Showing papers in "General Relativity and Gravitation in 2020"

Neutron-star tidal deformability and equation-of-state constraints

Abstract: Despite their long history and astrophysical importance, some of the key properties of neutron stars are still uncertain. The extreme conditions encountered in their interiors, involving matter of uncertain composition at extreme density and isospin asymmetry, uniquely determine the stars’ macroscopic properties within General Relativity. Astrophysical constraints on those macroscopic properties, such as neutron-star masses and radii, have long been used to understand the microscopic properties of the matter that forms them. In this article we discuss another astrophysically observable macroscopic property of neutron stars that can be used to study their interiors: their tidal deformation. Neutron stars, much like any other extended object with structure, are tidally deformed when under the influence of an external tidal field. In the context of coalescences of neutron stars observed through their gravitational-wave emission, this deformation, quantified through a parameter termed the tidal deformability, can be measured. We discuss the role of the tidal deformability in observations of coalescing neutron stars with gravitational waves and how it can be used to probe the internal structure of Nature’s most compact matter objects. Perhaps inevitably, a large portion of the discussion will be dictated by GW170817, the most informative confirmed detection of a binary neutron-star coalescence with gravitational waves as of the time of writing.

Prospects for Fundamental Physics with LISA

Abstract: In this paper, which is of programmatic rather than quantitative nature, we aim to further delineate and sharpen the future potential of the LISA mission in the area of fundamental physics. Given the very broad range of topics that might be relevant to LISA,we present here a sample of what we view as particularly promising fundamental physics directions. We organize these directions through a “science-first” approach that allows us to classify how LISA data can inform theoretical physics in a variety of areas. For each of these theoretical physics classes, we identify the sources that are currently expected to provide the principal contribution to our knowledge, and the areas that need further development. The classification presented here should not be thought of as cast in stone, but rather as a fluid framework that is amenable to change with the flow of new insights in theoretical physics.

Tracing the high energy theory of gravity: an introduction to Palatini inflation

Abstract: We present an introduction to cosmic inflation in the context of Palatini gravity, which is an interesting alternative to the usual metric theory of gravity. In the latter case only the metric $$g_{\mu u }$$ determines the geometry of space-time, whereas in the former case both the metric and the space-time connection $$\varGamma ^\lambda _{\mu u }$$ are a priori independent variables—a choice which can lead to a theory of gravity different from the metric one. In scenarios where the field(s) responsible for cosmic inflation are coupled non-minimally to gravity or the gravitational sector is otherwise extended, assumptions of the underlying gravitational degrees of freedom can have a big impact on the observational consequences of inflation. We demonstrate this explicitly by reviewing several interesting and well-motivated scenarios including Higgs inflation, $$R^2$$ inflation, and $$\xi $$-attractor models. We also discuss some prospects for future research and argue why $$r=10^{-3}$$ is a particularly important goal for future missions that search for signatures of primordial gravitational waves.

Neutron star merger remnants

Abstract: Binary neutron star mergers observations are a unique way to constrain fundamental physics and astrophysics at the extreme. The interpretation of gravitational-wave events and their electromagnetic counterparts crucially relies on general-relativistic models of the merger remnants. Quantitative models can be obtained only by means of numerical relativity simulations in $$3+1$$ dimensions including detailed input physics for the nuclear matter, electromagnetic and weak interactions. This review summarizes the current understanding of merger remnants focusing on some of the aspects that are relevant for multimessenger observations.

Non-inertial effects on a generalized DKP oscillator in a cosmic string space-time

Abstract: We examine the relativistic generalized DKP oscillator for a spin-zero field in a cosmic-string background space-time characterized by a stationary cylindrical symmetric metric. We solve the radial part of the wave function for linear, Coulomb (and shifted Coulomb) and Cornell functions. Thus we obtain the energy eigenvalues and eigenfunctions of the generalized DKP oscillator.

Fuzzballs and observations

Abstract: The advent of gravitational waves and black hole imaging has opened a new window into probing the horizon scale of black holes. An important question is whether string theory results for black holes can predict interesting and observable features that current and future experiments can probe. In this article I review the budding and exciting research being done on understanding the possibilities of observing signals from fuzzballs, where black holes are replaced by string-theoretic horizon-scale microstructure. In order to be accessible to both string theorists and black hole phenomenologists, I give a brief overview of the relevant observational experiments as well as the fuzzball paradigm in string theory and its explicitly constructable solutions called microstate geometries.

On Average Properties of Inhomogeneous Fluids in General Relativity III: General Fluid Cosmologies

Abstract: We investigate effective equations governing the volume expansion of spatially averaged portions of inhomogeneous cosmologies in spacetimes filled with an arbitrary fluid. This work is a follow-up to previous studies focused on irrotational dust models (Paper I) and irrotational perfect fluids (Paper II) in flow-orthogonal foliations of spacetime. It complements them by considering arbitrary foliations, arbitrary lapse and shift, and by allowing for a tilted fluid flow with vorticity. As for the first studies, the propagation of the spatial averaging domain is chosen to follow the congruence of the fluid, which avoids unphysical dependencies in the averaged system that is obtained. We present two different averaging schemes and corresponding systems of averaged evolution equations providing generalizations of Papers I and II. The first one retains the averaging operator used in several other generalizations found in the literature. We extensively discuss relations to these formalisms and pinpoint limitations, in particular regarding rest mass conservation on the averaging domain. The alternative averaging scheme that we subsequently introduce follows the spirit of Papers I and II and focuses on the fluid flow and the associated $$1+3$$ threading congruence, used jointly with the $$3+1$$ foliation that builds the surfaces of averaging. This results in compact averaged equations with a minimal number of cosmological backreaction terms. We highlight that this system becomes especially transparent when applied to a natural class of foliations which have constant fluid proper time slices.

The key role of magnetic fields in binary neutron star mergers

Abstract: The first multimessenger observation of a binary neutron star (BNS) merger in August 2017 demonstrated the huge scientific potential of these extraordinary events. This breakthrough led to a number of discoveries and provided the best evidence that BNS mergers can launch short gamma-ray burst (SGRB) jets and are responsible for a copious production of heavy r-process elements. On the other hand, the details of the merger and post-merger dynamics remain only poorly constrained, leaving behind important open questions. Numerical relativity simulations are a powerful tool to unveil the physical processes at work in a BNS merger and as such they offer the best chance to improve our ability to interpret the corresponding gravitational wave (GW) and electromagnetic emission. Here, we review the current theoretical investigation on BNS mergers based on general relativistic magnetohydrodynamics simulations, paying special attention to the magnetic field as a crucial ingredient. First, we discuss the evolution, amplification, and emerging structure of magnetic fields in BNS mergers. Then, we consider their impact on various critical aspects: (i) jet formation and the connection with SGRBs, (ii) matter ejection, r-process nucleosynthesis, and radioactively-powered kilonova transients, and (iii) post-merger GW emission.

Beyond $\Lambda $CDM with low and high redshift data: implications for dark energy

Abstract: Assuming that the Universe at higher redshifts ($$z \sim 4$$ and beyond) is consistent with $$\Lambda $$CDM model as constrained by the Planck measurements, we reanalyze the low redshift cosmological data to reconstruct the Hubble parameter as a function of redshift. This enables us to address the $$H_0$$ and other tensions between low z observations and high z Planck measurement from CMB. From the reconstructed H(z), we compute the energy density for the “dark energy” sector of the Universe as a function of redshift without assuming a specific model for dark energy. We find that the dark energy density has a minimum for certain redshift range and that the value of dark energy at this minimum $${\rho }_{_{\text {DE}}}^{\text {min}}$$ is negative. This behavior can most simply be described by a negative cosmological constant plus an evolving dark energy component. We discuss possible theoretical and observational implications of such a scenario.

Boundary effects in General Relativity with tetrad variables

Abstract: Varying the gravitational Lagrangian produces a boundary contribution that has various physical applications. It determines the right boundary terms to be added to the action once boundary conditions are specified, and defines the symplectic structure of covariant phase space methods. We study general boundary variations using tetrads instead of the metric. This choice streamlines many calculations, especially in the case of null hypersurfaces with arbitrary coordinates, where we show that the spin-1 momentum coincides with the rotational 1-form of isolated horizons. The additional gauge symmetry of internal Lorentz transformations leaves however an imprint: the boundary variation differs from the metric one by an exact 3-form. On the one hand, this difference helps in the variational principle: gluing hypersurfaces to determine the action boundary terms for given boundary conditions is simpler, including the most general case of non-orthogonal corners. On the other hand, it affects the construction of Hamiltonian surface charges with covariant phase space methods, which end up being generically different from the metric ones, in both first and second-order formalisms. This situation is treated in the literature gauge-fixing the tetrad to be adapted to the hypersurface or introducing a fine-tuned internal Lorentz transformation depending non-linearly on the fields. We point out and explore the alternative approach of dressing the bare symplectic potential to recover the value of all metric charges, and not just for isometries. Surface charges can also be constructed using a cohomological prescription: in this case we find that the exact 3-form mismatch plays no role, and tetrad and metric charges are equal. This prescription leads however to different charges whether one uses a first-order or second-order Lagrangian, and only for isometries one recovers the same charges.

Particle and entropy production in the running vacuum universe

Abstract: We study particle production and the corresponding entropy increase in the context of cosmology with dynamical vacuum. We focus on the particular form that has been called “running vacuum model” (RVM), which is known to furnish a successful description of the overall current observations at a competitive level with the concordance $$\Lambda $$CDM model. It also provides an elegant global explanation of the cosmic history from a non-singular initial state in the very early universe up to our days and further into the final de Sitter era. The model has no horizon problem and offers an alternative explanation for the early inflation and its graceful exit, as well as a powerful mechanism for generating the large entropy of the current universe. The energy–momentum tensor of matter is generally non-conserved in such context owing to particle creation or annihilation. We analyze general thermodynamical aspects of particle and entropy production in the RVM. We first study the entropy of particles in the comoving volume during the early universe and late universe. Then, in order to obtain a more physical interpretation, we pay attention to the entropy contribution from the cosmological apparent horizon, its interior and its surface. On combining the inner volume entropy with the entropy on the horizon, we elucidate with detailed calculations whether the evolution of the entropy of the RVM universe satisfies the generalized second law of thermodynamics. We find it is so and we prove that the essential reason for it is the existence of a positive cosmological constant.

Joule–Thomson expansion of the Bardeen-AdS black holes

Abstract: The Joule–Thomson expansion process is studied for Bardeen-AdS black holes in the extended phase space. We first get the Joule–Thomson coefficient, and then the inversion curves are obtained. In addition, the minimum inversion temperature and its corresponding mass are obtained. The ratio between minimum inversion and critical temperature for Bardeen-AdS black holes is also calculated. The isenthalpic curves are drawn in T–P graph and cooling-heating region are also demonstrated by the inversion curve. An interesting phenomenon we get is that black hole whose mass to charge ratio is below the critical value is always in heating process. The same phenomenon can be also obtained from the charged AdS black holes.

Cosmological perfect fluids in higher-order gravity

Abstract: We prove that in Robertson–Walker space-times (and in generalized Robertson–Walker spacetimes of dimension greater than 3 with divergence-free Weyl tensor) all higher-order gravitational corrections of the Hilbert–Einstein Lagrangian density $$F(R,\square R, \ldots , \square ^k R)$$ have the form of perfect fluids in the field equations. This statement definitively allows to deal with dark energy fluids as curvature effects.

Kerr–Newman–AdS black hole with quintessence and cloud of strings

Abstract: We obtain the solution corresponding to a Kerr–Newman–anti de Sitter black hole with quintessence and a spherically symmetric cloud of strings by using the Newman–Janis algorithm slightly modified. We analyze the horizon structure and the ergoregions, study the thermodynamics and the Hawking radiation as well. We discuss the role played by the different sources, namely, the quintessence, cosmological constant and cloud of strings on the horizons, ergoregions, thermodynamic quantities and in the flux of scalar particles associated to the Hawking radiation.

Non-perturbative 3D quantum gravity: quantum boundary states and exact partition function

Abstract: We push forward the investigation of holographic dualities in 3D quantum gravity formulated as a topological quantum field theory, studying the correspondence between boundary and bulk structures. Working with the Ponzano–Regge topological state-sum model defining an exact discretization of 3D quantum gravity, we analyse how the partition function for a solid twisted torus depends on the boundary quantum state. This configuration is relevant to the AdS$$_{3}$$/CFT$$_{2}$$ correspondence. We introduce boundary spin network states with coherent superposition of spins on a square lattice on the boundary surface. This allows for the first exact analytical calculation of Ponzano–Regge amplitudes with extended 2D boundary (beyond the single tetrahedron). We get a regularized finite truncation of the BMS character formula obtained from the one-loop perturbative quantization of 3D gravity. This hints towards the existence of an underlying symmetry and the integrability of the theory for finite boundary at the quantum level for coherent boundary spin network states.

Thermodynamic criticality of d-dimensional charged AdS black holes surrounded by quintessence with a cloud of strings background

Abstract: We focus on the study of exact solution corresponding to charged AdS black holes surrounded by quintessence with a cloud of strings present in higher dimensional spacetime. We then investigate its corresponding thermodynamic criticality in the extended phase space and show that the spacetime dimension has no effect on the existence of small/large phase transition for such black holes. The heat capacity is evaluated and the geothermodynamics of Quevedo analyzed for different spacetime dimensions with the cloud of strings and quintessence parameters. We calculate the critical exponents describing the behavior of relevant thermodynamic quantities near the critical point. Finally, we also discuss the uncharged case, show how it is sensitive to the quintessence and strings cloud parameters, and when the thermodynamic behavior of the uncharged black holes is similar to Van der Waals fluid.

Quasinormal modes and strong cosmic censorship in the regularised 4D Einstein–Gauss–Bonnet gravity

Abstract: The fate of strong cosmic censorship is ultimately linked to the extendibility of perturbation across the Cauchy Horizon and known to be violated in the near extremal region of a charged de Sitter black hole. Similar violations can also be realized in higher curvature theories, with the strength of violation becoming stronger as compared to general relativity. In this work, we extend this analysis further to study the validity of strong cosmic censorship conjecture in the context of the regularised four-dimensional Einstein Gauss–Bonnet theory with respect to both scalar and electromagnetic perturbation. We also study the late time tails of scalar fields.

Compact binary coalescences: constraints on waveforms

Abstract: Gravitational waveforms for compact binary coalescences (CBCs) have been invaluable for detections by the LIGO-Virgo collaboration. They are obtained by a combination of semi-analytical models and numerical simulations. So far systematic errors arising from these procedures appear to be less than statistical ones. However, the significantly enhanced sensitivity of the new detectors that will become operational in the near future will require waveforms to be much more accurate. This task would be facilitated if one has a variety of cross-checks to evaluate accuracy, particularly in the regions of parameter space where numerical simulations are sparse. Currently errors are estimated by comparing the candidate waveforms with the numerical relativity (NR) ones, which are taken to be exact. The goal of this paper is to propose a qualitatively different tool. We show that full non-linear general relativity (GR) imposes an infinite number of sharp constraints on the CBC waveforms. These can provide clear-cut measures to evaluate the accuracy of candidate waveforms against exact GR, help find systematic errors, and also provide external checks on NR simulations themselves.

Aharonov–Bohm effect for bound states in the cosmic string spacetime in the context of rainbow gravity

Abstract: We explore a scenario of the general relativity determined by the framework of the rainbow gravity with the purpose of searching for analogues of the Aharonov–Bohm effect. By focusing on the confinement of the Dirac field and the scalar field to a hard-wall confining potential in a modified background of the cosmic string spacetime, we examine the effects of the rainbow gravity and the topology of the cosmic string spacetime. Then, we compare each spectrum of energy with the cases where the rainbow gravity is absent.

Stability and horizon formation during dissipative collapse

Abstract: We investigate the role played by density inhomogeneities and dissipation on the final outcome of collapse of a self-gravitating sphere. By imposing a perturbative scheme on the thermodynamical variables and gravitational potentials we track the evolution of the collapse process starting off with an initially static perfect fluid sphere which is shear-free. The collapsing core dissipates energy in the form of a radial heat flux with the exterior spacetime being filled with a superposition of null energy and an anisotropic string distribution. The ensuing dynamical process slowly evolves into a shear-like regime with contributions from the heat flux and density fluctuations. We show that the anisotropy due to the presence of the strings drives the stellar fluid towards instability with this effect being enhanced by the density inhomogeneity. An interesting and novel consequence of this collapse is the earlier formation of the horizon.

Test fields cannot destroy extremal de Sitter black holes

Abstract: We determine the timelike Killing vector field that gives the correct definition of energy for test fields propagating in a Kerr–Newman–de Sitter spacetime, and use this result to prove that test fields cannot destroy extremal Kerr–Newman–de Sitter black holes.

Higher dimensional charged black hole surrounded by quintessence in massive gravity

Abstract: In this paper, we have found a D-dimensional static and spherically symmetric charged black hole solution with the quintessence matter surrounding in the context of the massive gravity. We studied the horizon properties of this black hole solution, which depend crucially on the sign of the coupling parameters of the massive gravity. In addition, we investigated the black hole thermodynamics in details from the local and global viewpoints at which various thermodynamic quantities are computed. The thermodynamic stability and phase transition of the black hole are analyzed by studying the behavior of the heat capacity at constant normalization factor which is related to the energy density of the quintessence matter.

Stability of a modified Jordan–Brans–Dicke theory in the dilatonic frame

Abstract: We investigate the Jordan–Brans–Dicke action in the cosmological scenario of FLRW spacetime with zero spatially curvature and with an extra scalar field minimally coupled to gravity as matter source. The field equations are studied in two ways. The method of group invariant transformations, i.e., symmetries of differential equations apply in order to constraint the free functions of the theory and determine conservation laws for the gravitational field equations. The second method that we apply for the study of the evolution of the field equations is the stability analysis of equilibrium points. Particularly, we find solutions with $$w_{\text {tot}}=-\,1$$ , and we study their stability by means of the Center Manifold Theorem. We show this solution is an attractor in the dilatonic frame but it is an intermediate accelerated solution $$a \simeq e^{A t^p}, p:=\frac{2}{2+l}, \quad \frac{32}{57+6 \omega _0}

Light cone and Weyl compatibility of conformal and projective structures

Abstract: In the literature different concepts of compatibility between a projective structure $${\mathscr {P}}$$ and a conformal structure $${\mathscr {C}}$$ on a differentiable manifold are used. In particular compatibility in the sense of Weyl geometry is slightly more general than compatibility in the Riemannian sense. In an often cited paper (Ehlers et al. in: O’Raifertaigh (ed) General Relativity, Papers in Honour of J.L. Synge, Clarendon Press, Oxford, 2012) Ehlers/Pirani/Schild introduce still another criterion which is natural from the physical point of view: every light like geodesics of $${\mathscr {C}}$$ is a geodesics of $${\mathscr {P}}$$ . Their claim that this type of compatibility is sufficient for introducing a Weylian metric has recently been questioned (Trautman in Gen Relativ Gravit 44:1581–1586, 2012); (Vladimir in Commun Math Phys 329:821–825, 2014); as reported by Scholz (in: A scalar field inducing a non-metrical contribution to gravitational acceleration and a compatible add-on to light deflection, 2019). Here it is proved that the conjecture of EPS is correct.

Aspects of C 0 causal theory

Abstract: This paper serves as an introduction to $$C^0$$ causal theory. We focus on those parts of the theory which have proven useful for establishing spacetime inextendibility results in low regularity—a question which is motivated by the strong cosmic censorship conjecture in general relativity. This paper is self-contained; prior knowledge of causal theory is not assumed.

Times of arrival (TOA) of signals in the Kerr-MOG black hole

Abstract: Modified gravity (MOG) theories are alternatives to general relativity (GR) that arose primarily from the need to explain the observed galactic flat rotation curves without invoking the elusive dark matter hypothesized by GR. A well known MOG is the Scalar–Tensor–Vector–Gravity developed by Moffat, who has also found a spinning solution called the Kerr-MOG black hole (BH) characterized by the spin a and MOG parameter $$\alpha $$, the latter determining the strength of the gravitational vector forces. We consider the static-MOG metric ($$a=0$$) to first understand how the nature of geometry drastically changes depending on different sectors of $$\alpha $$. Then we study the influence of $$\alpha $$ in each sector on a new astrophysical diagnostic caused by frame dragging, viz., the difference $$\varDelta t$$ in the times of arrival at the observer of signals emanating from a variable pulsar (PSR) passing behind a Kerr-MOG lens in a PSR-BH binary system. The study generalizes the zeroth order Laguna–Wolszczan formula up to third PPN order in $$\left( 1/r\right) $$ using thin-lens approximation, which reveals how $$\varDelta t$$ is influenced both by a and $$\alpha $$. The magnitude and sign of $$\alpha $$ indicate deviations from GR ($$\alpha =0$$) and future measurements may constrain $$\alpha $$ provided a suitable binary is identified.

Recovering $\Lambda$CDM Model From a Cosmographic Study

Abstract: Using the mathematical definitions of deceleration and jerk parameters we obtain a general differential equation for squared Hubble parameter. For a constant jerk, this differential equation leads to an exact function for Hubble parameter. By the aid of this exact Hubble function we can exactly reconstruct any other cosmographic parameters. We also obtained a general function for transition redshift as well as spacetime curvature. Our derived functions clearly impose a lower limit on the jerk parameter which is $$j_{min}\ge -\,0.125$$. Moreover, we found that the jerk parameter indicates the geometry of the spacetime i.e any deviation from $$j=1$$ imply to a non-flat spacetime. In other word $$j e 1$$ refers to a dynamical, time varying, dark energy. From obtained Hubble function we recover the analogue of $$\Lambda $$CDM model. To constrain cosmographic parameters as well as transition redshift and spacetime curvature of the recovered $$\Lambda $$CDM model, we used Metropolis–Hasting algorithm to perform Monte Carlo Markov Chain analysis by using observational Hubble data obtained from cosmic chronometric technique, BAO data, Pantheon compilation of Supernovae type Ia, and their joint combination. The only free parameters are H, $$A(\Omega _{m})$$ and j. From joint analysis we obtained $$H_{0}=69.9\pm 1.7$$, $$A(\sim \Omega _{0m})=0.279^{+0.013}_{-0.017}$$, $$B(\sim \Omega _{0X})=0.721^{+0.017}_{-0.013}$$, $$j_{0}=1.038^{+0.061}_{-0.023}$$ and $$z_{t}=0.706^{+0.031}_{-0.034}$$.

Proof of the weak cosmic censorship conjecture for several extremal black holes

Abstract: We show explicitly, for different types of extremal black holes, that test fields satisfying the null energy condition at the event horizon cannot violate the weak cosmic censorship conjecture. This is done by checking, in each case, that the hypotheses for a general theorem proved in a previous paper are satisfied.

Phase transitions in Born-Infeld AdS black holes in D-dimensions

Abstract: In this paper, we have investigated phase transitions in arbitrary spacetime dimensions for Born-Infeld AdS black holes. The phase transition points are characterised from the divergence of heat capacity of the black hole. Two well established techniques, namely, the Ehrenfest scheme and the Ruppeiner state space geometry approach are used to identify the order of phase transition the black hole undergoes. It is observed that the results obtained from these two methods agree with each other. Our analysis reveals that the phase transition is of second order. It is also observed from the variation of the heat capacity with entropy that the small unstable black hole phase becomes more and more stable with increase in the spacetime dimensions. We speculate that this dependence of the stability of the black hole on the spacetime dimension can put an upper limit to the dimension of spacetime from the physical condition of the improbability of the formation of a small stable black hole. We have also derived a Smarr relation in D-spacetime dimensions using scaling arguments and first law of black hole thermodynamics which includes the cosmological constant and the Born-Infeld parameters as thermodynamic variables.

Exact analytical solution for an Israel–Stewart cosmology

Abstract: In this article we report a novel analytic solution describing a cosmological model with a matter content represented by a one dissipative fluid component, in the framework of the causal Israel–Stewart theory The dissipative fluid is described by a barotropic equation of state $$p= (\gamma -1) \rho $$ and the bulk viscosity has been assumed of the form $$\xi =\xi _{0}\rho ^{s}$$ We study within the parameter space which label the solution, a suited region compatible with an accelerated expansion of the universe for late times, as well as stability properties of the solution at the critical parameter value $$ \gamma = 1$$ and for $$ s = 1/2 $$ We study as well the consequences that arise from the positiveness of the entropy production along the time evolution We found that the solution for pressureless dark matter, $$ \gamma = 1$$ , can well describe a universe with a transition from a decelerated expansion to an accelerated one at late times, but with a very large non-adiabatic contribution to speed of sound Finally, the kinematics and thermodynamics properties of the solutions are discussed in terms of the type of expansion and entropy production