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 it is plausible that the effective stress-energy tensor in the first stages of evolution is generated by semiclassical effects, in agreement with independent works analyzing semiclassical backreaction.

Nested solutions of gravitational condensate stars

“…Although these equilibria are admittedly on the exotic side of astrophysical compact objects, it is interesting to note that more than 100 yr after the introduction of general relativity, new solutions can be still found in one of the simplest spacetimes: spherically symmetric, static, and not in vacuum. The results presented here can be further explored in a number of different directions, which include: the study of different EOSs, the stability of these solutions against perturbations and the corresponding quasi-normal modes, the existence of stable and unstable circular photon orbits (see [28] for a comprehensive discussion), the extension to rotating configurations (the different shells can have the same or different angular velocities) and their stability [29], and, of course, the exploration of similar equilibria in other theories of gravity. Among these possible developments, the exploration of the response of nestars to perturbations probably represents the most interesting avenue, as it will offer, besides a confirmation about the stability of these compact objects, also precise tools to distinguish nestars from gravastars and hence from black holes.…”

“…where the constants α 2 and α 4 are not constrained yet, but play the same role of the constant α in the EOS (28). In particular, we can require that the newly proposed EOS continues to satisfy the following set of conditions [26] (i) it must satisfy at least the less stringent energy conditions, namely, the DEC (e ⩾ |p r |) and the WEC 7 (e ⩾ 0 and e + p r ⩾ 0) [26]; (ii) the corresponding sound speed must always be subliminal; (iii) the corresponding sound speed does not have a maximum at e = 0.…”

“…The first one is reminiscent of a 'copy-paste' operation, in that the energy density of regions I and III are set to be the same, namely, e 1 = e 3 . In this case, because the EOS (51) in region II would have a zero in the denominator, we need to use in such a region the same EOS used for a single gravastar (28), as it is capable of connecting two de-Sitter shells with the same energy density.…”

Constraints on thermalizing surfaces from infrared observations of supermassive black holes

“…Quite remarkably, the viable resulting geometries endowed with an outer horizon were found to belong to basically two families of solutions [1,2]. Both these families admit as limiting cases horizonless ultracompact configurations (see [3] for details). Similar static solutions for ultra-compact quasi-black hole configurations can be found in the literature independently from the aforementioned limiting procedure (see e.g.…”

From regular black holes to horizonless objects: quasi-normal modes, instabilities and spectroscopy

“…For both families, depending on the value of the regularization parameter ℓ, regular, horizonless configurations can appear [7]. The SV metric can actually describe a traversable wormhole connecting two symmetric regions of our universe or two universes, while Bardeenlike RBHs can be continuously deformed into horizonless objects.…”