result to be equivalent, on-shell, to a Vector-Tensor theory. We also discuss the sub-cases in which the contribution to the Lagrangian quadratic in the field-strengths of the torsion and non-metricity vectors just exhibits one of the aforementioned quadratic terms. We then report on implications of our findings in the context of $F(R,T,Q,\mathcal{T},\mathcal{D})$ gravity.

Tornado-induced collapse analysis of a super-large reinforced concrete cooling tower

Wind loads are a predominant design factor for the gravity, thermal effects, and seismic actions of thin-walled super-large cooling towers. The current load code provisions relating to cooling towers in various countries are only suitable for straight-line boundary-layer wind fields under normal climates, rather than non-synoptic winds such as tornadoes, which have quite different wind field characteristics and are more vulnerable to structural safety. To further understand the tornado-resistant performances of super-large reinforced concrete cooling towers (RCCTs), wind tunnel tests with respect to a reduced-scale model of a prototype structure with a height of 215 m were conducted using a tornado vortex simulator. Wind loads acting on the tower shell due to tornadoes with different swirl ratios were studied, and then applied to a refined full-scale finite element model. The entire process of tornado-induced RCCT collapse was simulated and the critical failure wind velocities considering nonlinear collapse effects were compared with the results from the buckling stress state (BSS) approach, which is recommended in national codes and linear bifurcation buckling analysis. The results revealed that structural failure is more likely to occur when the RCCT is located at the tornado core radius because the structure experiences the most unfavorable tornadic wind loads within this region. The loss of material strength rather than the loss of stability is the key factor influencing tornado-induced RCCT failure. The structural collapse is initiated by the appearance and development of circumferential cracks in the windward region of the tangential wind flow owing to the tensile failure of the meridian reinforcement inside the tower shell. The cracks around the tower shell lead to the degradation of the natural frequencies of the structure and the change in vibration modes, and finally affect the tornado-resistant performance of the RCCT.

Theoretical Model and Solution of Dynamic Evolution in Initial Stage of Lacustrine Delta

Sediment is carried by flows into lake during the formation of lacustrine deltas. The subsequent movement of sediment-laden flow is maintained by the initial momentum at the early stage of delta growth. A theoretical model of the plane jet boundary layer in the initial stage of a lacustrine delta, including the consideration of gravity action on the slope and bottom friction, was established according to characteristics of this process, and a similarity solution method was used to solve the model. A coefficient ε was introduced that describes the characteristics of sediment-laden flow spreading along a straight line and its value is given. The theoretical expression of morphological characteristics of the initial stage was derived based on the general mathematical model of river bed evolution, and the erosion and deposition patterns were quantitatively analyzed. It was verified through experimental data that the theoretical model can be solved accurately and, can satisfactorily describe the trend in erosion and deposition and morphological characteristics of the initial stage of a delta.

Analysis of parametric effects in the wave profile of the variant Boussinesq equation through two analytical approaches

Abstract The variant Boussinesq equation has significant application in propagating long waves on the surface of the liquid layer under gravity action. In this article, the improved Bernoulli subequation function (IBSEF) method and the new auxiliary equation (NAE) technique are introduced to establish general solutions, some fundamental soliton solutions accessible in the literature, and some archetypal solitary wave solutions that are extracted from the broad-ranging solution to the variant Boussinesq wave equation. The established soliton solutions are knowledgeable and obtained as a combination of hyperbolic, exponential, rational, and trigonometric functions, and the physical significance of the attained solutions is speculated for the definite values of the included parameters by depicting the 3D profiles and interpreting the physical incidents. The wave profile represents different types of waves associated with the free parameters that are related to the wave number and velocity of the solutions. The obtained solutions and graphical representations visualize the dynamics of the phenomena and build up the mathematical foundation of the wave process in dissipative and dispersive media. It turns out that the IBSEF method and the NAE are powerful and might be used in further works to find novel solutions for other types of nonlinear evolution equations ascending in physical sciences and engineering.

Complex saddles and Euclidean wormholes in the Lorentzian path integral

We study complex saddles of the Lorentzian path integral for 4D axion gravity and its dual description in terms of a 3-form flux, which include the Giddings-Strominger Euclidean wormhole. Transition amplitudes are computed using the Lorentzian path integral and with the help of Picard-Lefschetz theory. The number and nature of saddles is shown to qualitatively change in the presence of a bilocal operator that could arise, for example, as a result of considering higher-topology transitions. We also analyze the stability of the Giddings-Strominger wormhole in the 3-form picture, where we find that it represents a perturbatively stable Euclidean saddle of the gravitational path integral. This calls into question the ultimate fate of such solutions in an ultraviolet-complete theory of quantum gravity.

Black holes in the limiting curvature theory of gravity

We discuss a recently proposed limiting curvature theory of gravity and its application to the problem of singularities inside black holes. In this theory the growth of the curvature is suppressed by specially chosen inequality constraints included in the gravity action. We consider a case of a spherically symmetric four-dimensional black hole and demonstrated that imposed curvature constraints modify a solution in the black hole interior. Instead of forming the curvature singularity the modified metric describes a space which is exponentially expanding in one direction and oscillating the other two directions. The space–time is complete and its polynomial curvature invariants are uniformly bounded.

Spontaneous Lorentz symmetry breaking and one-loop effective action in the metric-affine bumblebee gravity

The metric-affine bumblebee model in the presence of fermionic matter minimally coupled to the connection is studied. We show that the model admits an Einstein frame representation in which the matter sector is described by a non-minimal Dirac action without any analogy in the literature. Such non-minimal terms involve unconventional couplings between the bumblebee and the fermion field. We then rewrite the quadratic fermion action in the Einstein frame in the basis of 16 Dirac matrices in order to identify the coefficients for Lorentz/CPT violation in all orders of the non-minimal coupling ξ. The exact result for the fermionic determinant in the Einstein frame, including all orders in ξ, is also provided. We demonstrate that the axial contributions are at least of second order in the perturbative expansion of ξ. Furthermore, we compute the one-loop effective potential within the weak field approximation.

Fabrication of laminated composite filter with nonwoven fabrics for oil/water separation

Various materials have been prepared and applied for the oil-water mixture separation, but there are some problems such as high cost, complicated formulation process and inferior durability. Nonwoven fabrics are widely used in industrial fields because of their low production cost and high economic benefit. In this study, a sandwich-structure filter was prepared for oil/water separation based on laminated nonwovens. Spunbond-meltblown-spunbond (SMS) nonwoven treated with silica and fluorine resin was selected for the hydrophobic top layer. The spunlaced viscose (VS) impregnated with citrate was used for the hygroscopic core layer while spunbond polypropylene (PP) prepared for the bottom substrate. These three layers were reinforced on an ultrasonic-bonding machine and tested for oil-water separation. Surface morphology and chemical group changes of treated fiber were recorded by scanning electron microscope (SEM), atomic force microscope (AFM), energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). The effect of surface hydrophobicity and core hydrophilicity on separation efficiency was analyzed. The durability after recycles was evaluated and discussed. The possible separation mechanism was proposed. The results demonstrated that coating and spraying methods achieved uniform distribution of silica and fluorine resin onto the fiber surface. The suitable conditions for the preparation of composite filter were obtained as follows: the surface SMS layer was coated with silica, then sprayed with fluorine resin for a 6.0%weight gain to get a hydrophobic layer. The core layer viscose fiber impregnated with 3.0% citrate to enhance its hygroscopicity. The three layers were laminated and reinforced for the composite filter. Improving the hydrophobicity of top layer and the hydrophilicity of core layer both contributed to the enhance of separation efficiency and stability. The pore size of composite filter was mainly in the range of 9-11μm with a 35% distribution. After 15 times recycles, the water recovery ratio remains over 95% while water contact angle decreased from 150° to 135°. After layers laminated and bonding, the hydrophobic surface trapped oil and water in the mixture, hygroscopicity layer enabled a connection with the water in mixture through SMS layer and formed a capillary water channel. The water transferred to the sublayer by gravity and capillary action to actualize oil-water separation. These results provided new insights for the application of nonwoven fabrics on oil-water separation.

Dynamical dark energy and infinite statistics

In the [Formula: see text]CDM model, dark energy is viewed as a constant vacuum energy density, the cosmological constant in the Einstein–Hilbert action. This assumption can be relaxed in various models that introduce a dynamical dark energy. In this paper, we argue that the mixing between infrared (IR) and ultraviolet (UV) degrees of freedom in quantum gravity leads to infinite statistics, the unique statistics consistent with Lorentz invariance in the presence of nonlocality, and yields a fine structure for dark energy. Introducing IR and UV cutoffs into the quantum gravity action, we deduce the form of [Formula: see text] as a function of redshift and translate this to the behavior of the Hubble parameter.