Collapse In Gravity Research Articles

Shell-crossings and shock formation during gravitational collapse in effective loop quantum gravity

Shell-crossings and shock formation during gravitational collapse in effective loop quantum gravity

Gravitational collapse in energy-momentum squared gravity: Nature of singularities

In this paper we explore a collapsing scenario in the background of energy-momentum squared gravity (EMSG). EMSG claims to have terms that originate from the quantum gravity effects mimicking loop quantum gravity. As a result the framework admits a bounce at a finite time thus avoiding a singularity. So the question that naturally arises: Is there any realistic chance of formation of a black hole or the quantum gravity effects are strong enough to totally avoid such a pathology? Motivated from this we are interested in studying a gravitational collapse mechanism in the background of EMSG and investigate the fate of such a process. We model the spacetime of a massive star by the Vaidya metric and derive the field equations in EMSG. Then using the equations we go on to study a gravitational collapse mechanism, on two specific models of EMSG with different forms of curvature-matter coupling. The prime objective is to probe the nature of singularity (if formed) as the end state of the collapse. We see that none of the models generically admit the formation of black holes as the end state of collapse, but on the contrary they support the formation of naked singularities. This can be attributed to the quantum fluctuations of the gravitational interactions at the fundamental level.

Model metrics for quantum black hole evolution: Gravitational collapse, singularity resolution, and transient horizons

It is widely accepted that curvature singularity resolution should be a feature of quantum gravity. We present a class of time-dependent asymptotically flat spherically symmetric metrics that model gravitational collapse in quantum gravity. The metrics capture intuitions associated with the dynamics of singularity resolution, and horizon formation and evaporation following a matter bounce. A parameter in the metric associated with the speed of the bounce determines black hole lifetime as a power of its mass; this includes the Hawking evaporation time $M^{3}$.

Dynamics of charged string cloud collapse in rainbow gravity

This work will proceed to examine the dynamics of collapsing charged string cloud in a spherically symmetric rainbow metric. This is the framework that allows new stars and compact objects to emerge. The dynamical variables (mass density, string tension) and their physical existence as energy conditions are computed and visually depicted. We also calculated the duration and radius of development of the collapsing string cloud’s apparent horizons. The mass density, string tension, expected time and radius of apparent horizon creation are dependent on the Rainbow parameter [Formula: see text] At the center of the configuration [Formula: see text] the mass density, string tension and electric field intensity are at their maximum leading to the formation of a compact object (black hole). The fluid’s mass density and string tension are reduced by the electric field. Furthermore, the presence of an electric field delays the formation of a compact object (black hole).

Replica wormholes for an evaporating 2D black hole

Quantum extremal islands reproduce the unitary Page curve of an evaporating black hole. This has been derived by including replica wormholes in the gravitational path integral, but for the transient, evaporating black holes most relevant to Hawking’s paradox, these wormholes have not been analyzed in any detail. In this paper we study replica wormholes for black holes formed by gravitational collapse in Jackiw-Teitelboim gravity, and confirm that they lead to the island rule for the entropy. The main technical challenge is that replica wormholes rely on a Euclidean path integral, while the quantum extremal islands of an evaporating black hole exist only in Lorentzian signature. Furthermore, the Euclidean equations for the Schwarzian mode are non-local, so it is unclear how to connect to the local, Lorentzian dynamics of an evaporating black hole. We address these issues with Schwinger-Keldysh techniques and show how the non-local equations reduce to the local ‘boundary particle’ description in special cases.

Dynamics of charged anisotropic spherical collapse in energy-momentum squared gravity

This paper investigates the dynamics of charged spherical collapse with anisotropic matter configuration in the context of energy-momentum squared gravity. This newly developed proposal resolves the big-bang singularity and yields the physically viable cosmological results in the early time universe. We establish dynamical equations through Misner-Sharp technique and analyze the effects of charge, anisotropy, effective matter variables and dark source terms on the collapse rate. A relation between Weyl scalar, fluid parameters and dark source terms is also established. The spacetime is not conformally flat due to the presence of anisotropic pressure, multivariate functions and their derivatives. In order to obtain conformally flat spacetime, we consider a specific model of this gravity, neglect the impact of charge and assume the isotropic matter distribution which yields homogeneity of the energy density and conformally flat spacetime. We conclude that positive dark source terms, anisotropy and charge yield the action of a repulsive force which enhances the stability of the system and hence diminishes the collapse rate.

Black hole collapse and bounce in effective loop quantum gravity

We derive effective equations with loop quantum gravity corrections for the Lemaître–Tolman–Bondi family of space-times, and use these to study quantum gravity effects in the Oppenheimer–Snyder collapse model. For this model, after the formation of a black hole with an apparent horizon, quantum gravity effects become important in the space-time region where the energy density and space-time curvature scalars become comparable to the Planck scale. These quantum gravity effects first stop the collapse of the dust matter field when its energy density reaches the Planck scale, and then cause the dust field to begin slowly expanding. Due to this continued expansion, the matter field will eventually extend beyond the apparent horizon, at which point the horizon disappears and there is no longer a black hole. There are no singularities anywhere in this space-time. In addition, in the limit that edge effects are neglected, we show that the dynamics for the interior of the star of uniform energy density follow the loop quantum cosmology effective Friedman equation for the spatially flat Friedman–Lemaître–Robertson–Walker space-time. Finally, we estimate the lifetime of the black hole, as measured by a distant observer, to be ∼(GM)2/ℓ Pl.

Gravitational collapse for the K-essence emergent Vaidya spacetime

In this paper, we study the gravitational collapse in the k-essence emergent gravity using a generalized Vaidya-type metric as a background. We also analyze the cosmic censorship hypothesis for this system. We show that the emergent gravity metric resembles closely to the new type of the generalized Vaidya metrics for null fluid collapse with the k-essence emergent mass function, where we consider the k-essence scalar field being a function solely of the advanced or the retarded time. This new type of k-essence emergent Vaidya metric has satisfied the required energy conditions. The existence of the locally naked central singularity, the strength and the strongness of the singularities for the k-essence emergent Vaidya metric are the interesting outcomes of the present work.

Core collapse in massive scalar-tensor gravity

This paper provides an extended exploration of the inverse-chirp gravitational-wave signals from stellar collapse in massive scalar-tensor gravity reported in [Phys. Rev. Lett. {\bf 119}, 201103]. We systematically explore the parameter space that characterizes the progenitor stars, the equation of state and the scalar-tensor theory of the core collapse events. We identify a remarkably simple and straightforward classification scheme of the resulting collapse events. For any given set of parameters, the collapse leads to one of three end states, a weakly scalarized neutron star, a strongly scalarized neutron star or a black hole, possibly formed in multiple stages. The latter two end states can lead to strong gravitational-wave signals that may be detectable in present continuous-wave searches with ground-based detectors. We identify a very sharp boundary in the parameter space that separates events with strong gravitational-wave emission from those with negligible radiation.

Electromagnetic effect on scalar field collapse in higher curvature gravity

We consider a “Scalar-Maxwell–Einstein–Gauss–Bonnet” theory in four dimension, where the scalar field couples non-minimally with the Gauss–Bonnet (GB) term. This coupling with the scalar field ensures the non topological character of the GB term. In such higher curvature scenario, we explore the effect of electromagnetic field on scalar field collapse. Our results reveal that the presence of a time dependent electromagnetic field requires an anisotropy in the background spacetime geometry and such anisotropic spacetime allows a collapsing solution for the scalar field. The singularity formed as a result of the collapse is found to be a curvature singularity which may be point like or line like depending on the strength of the anisotropy. We also show that the singularity is always hidden from exterior by an apparent horizon.

Inverse-chirp signals and spontaneous scalarisation with self-interacting potentials in stellar collapse

We study how the gravitational wave signal from stellar collapse in scalar-tensor gravity varies under the influence of scalar self-interaction. To this end, we extract the gravitational radiation from numerical simulations of stellar collapse for a range of potentials with higher-order terms in addition to the quadratic mass term. Our study includes collapse to neutron stars and black holes and we find the strong inverse-chirp signals obtained for the purely quadratic potential to be exceptionally robust under changes in the potential at higher orders; quartic and sextic terms in the potential lead to noticeable differences in the wave signal only if their contribution is amplified, implying a relative fine-tuning to within five or more orders of magnitude between the mass and self-interaction parameters.

Vaidya spacetime for Galileon gravity's rainbow

In this paper, we analyze Vaidya spacetime with an energy dependent metric in Galileon gravity's rainbow. This will be done using the rainbow functions which are motivated from the results obtained in loop quantum gravity approach and noncommutative geometry. We will investigate the Gravitational collapse in this Galileon gravity's rainbow. We will discuss the behavior of singularities formed from the gravitational collapse in this rainbow deformed Galileon gravity.

Viscous Chaplygin gas models as spherical top-hat collapsing fluids

We study the spherical top-hat collapse in Einstein gravity and loop quantum cosmology (LQC) by taking the nonlinear evolution of viscous modified variable Chaplygin gas (CG) and viscous generalized cosmic chaplygin gas (GCCG). We calculate the equation of state (EoS) parameter, square speed of sound, perturbed (EoS) parameter, perturbed square speed of sound, density contrast and divergence of peculiar velocity in perturbed region and discussed their behavior. It is observed that both CG models support the spherical collapse (SC) in Einstein as well as LQC because density contrast remains positive in both cases and the perturbed EoS parameter remains positive at the present epoch as well as near future. It is remarked here that these parameters provide consistent results for both CG models in both gravities.

Gravitational collapse in gravity's rainbow

In this paper, we will analyze the gravitational collapse in the framework of gravity's rainbow. We will demonstrate that the position of the horizon for a particle inside the black hole depends on the energy of that particle. It will also be observe that the position of the horizon for a particle falling radially into the black hole also depends on its energy. Thus, it is possible for a particle coming from outside to interact with a particle inside the black, and take some information outside the black hole. This is because for both these particles the position of horizon is different. So, even though the particle from inside the black hole is in its own horizon, it is not in the horizon of the particle coming from outside. Thus, we will demonstrate that in gravity's rainbow information can get out of a black hole.

New Cosmological Solutions in Massive Gravity

In this talk, we have presented new and simple cosmological solutions with flat, open, and closed spatial geometries, recently found by the present authors. The metric and the Stückelberg fields are given explicitly, showing nontrivial configurations of the Stückelberg in the usual Friedmann-Lemaître-Robertson-Walker coordinates. The solutions exhibit self-acceleration, while being free from ghost instabilities. Our solutions can be extended to include inhomogeneous dust collapse represented by the Lemaître-Tolman-Bondi metric as well. Thus, our results can be used not only to describe homogeneous and isotropic cosmology but also to study gravitational collapse in massive gravity.

Spherical collapse in Galileon gravity: fifth force solutions, halo mass function and halo bias

We study spherical collapse in the Quartic and Quintic Covariant Galileon gravity models within the framework of the excursion set formalism. We derive the nonlinear spherically symmetric equations in the quasi-static and weak-field limits, focusing on model parameters that fit current CMB, SNIa and BAO data. We demonstrate that the equations of the Quintic model do not admit physical solutions of the fifth force in high density regions, which prevents the study of structure formation in this model. For the Quartic model, we show that the effective gravitational strength deviates from the standard value at late times (z≲1), becoming larger if the density is low, but smaller if the density is high. This shows that the Vainshtein mechanism at high densities is not enough to screen all of the modifications of gravity. This makes halos that collapse at z≲1 feel an overall weaker gravity, which suppresses halo formation. However, the matter density in the Quartic model is higher than in standard ΛCDM, which boosts structure formation and dominates over the effect of the weaker gravity. In the Quartic model there is a significant overabundance of high-mass halos relative to ΛCDM. Dark matter halos are also less biased than in ΛCDM, with the difference increasing appreciably with halo mass. However, our results suggest that the bias may not be small enough to fully reconcile the predicted matter power spectrum with LRG clustering data.

Gravitational collapse in pure Lovelock gravity in higher dimensions

We study collapse of inhomogeneous dust and null dust (Vaidya radiation) in pure Lovelock gravity in higher dimensions. Since pure Lovelock gravity is kinematic in odd d=2N+1 dimension, hence pertinent dimension for the study is even d=2N+2, where N is degree of Lovelock polynomial. It turns out that pure Lovelock collapse favors naked singularity as against black hole for the Einstein case in the same dimension while strength of singularity as measured by divergence of Kretshmann scalar is interestingly the same in the two cases; i.e. the corresponding scalars have the same fall off behavior.

DOES PRESSURE ACCENTUATE GENERAL RELATIVISTIC GRAVITATIONAL COLLAPSE AND FORMATION OF TRAPPED SURFACES?

It is widely believed that though pressure resists gravitational collapse in Newtonian gravity, it aids the same in general relativity (GR) so that GR collapse should eventually be similar to the monotonous free fall case. But we show that, even in the context of radiationless adiabatic collapse of a perfect fluid, pressure tends to resist GR collapse in a manner which is more pronounced than the corresponding Newtonian case and formation of trapped surfaces is inhibited. In fact there are many works which show such collapse to rebound or become oscillatory implying a tug of war between attractive gravity and repulsive pressure gradient. Furthermore, for an imperfect fluid, the resistive effect of pressure could be significant due to likely dramatic increase of tangential pressure beyond the "photon sphere." Indeed, with inclusion of tangential pressure, in principle, there can be static objects with surface gravitational redshift z → ∞. Therefore, pressure can certainly oppose gravitational contraction in GR in a significant manner in contradiction to the idea of Roger Penrose that GR continued collapse must be unstoppable.

New cosmological solutions in massive gravity

We find new, simple cosmological solutions with flat, open, and closed spatial geometries, contrary to the previous wisdom that only the open model is allowed. The metric and the St\"{u}ckelberg fields are given explicitly, showing nontrivial configurations of the St\"{u}ckelberg in the usual Friedmann-Lema\^{i}tre-Robertson-Walker coordinates. The solutions exhibit self-acceleration, while being free from ghost instabilities. Our solutions can accommodate inhomogeneous dust collapse represented by the Lema\^{i}tre-Tolman-Bondi metric as well. Thus, our results can be used not only to describe homogeneous and isotropic cosmology but also to study gravitational collapse in massive gravity.

Spherical collapse and the halo model in braneworld gravity

We present a detailed study of the collapse of a spherical perturbation in DGP braneworld gravity for the purpose of modeling simulation results for the halo mass function, bias and matter power spectrum. The presence of evolving modifications to the gravitational force in form of the scalar brane-bending mode lead to qualitative differences to the collapse in ordinary gravity. In particular, differences in the energetics of the collapse necessitate a new, generalized method for defining the virial radius which does not rely on strict energy conservation. These differences and techniques apply to smooth dark energy models with w unequal -1 as well. We also discuss the impact of the exterior of the perturbation on collapse quantities due to the lack of a Birkhoff theorem in DGP. The resulting predictions for the mass function, halo bias and power spectrum are in good overall agreement with DGP N-body simulations on both the self-accelerating and normal branch. In particular, the impact of the Vainshtein mechanism as measured in the full simulations is matched well. The model and techniques introduced here can serve as practical tools for placing consistent constraints on braneworld models using observations of large scale structure.