Showing papers in "International Journal of Modern Physics D in 2017"
TL;DR: The BlackHoleCam project as mentioned in this paper provides a unique fundamental-physics laboratory for experimental tests of BH accretion and theories of gravity in their most extreme limits in the center of our own Milky Way.
Abstract: Einstein’s General theory of relativity (GR) successfully describes gravity. Although GR has been accurately tested in weak gravitational fields, it remains largely untested in the general strong field cases. One of the most fundamental predictions of GR is the existence of black holes (BHs). After the recent direct detection of gravitational waves by LIGO, there is now near conclusive evidence for the existence of stellar-mass BHs. In spite of this exciting discovery, there is not yet direct evidence of the existence of BHs using astronomical observations in the electromagnetic spectrum. Are BHs observable astrophysical objects? Does GR hold in its most extreme limit or are alternatives needed? The prime target to address these fundamental questions is in the center of our own Milky Way, which hosts the closest and best-constrained supermassive BH candidate in the universe, Sagittarius A* (Sgr A*). Three different types of experiments hold the promise to test GR in a strong-field regime using observations of Sgr A* with new-generation instruments. The first experiment consists of making a standard astronomical image of the synchrotron emission from the relativistic plasma accreting onto Sgr A*. This emission forms a “shadow” around the event horizon cast against the background, whose predicted size (∼50μas) can now be resolved by upcoming very long baseline radio interferometry experiments at mm-waves such as the event horizon telescope (EHT). The second experiment aims to monitor stars orbiting Sgr A* with the next-generation near-infrared (NIR) interferometer GRAVITY at the very large telescope (VLT). The third experiment aims to detect and study a radio pulsar in tight orbit about Sgr A* using radio telescopes (including the Atacama large millimeter array or ALMA). The BlackHoleCam project exploits the synergy between these three different techniques and contributes directly to them at different levels. These efforts will eventually enable us to measure fundamental BH parameters (mass, spin, and quadrupole moment) with sufficiently high precision to provide fundamental tests of GR (e.g. testing the no-hair theorem) and probe the spacetime around a BH in any metric theory of gravity. Here, we review our current knowledge of the physical properties of Sgr A* as well as the current status of such experimental efforts towards imaging the event horizon, measuring stellar orbits, and timing pulsars around Sgr A*. We conclude that the Galactic center provides a unique fundamental-physics laboratory for experimental tests of BH accretion and theories of gravity in their most extreme limits.
207 citations
TL;DR: Hořava's quantum gravity at a Lifshitz point as discussed by the authors is a theory intended to quantize gravity by using traditional quantum field theories, and various modifications have been proposed, since the first incarnation of the theory in 2009.
Abstract: Hořava’s quantum gravity at a Lifshitz point is a theory intended to quantize gravity by using traditional quantum field theories. To avoid Ostrogradsky’s ghosts, a problem that has been facing in quantization of general relativity since the middle of 1970’s, Hořava chose to break the Lorentz invariance by a Lifshitz-type of anisotropic scaling between space and time at the ultra-high energy, while recovering (approximately) the invariance at low energies. With the stringent observational constraints and self-consistency, it turns out that this is not an easy task, and various modifications have been proposed, since the first incarnation of the theory in 2009. In this review, we shall provide a progress report on the recent development of Hořava gravity. In particular, we first present four so far most-studied versions of Hořava gravity, by focusing first on their self-consistency and then their consistency with experiments, including the solar system tests and cosmological observations. Then, we provide a general review on the recent development of the theory in three different but also related areas: (i) universal horizons, black holes and their thermodynamics; (ii) nonrelativistic gauge/gravity duality and (iii) quantization of the theory. The studies in these areas can be easily generalized to other gravitational theories with broken Lorentz invariance.
183 citations
TL;DR: Intermediate-mass black holes (IMBHs), with masses in the range 100-106M⊙, are the link between stellar-mass BHs and supermassive BHS (SMBHs) as mentioned in this paper.
Abstract: Intermediate-mass black holes (IMBHs), with masses in the range 100–106M⊙, are the link between stellar-mass BHs and supermassive BHs (SMBHs). They are thought to be the seeds from which SMBHs grow...
174 citations
TL;DR: In this paper, the authors discuss a variety of topics in cosmology when it is enlarged in order to contain a bulk viscosity, when expressed in terms of the fluid density or the Hubble parameter.
Abstract: From a hydrodynamicist’s point of view the inclusion of viscosity concepts in the macroscopic theory of the cosmic fluid would appear most natural, as an ideal fluid is after all an abstraction (exluding special cases such as superconductivity). Making use of modern observational results for the Hubble parameter plus standard Friedmann formalism, we may extrapolate the description of the universe back in time up to the inflationary era, or we may go to the opposite extreme and analyze the probable ultimate fate of the universe. In this review, we discuss a variety of topics in cosmology when it is enlarged in order to contain a bulk viscosity. Various forms of this viscosity, when expressed in terms of the fluid density or the Hubble parameter, are discussed. Furthermore, we consider homogeneous as well as inhomogeneous equations of state. We investigate viscous cosmology in the early universe, examining the viscosity effects on the various inflationary observables. Additionally, we study viscous cosmology in the late universe, containing current acceleration and the possible future singularities, and we investigate how one may even unify inflationary and late-time acceleration. Finally, we analyze the viscosity-induced crossing through the quintessence-phantom divide, we examine the realization of viscosity-driven cosmological bounces, and we briefly discuss how the Cardy–Verlinde formula is affected by viscosity.
160 citations
TL;DR: In this paper, a class of quantum gravitational theories based on weakly nonlocal analytic classical actions is reviewed and it is proved that the theory is superrenormalizable in any dimension, i.e. only one-loop divergences survive, and is asymptotically free.
Abstract: We hereby review a class of quantum gravitational theories based on weakly nonlocal analytic classical actions. The most general action is characterized by two nonpolynomial entire functions (form-factors) in terms quadratic in curvature. The form-factors avert the presence of poltergeists, that plague any local higher derivative theory of gravity and improve the high-energy behavior of loop amplitudes. For pedagogical purposes, it is proved that the theory is super-renormalizable in any dimension, i.e. only one-loop divergences survive, and is asymptotically free. Furthermore, due to dimensional reasons, in odd dimensions, there are no counterterms for pure gravity and the theory turns out to be finite. Moreover, we show that it is always possible to choose the additional terms in the action (higher in curvature) in such a way to make the full theory UV-finite and therefore, scale-invariant in quantum realm, also in even dimension.
145 citations
TL;DR: In this article, the shape and size of the shadow of a rotating black hole surrounded by plasma was studied in vacuum and in the presence of radial power-law density, respectively.
Abstract: We study the shadow of the rotating black hole with quintessential energy (i) in vacuum, (ii) in the presence of plasma with radial power-law density. For the vacuum case, the quintessential field parameter of the rotating black hole significantly changes the shape of the shadow. With increasing quintessential field parameter, the radius of the shadow also increases. With the increase of the radius of the shadow of the rotating black hole, the quintessential field parameter causes decrease of the distortion of the shadow shape: in the presence of the quintessential field parameter, the shadow of the fast rotating black hole becomes too close to the circle. We assume the distant observer of the black hole shadow to be located near the so-called static radius where the gravitational attraction of the black hole is just balanced by the cosmic repulsion. The shape and size of the shadow of quintessential rotating black hole surrounded by plasma depends on (i) plasma parameters, (ii) black hole spin and (iii) quintessential field parameter. With the increase of the plasma refraction index, the apparent radius of the shadow increases. However, for the large values of the quintessential field parameter, the change of the black hole shadow shape due to the presence of plasma is not significant, i.e. the effect of the quintessential field parameter dominates over the plasma effect.
133 citations
TL;DR: In this article, a review of the status of research on the nature of initial conditions required to obtain a period of cosmological inflation is given, and it is shown that in the case of large field models, the inflationary slow-roll trajectory is a local attractor in initial condition space.
Abstract: I give a brief review of the status of research on the nature of initial conditions required to obtain a period of cosmological inflation. It is shown that there is good evidence that in the case of large field models, the inflationary slow-roll trajectory is a local attractor in initial condition space, whereas it is not in the case of small field models.
103 citations
TL;DR: A review of the observational evidence for the existence of dark matter can be found in this article, where a consensus picture has emerged in which roughly a quarter of the universe consists of dark mass.
Abstract: Over the past few decades, a consensus picture has emerged in which roughly a quarter of the universe consists of dark matter. I begin with a review of the observational evidence for the existence ...
86 citations
TL;DR: In this paper, the effect of the gravitational field and the plasma on the deflection angle of the photons was studied in the weak field approximation of the ABG regular black hole.
Abstract: In the weak field approximation, we study the gravitational lensing near the regular Bardeen, Hayward and Ayon-Beato–Garcia (ABG) black holes surrounded by plasma. The exact expressions for the deflection angle of the photons due to the effect of the gravitational field and the plasma have been obtained. The analysis of the image source brightness magnification in the background spacetimes of (i) Bardeen, (ii) Hayward and (iii) ABG regular black holes have shown that the increase of the corresponding charge of regular black hole causes the increase in the magnification of the source image. In addition to the primary ring, one may observe the secondary ring with smaller magnification. The influence of the plasma with (i) constant and (ii) radial power law electron density to the magnification of the source image has been studied.
75 citations
TL;DR: In this article, the authors considered the critical behaviors and phase transitions of Gauss-Bonnet-Born-Infeld-AdS black holes for d = 5, 6 and the extended phase space.
Abstract: We consider the critical behaviors and phase transitions of Gauss–Bonnet–Born–Infeld-AdS black holes (GB–BI-AdS) for d = 5, 6 and the extended phase space. We assume the cosmological constant, Λ, the coupling coefficient α, and the BI parameter β to be thermodynamic pressures of the system. Having made these assumptions, the critical behaviors are then studied in the two canonical and grand canonical ensembles. We find “reentrant and triple point phase transitions” (RPT-TP) and “multiple reentrant phase transitions” (multiple RPT) with increasing pressure of the system for specific values of the coupling coefficient α in the canonical ensemble. Also, we observe a reentrant phase transition (RPT) of GB–BI-AdS black holes in the grand canonical ensemble and for d = 6. These calculations are then expanded to the critical behavior of Born–Infeld-AdS (BI-AdS) black holes in the third-order of Lovelock gravity and in the grand canonical ensemble to find a van der Waals (vdW) behavior for d = 7 and a RPT for d = 8 for specific values of potential ϕ in the grand canonical ensemble. Furthermore, we obtain a similar behavior for the limit of β →∞, i.e. charged-AdS black holes in the third-order of the Lovelock gravity. Thus, it is shown that the critical behaviors of these black holes are independent of the parameter β in the grand canonical ensemble.
71 citations
TL;DR: In this paper, a regular black hole solution is presented in the context of scale-dependent General Relativity, satisfying the weak energy condition, which can be considered as a semiclassical extension of the Schwarzschild solution.
Abstract: In this work, we present a regular black hole solution, in the context of scale-dependent General Relativity, satisfying the weak energy condition. The source of this solution is an anisotropic effective energy–momentum tensor which appears when the scale dependence of the theory is turned-on. In this sense, the solution can be considered as a semiclassical extension of the Schwarzschild one.
TL;DR: In this paper, the authors presented a regular black hole with a positive cosmological constant, which is a solution to the Einstein equations coupled to nonlinear electrodynamics with a magnetic monopole.
Abstract: In this paper, we present a regular black hole with a positive cosmological constant. The regular black hole considered is the well known Bardeen black hole and it is a solution to the Einstein equations coupled to nonlinear electrodynamics with a magnetic monopole. The paper discusses the properties of the Bardeen–de Sitter black hole. We have computed the gray body factors and partial absorption cross-sections for massless scalar field impinges on this black hole with the third-order WKB approximation. A detailed discussion on how the behavior of the gray body factors depend on the parameters of the theory such as the mass, charge and the cosmological constant is given. Possible extensions of the work is discussed at the end of the paper.
TL;DR: In this paper, the evolution of the Hubble radius and the issue of primordial cosmological perturbations in detail were investigated, and the viability of the singular bounce model was discussed.
Abstract: An alternative to the Big Bang cosmologies is obtained by the Big Bounce cosmologies. In this paper, we study a bounce cosmology with a Type IV singularity occurring at the bouncing point in the context of F(R) modified gravity. We investigate the evolution of the Hubble radius and we examine the issue of primordial cosmological perturbations in detail. As we demonstrate, for the singular bounce, the primordial perturbations originating from the cosmological era near the bounce do not produce a scale-invariant spectrum and also the short wavelength modes after these exit the horizon, do not freeze, but grow linearly with time. After presenting the cosmological perturbations study, we discuss the viability of the singular bounce model, and our results indicate that the singular bounce must be combined with another cosmological scenario, or should be modified appropriately, in order that it leads to a viable cosmology. The study of the slow-roll parameters leads to the same result indicating that the singular bounce theory is unstable at the singularity point for certain values of the parameters. We also conformally transform the Jordan frame singular bounce, and as we demonstrate, the Einstein frame metric leads to a Big Rip singularity. Therefore, the Type IV singularity in the Jordan frame becomes a Big Rip singularity in the Einstein frame. Finally, we briefly study a generalized singular cosmological model, which contains two Type IV singularities, with quite appealing features.
TL;DR: In this paper, the spectrum of quasinormal frequencies of four-dimensional (4D) charged black holes (BHs) in EpM theory was computed and an analytical expression for the spectrum in the eikonal limit was provided.
Abstract: In the present work, we compute the spectrum of quasinormal frequencies of four-dimensional (4D) charged black holes (BHs) in Einstein-power-Maxwell (EpM) theory. In particular, we study scalar perturbations adopting the sixth-order WKB approximation. We analyze in detail, the behavior of the spectrum depending on the charge of the BH, the quantum number of angular momentum and the overtone number. In addition, a comparison is made between the results obtained here and the results valid for charged BHs in other theories as well as for the Reissner–Nordstrom BH. Finally, we have provided an analytical expression for the quasinormal spectrum in the eikonal limit.
TL;DR: In this paper, the authors considered a compact spherical celestial star undergoing expansion due to the presence of higher curvature invariants of f(R) gravity and imperfect fluid and explored some dynamical variables from splitting the Riemann curvature tensor.
Abstract: One of the striking feature of inhomogeneous matter distribution under the effects of fourth-order gravity and electromagnetic field have been discussed in this manuscript. We have considered a compact spherical celestial star undergoing expansion due to the presence of higher curvature invariants of f(R) gravity and imperfect fluid. We have explored the dynamical equations and field equations in f(R) gravity. An explicit expression have been found for Weyl tensor and material variables under the dark dynamical effects. Using a viable f(R) model, some dynamical variables have been explored from splitting the Riemann curvature tensor. These dark dynamical variables are also studied for charged dust cloud with and without the constraint of constant Ricci scalar.
TL;DR: In this paper, a new well-behaved charged anisotropic solution of the field equations was discovered, which can represent a physically possible configuration with the inclusion of some net electric charge, i.e. the solution can become a wellbehaved solution with decreasing sound speed radially outward.
Abstract: In the present paper, we discover a new well-behaved charged anisotropic solution of Einstein–Maxwell’s field equations. We ansatz the metric potential g00 of the form given by Maurya el al. (Eur. Phys. J. C 76(2) (2016) 693) with n = 2. In their paper, it is mentioned that for n = 2, the solution is not well-behaved for neutral configuration as the speed of sound is nondecreasing radially outward. However, the solution can represent a physically possible configuration with the inclusion of some net electric charge, i.e. the solution can become a well-behaved solution with decreasing sound speed radially outward for a charged configuration. Due to the inclusion of electric charge, the solution leads to a very stiff equation-of-state (EoS) with the velocity of sound at the center vr02 = 0.819, vt02 = 0.923 and the compactness parameter u = 0.823 is close to the Buchdahl limit 0.889. This stiff EoS support a compact star configuration of mass 5.418M⊙ and radius of 10.1km.
TL;DR: In this paper, the authors provide a concise review on multi-field inflation and cosmological perturbations, and give formal accounts on the gauge fixing conditions and present the perturbation action in two gauges.
Abstract: We provide a concise review on multi-field inflation and cosmological perturbations. We discuss convenient and physically meaningful bases in terms of which perturbations can be systematically studied. We give formal accounts on the gauge fixing conditions and present the perturbation action in two gauges. We also briefly review nonlinear perturbations.
TL;DR: In this article, the authors presented new anisotropic models for Buchdahl type perfect fluid solution and determined the new pressure anisotropy factor Δ with the help of both the metric potentials eλ and eν.
Abstract: We present new anisotropic models for Buchdahl [H. A. Buchdahl, Phys. Rev. 116 (1959) 1027.] type perfect fluid solution. For this purpose, we started with metric potential eλ same as Buchdahl [H. A. Buchdahl, Phys. Rev. 116 (1959) 1027.] and eν is monotonically increasing function as suggested by Lake [K. Lake, Phys. Rev. D 67 (2003) 104015]. After that we determine the new pressure anisotropy factor Δ with the help of both the metric potentials eλ and eν and propose new well behaved general solution for anisotropic fluid distribution. The physical quantities like energy density, radial and tangential pressures, velocity of sound and redshift etc. are positive and finite inside the compact star. In this connection, we have studied the stability of the models, which is most vital one and also we determined the equation of state p = f(ρ) for the realistic compact star models. It is noted that the mass and radius of our models can represent the structure of realistic astrophysical objects such as Her X-1 and RXJ 1856-37.
TL;DR: In this paper, the extended phase-space thermodynamics and heat engines for static spherically symmetric black hole solutions of four-dimensional conformal gravity are studied in detail, and it is argued that the equation of states (EOS) for such black holes is always branched, any continuous thermodynamical process cannot drive the system from one branch of the EOS into another branch.
Abstract: The extended phase-space thermodynamics and heat engines for static spherically symmetric black hole solutions of four-dimensional conformal gravity are studied in detail. It is argued that the equation of states (EOS) for such black holes is always branched, any continuous thermodynamical process cannot drive the system from one branch of the EOS into another branch. Meanwhile, the thermodynamical volume is bounded from above, making the black holes always super-entropic in one branch and may also be super-entropic in another branch in certain range of the temperature. The Carnot and Stirling heat engines associated to such black holes are shown to be distinct from each other. For rectangular heat engines, the efficiency always approaches zero when the rectangle becomes extremely narrow, and given the highest and lowest working temperatures fixed, there is always a maximum for the efficiency of such engines.
TL;DR: In this paper, several new families of exact solution to the Einstein-Maxwell system of differential equations are found for anisotropic charged matter. But the spacetime geometry is that of Finch and Skea which satisfies all criteria for physical acceptability.
Abstract: Several new families of exact solution to the Einstein–Maxwell system of differential equations are found for anisotropic charged matter. The spacetime geometry is that of Finch and Skea which satisfies all criteria for physical acceptability. The exact solutions can be expressed in terms of elementary functions, Bessel functions and modified Bessel functions. When a parameter is restricted to be an integer then the special functions reduce to simple elementary functions. The uncharged model of Finch and Skea [R. Finch and J. E. F. Skea, Class. Quantum Grav. 6 (1989) 467.] and the charged model of Hansraj and Maharaj [S. Hansraj and S. D. Maharaj, Int. J. Mod. Phys. D 15 (2006) 1311.] are regained as special cases. The solutions found admit a barotropic equation of state. A graphical analysis indicates that the matter and electric quantities are well behaved.
TL;DR: It is argued that the emergence of holographic gravitational theory is related to deep learning process of the quantum field theory, and the structure of entanglement encoded in the graph of DNN is of Ryu-Takayanagi form.
Abstract: Quantum many-body problem with exponentially large degrees of freedom can be reduced to a tractable computational form by neural network method [G. Carleo and M. Troyer, Science 355 (2017) 602, arX...
TL;DR: In this paper, a relativistic anisotropic compact star model with a spherically symmetric metric of embedding class one has been constructed and the model is free from central singularities and satisfies all energy conditions.
Abstract: In the present paper, we have constructed a new relativistic anisotropic compact star model having a spherically symmetric metric of embedding class one. Here we have assumed an arbitrary form of metric function eλ and solved the Einstein’s relativistic field equations with the help of Karmarkar condition for an anisotropic matter distribution. The physical properties of our model such as pressure, density, mass function, surface red-shift, gravitational redshift are investigated and the stability of the stellar configuration is discussed in details. Our model is free from central singularities and satisfies all energy conditions. The model we present here satisfy the static stability criterion, i.e. dM/dρc > 0 for 0 ≤ ρc ≤ 4.04 × 1017g/cm3 (stable region) and for ρc ≥ 4.04 × 1017g/cm3, the region is unstable i.e. dM/dρc ≤ 0.
TL;DR: In this article, it was shown that the MPTD equations have unexpected behavior: the acceleration in the direction of the velocity grows up to infinity in the ultra-relativistic limit.
Abstract: Mathisson–Papapetrou–Tulczyjew–Dixon (MPTD) equations in the Lagrangian formulation correspond to the minimal interaction of spin with gravity. Due to the interaction, in the Lagrangian equations instead of the original metric g emerges spin-dependent effective metric G = g + h(S). So we need to decide, which of them the MPTD particle sees as the spacetime metric. We show that the MPTD equations, if considered with respect to the original metric (using the standard Landau–Lifshitz spacetime decomposition), have unexpected behavior: the acceleration in the direction of the velocity grows up to infinity in the ultra-relativistic limit. If considered with respect to G, the theory does not have this problem. But the metric now depends on spin, so there is no unique spacetime manifold for the universe of spinning particles: each particle probes its own three-dimensional (3D) geometry. This can be improved by adding a nonminimal interaction, given the modified MPTD equations with reasonable behavior within the original metric.
TL;DR: In this paper, the influence of polynomial f(R) dark sector cosmic terms on the collapse of electrically charged Lemaitre-Tolman-Bondi geometry was examined.
Abstract: The objective of this paper is to examine the influence of polynomial f(R) dark sector cosmic terms on the collapse of electrically charged Lemaitre–Tolman–Bondi geometry. We explored a class of solutions for f(R) field equations in the existence of electromagnetic field and under the constraint of constant curvature scalar. The influence of f(R) model on the dynamics of collapsing object have been discussed by studying its black hole and cosmological horizons. Also, the effects of these dark sources on the time interval between the corresponding singularities and horizons have been studied. We investigated that the process of collapse slows down due to the higher order curvature invariants of polynomial f(R) model and electromagnetic field.
TL;DR: In this paper, an interaction term between matter fields and the gravitational field in the total Lagrangian, analogous to that for Electromagnetism, could possibly provide the dynamical effect for which the dark matter is postulated, on the one hand and a Quantum-Field Theory (QFT) incorporating gravity, that does not have unmanageable divergences.
Abstract: One of the major problems in Cosmology is the fact that there is no good candidate of dark matter in the Standard Model of Particle Physics or any experimentally supported modifications of it. At the same time, one of the major problems of General Relativity is that it cannot be unified with Quantum Theory. Here, we present a program to see if there is not a common source of both problems. The idea is that an interaction term between matter fields and the gravitational field in the total Lagrangian, analogous to that for Electromagnetism, could possibly provide the dynamical effect for which the dark matter is postulated, on the one hand and a Quantum-Field Theory (QFT) incorporating Gravity, that does not have unmanageable divergences, on the other. One could first check that the modified relativistic dynamics, if fitted for the dark matter in individual galaxies fits also for systems and clusters of galaxies, at all scales. If there is no problem with the explanation of the dynamics usually explained by...
TL;DR: In this article, it was shown that the weak gravity conjecture can be inferred directly from Bekenstein's generalized second law of thermodynamics, a law which is widely believed to reflect a fundamental aspect of the elusive theory of quantum gravity.
Abstract: The weak gravity conjecture suggests that, in a self-consistent theory of quantum gravity, the strength of gravity is bounded from above by the strengths of the various gauge forces in the theory. In particular, this intriguing conjecture asserts that in a theory describing a U(1) gauge field coupled consistently to gravity, there must exist a particle whose proper mass is bounded (in Planck units) by its charge: m/mP < q. This beautiful and remarkably compact conjecture has attracted the attention of physicists and mathematicians over the last decade. It should be emphasized, however, that despite the fact that there are numerous examples from field theory and string theory that support the conjecture, we still lack a general proof of its validity. In the present paper, we prove that the weak gravity conjecture (and, in particular, the mass–charge upper bound m/mP < q) can be inferred directly from Bekenstein’s generalized second law of thermodynamics, a law which is widely believed to reflect a fundamental aspect of the elusive theory of quantum gravity.
TL;DR: In this paper, the authors show that the firewall problem can be solved by assuming an extra local symmetry, conformal invariance, spontaneously broken by the vacuum, in a way similar to the Brout-Englert-Higgs mechanism.
Abstract: The black hole information problem and the firewall problem can be addressed by assuming an extra local symmetry: conformal invariance. It must be an exact symmetry, spontaneously broken by the vacuum, in a way similar to the Brout–Englert–Higgs (BEH) mechanism. We note how this symmetry formally removes the horizon and the singularity inside black holes. For the Standard Model this symmetry is severely restrictive, demanding all coupling constants, masses and even the cosmological constant to be computable, in principle. Finally, this symmetry suggests that the Weyl action (the square of the Weyl curvature) should be added to the Einstein–Hilbert action. The ensuing indefinite metric states are briefly studied, and we conclude with some remarks concerning the interpretation of quantum mechanics.
TL;DR: The field of inhomogeneities via nonlinear effects can backreact and alter the properties of the universe on its largest scales, leading to a non-Friedmannian evolution as discussed by the authors.
Abstract: Astronomical observations reveal hierarchical structures in the universe, from galaxies, groups of galaxies, clusters and superclusters, to filaments and voids. On the largest scales, it seems that some kind of statistical homogeneity can be observed. As a result, modern cosmological models are based on spatially homogeneous and isotropic solutions of the Einstein equations, and the evolution of the universe is approximated by the Friedmann equations. In parallel to standard homogeneous cosmology, the field of inhomogeneous cosmology and backreaction is being developed. This field investigates whether small scale inhomogeneities via nonlinear effects can backreact and alter the properties of the universe on its largest scales, leading to a non-Friedmannian evolution. This paper presents the current status of inhomogeneous cosmology and backreaction. It also discusses future prospects of the field of inhomogeneous cosmology, which is based on a survey of 50 academics working in the field of inhomogeneous cosmology.
TL;DR: In this paper, the quasinormal modes for a scalar field in the background spacetime corresponding to a black hole, with a cloud of strings, in the Einstein-Gauss-Bonnet gravity, and the tensor modes corresponding to perturbations in such spacetime, were both calculated using the WKB approximation.
Abstract: The quasinormal modes for a scalar field in the background spacetime corresponding to a black hole, with a cloud of strings, in Einstein–Gauss–Bonnet gravity, and the tensor quasinormal modes corresponding to perturbations in such spacetime, were both calculated using the WKB approximation. In the obtained results, we emphasize the role played by the parameter associated with the string cloud, comparing them with the results already obtained for the Boulware–Deser metric. We also study how the Gauss–Bonnet correction to general relativity affects the results for the quasinormal modes, comparing them with the same background in general relativity.
TL;DR: In this paper, the stability of the universe against linear homogeneous perturbations in the background of f(𝒢,T) gravity was investigated.
Abstract: This paper explores the stability of the Einstein universe against linear homogeneous perturbations in the background of f(𝒢,T) gravity. We construct static as well as perturbed field equations and investigate stability regions for the specific forms of generic function f(𝒢,T) corresponding to conserved as well as nonconserved energy-momentum tensor. We use the equation-of-state parameter to parameterize the stability regions. The graphical analysis shows that the suitable choice of parameters lead to stable regions of the Einstein universe.