Showing papers by "Matt Visser published in 2022"

On the Inner Horizon Instability of Non-Singular Black Holes

Abstract: Regular black holes represent a conservative model in which the classical singularity is replaced by a non-singular core without necessarily modifying the spacetime outside the trapping horizon. Given the possible lack of phenomenological signatures, it is crucial to study the consistency of the model. In this short work, we review the physical mechanism leading to the instability of the central core, arguing that that non-perturbative backreation is non-negligible and must be taken into account to provide a meaningful description of physical black holes.

Astrophysically viable Kerr-like spacetime

Abstract: Based on carefully selected physical requirements, we develop and analyze a rotating regular black hole with asymptotically Minkowski core. This Kerr-like geometry possesses the full ``Killing tower'' of nontrivial Killing tensor, Killing-Yano tensor, and principal tensor. The Hamilton-Jacobi equation, Klein-Gordon equation, and Maxwell's equations are separable. Energy-condition violating physics is pushed into an arbitrarily small region in the deep core. Extraction of astrophysical observables falsifiable/verifiable by the observational community is straightforward.

Constant-r geodesics in the Painlevé–Gullstrand form of Lense–Thirring spacetime

Abstract: Abstract Herein we explore the non-equatorial constant-r (“quasi-circular”) geodesics (both timelike and null) in the Painlevé–Gullstrand variant of the Lense–Thirring spacetime recently introduced by the current authors. Even though the spacetime is not spherically symmetric, shells of constant- r geodesics still exist. Whereas the radial motion is (by construction) utterly trivial, determining the allowed locations of these constant- r geodesics is decidedly non-trivial, and the stability analysis is equally tricky. Regarding the angular motion, these constant- r orbits will be seen to exhibit both precession and nutation — typically with incommensurate frequencies. Thus this constant- r geodesic motion, though integrable in the precise technical sense, is generically surface-filling, with the orbits completely covering a symmetric equatorial band which is a segment of a spherical surface, (a so-called “spherical zone”), and whose latitudinal extent is governed by delicate interplay between the orbital angular momentum and the Carter constant. The situation is qualitatively similar to that for the (exact) Kerr spacetime — but we now see that any physical model having the same slow-rotation weak-field limit as general relativity will still possess non-equatorial constant- r geodesics.

Painlevé–Gullstrand coordinates versus Kerr spacetime geometry

Abstract: We discuss the tension between the possible existence of Painleve-Gullstrand coordinate systems versus the explicit geometrical features of the Kerr spacetime; a subject of interest to Professor Thanu Padmanabhan in the weeks immediately preceding his unexpected death. We shall carefully distinguish strong and weak Painleve-Gullstrand coordinate systems, and conformal variants thereof, cataloguing what we know can and cannot be done -- sometimes we can make explicit global statements, sometimes we must resort to implicit local statements. For the Kerr spacetime the best that seems to be achievable is to set the lapse function to unity and represent the spatial slices with a 3-metric in factorized unimodular form; this arises from considering the Doran version of Kerr spacetime in Cartesian coordinates. We finish by exploring the (limited) extent to which this construction might possibly lead to implementing an "analogue spacetime" model suitable for laboratory simulations of the Kerr spacetime.

ADM mass in warp drive spacetimes

Abstract: Abstract What happens when a warp bubble has mass? This seemingly innocent question forces one to carefully formalize exactly what one means by a warp bubble, exactly what one means by having the warp bubble “move” with respect to the fixed stars, and forces one to more carefully examine the notion of mass in warp-drive spacetimes. This is the goal of the present article. In this process, we will see that often-made throw-away comments regarding “payloads” are even simpler than commonly assumed, while there are two further, distinct yet subtle ways in which a mass can appear in connection with a warp drive space-time: One, that the warp bubble (not its payload) has the mass; two, that the mass is a background feature in front of which the warp drive moves. For simplicity, we consider generic Natário warp drives with zero-vorticity flow field. The resulting spacetimes are sufficiently simple to allow an exact and fully explicit computation of all of the stress-energy components, and verify that (as expected) the null energy condition (NEC) is violated. Likewise the weak, strong, and dominant energy conditions (WEC, SEC, DEC) are violated. Indeed, this confirms the community’s folk wisdom, and recent (fully general, but implicit) results of the present authors which closed previous gaps in the argument. However, folk wisdom should be carefully and critically examined before being believed, and the present examples for general results will greatly aid physical intuition.

Physically motivated ansatz for the Kerr spacetime

Abstract: Despite some 60 years of work on the subject of the Kerr rotating black hole there is as yet no widely accepted physically based and pedagogically viable ansatz suitable for deriving the Kerr solution without significant computational effort. (Typically involving computer-aided symbolic algebra.) Perhaps the closest one gets in this regard is the Newman–Janis trick; a trick which requires several physically unmotivated choices in order to work. Herein we shall try to make some progress on this issue by using a non-ortho-normal tetrad based on oblate spheroidal coordinates to absorb as much of the messy angular dependence as possible, leaving one to deal with a relatively simple angle-independent tetrad-component metric. That is, we shall write gab=gABeAaeBb seeking to keep both the tetrad-component metric g AB and the non-ortho-normal co-tetrad eAa relatively simple but non-trivial. We shall see that it is possible to put all the mass dependence into g AB , while the non-ortho-normal co-tetrad eAa can be chosen to be a mass-independent representation of flat Minkowski space in oblate spheroidal coordinates: (gMinkowski)ab=ηABeAaeBb . This procedure separates out, to the greatest extent possible, the mass dependence from the rotational dependence, and makes the Kerr solution perhaps a little less mysterious.

Comment on"Stability properties of Regular Black Holes"

Abstract: Regular black holes are generically unstable because of the phenomenon that goes by the name of “mass inflation” which destabilizes the inner horizon. In recent works, [arXiv:2209.10612v1 and arXiv:2211.09192v1], it is argued that Hawking radiation can cure this instability and some concerns are raised against the validity of the previous analyses showing its existence in first place. In this short comment, we explain our reservations regarding these recent claims and reiterate the relevance of the mass inflation instability for regular black holes of astrophysical interest.

A connection between regular black holes and horizonless ultracompact stars

Abstract: We illustrate that regular black holes and horizonless stars, typically considered as quite distinct families of black hole mimickers, are intimately intertwined. We show that any spherically symmetric regular black hole can be continuously deformed into a horizonless star under the mild conditions of non-negativity of gravitational energy (Misner–Sharp quasi-local mass), and an assumed linear relation between the latter and the Arnowitt–Deser–Misner (ADM) mass. We illustrate this general result by considering the family of geometries proposed by Hayward as the description of regular black holes, and we also describe the properties of the corresponding horizonless stars. The form of the associated effective stress-energy tensor shows that these horizonless stars can be identified as anisotropic gravastars with a soft surface and inner/outer light rings. We also construct dynamical geometries that could describe the evolution of regular black holes towards horizonless stars, and show that semiclassical physics contains the necessary ingredients to trigger the early stages of such dynamical evolution.

Cosmology in Painlevé-Gullstrand coordinates

Abstract: Cosmology is most typically analyzed using standard co-moving coordinates, in which the galaxies are (on average, up to presumably small peculiar velocities) “at rest”, while “space” is expanding. But this is merely a specific coordinate choice; and it is important to realise that for certain purposes other, (sometimes radically, different) coordinate choices might also prove useful and informative, but without changing the underlying physics. Specifically, herein we shall consider the k= 0 spatially flat FLRW cosmology but in Painlevé-Gullstrand coordinates — these coordinates are very explicitly not co-moving: “space” is now no longer expanding, although the distance between galaxies is still certainly increasing. Working in these Painlevé-Gullstrand coordinates provides an alternate viewpoint on standard cosmology, and the symmetries thereof, and also makes it somewhat easier to handle cosmological horizons. With a longer view, we hope that investigating these Painlevé-Gullstrand coordinates might eventually provide a better framework for understanding large deviations from idealized FLRW spacetimes. We illustrate these issues with a careful look at the Kottler and McVittie spacetimes.

Null affine parameter in spacetimes conformal to spacetimes exhibiting a timelike conformal Killing vector

Abstract: : Finding affine parameters for null geodesics is often quite tedious, and can sometimes even be somewhat tricky. Herein we shall demonstrate that the existence of a conformally related spacetime containing a conformal Killing vector, timelike in the domain of outer communication, is sufficient to define a preferred set of spatial 3-slices — on which a well-defined “affine” 3-metric can be introduced to capture the notion of affine null parameter — before explicitly finding the null geodesics. The construction depends on the properties of conformal transformations, and on the conserved quantity associated with the conformal Killing vector. Having the affine null parameter in hand before attempting to find the actual null geodesics often simplifies other parts of the analysis.

Dynamical analysis of the redshift drift in FLRW universes

Abstract: : Redshift drift is the phenomenon whereby the observed redshift between an emitter and observer comoving with the Hubble flow in an expanding FLRW universe will slowly evolve — on a timescale comparable to the Hubble time. In a previous article [JCAP 04 (2020) 043; arXiv:2001.11964] three of the current authors had performed a cosmographic analysis of the redshift drift in a FLRW universe, temporarily putting aside the issue of dynamics (the Friedmann equations). In the current article we now add dynamics, still within the framework of an exact FLRW universe. We shall develop a suitable generic matter model, and study both the low-redshift asymptotic behaviour and the utility of using alternative variables to describe the redshift.

General-relativistic thin-shell Dyson megaspheres

Abstract: : Loosely inspired by the somewhat fanciful notion of detecting an arbitrarily advanced alien civilization, we consider a general-relativistic thin-shell Dyson mega-sphere completely enclosing a central star-like object, and perform a full general-relativistic analysis using the Israel–Lanczos–Sen junction conditions. We focus attention on the surface mass density, the surface stress, the classical energy conditions, and the forces between hemispheres. We find that in the physically acceptable region the NEC, WEC, and SEC are always satisfied, while the DEC can be violated if the Dyson mega-sphere is sufficiently close to forming a black hole. We also demonstrate that the original version of the maximum force conjecture, F ≤ 14 F Stoney = 14 F Planck , can easily be violated if the Dyson mega-sphere is sufficiently compact, that is, sufficiently close to forming a black hole. Interestingly there is a finite region of parameter space where one can violate the original version of the maximum force conjecture without violating the DEC. Finally, we very briefly discuss the possibility of nested thin-shell mega-spheres (Matrioshka configurations) and thick-shell Dyson mega-spheres.

Effective exponential bounds on the prime gaps

Abstract: : Over the last 50 years a large number of effective exponential bounds on the first Chebyshev function ϑ ( x ) have been obtained. Specifically we shall be interested in effective exponential bounds of the form Herein we shall convert these effective bounds on ϑ ( x ) into effective exponential bounds on the prime gaps g n = p n +1 − p n . Specifically we shall establish a number of effective exponential bounds of the form and for some effective computable x ∗ . It is the explicit presence of the exponential factor, with known coefficients and known range of validity for the bound, that makes these bounds particularly interesting.

9 60 40 09 v 1 3 A pr 1 99 6 Gravitational vacuum polarization III : Energy conditions in the ( 1 + 1 ) Schwarzschild spacetime gr-qc / 9604009

Abstract: Building on techniques developed in a pair of earlier papers, I investigate the various pointwise and averaged energy conditions for the quantum stress-energy tensor corresponding to a conformally-coupled massless scalar field in the in the (1+1)-dimensional Schwarzschild spacetime. Because the stress-energy tensors are analytically known, I can get exact results for the Hartle–Hawking, Boulware, and Unruh vacua. This exactly solvable model serves as a useful sanity check on my (3+1)-dimensional investigations wherein I had to resort to a mixture of analytic approximations and numerical techniques. Key results in (1+1) dimensions are: (1) NEC is satisfied outside the event horizon for the Hartle–Hawking vacuum, and violated for the Boulware and Unruh vacua. (2) DEC is violated everywhere in the spacetime (for any quantum state, not just the standard vacuum states).