Light rings as observational evidence for event horizons: Long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects

By V. Cardoso, L. Crispino, C. F. Macedo, Hirotada Okawa, Paolo Pani. 2014

219 citations recorded by the source corpus.

Ultracompact objects are self-gravitating systems with a light ring. It was recently suggested that fluctuations in the background of these objects are extremely long-lived and might turn unstable at the nonlinear level, if the object is not endowed with a horizon. If correct, this result has important consequences: objects with a light ring are black holes. In other words, the nonlinear instability of ultracompact stars would provide a strong argument in favor of the "black hole hypothesis," once electromagnetic or gravitational-wave observations confirm the existence of light rings. Here we explore in some depth the mode structure of ultracompact stars, in particular constant-density stars and gravastars. We show that the existence of very long-lived modes -- localized near a second, stable null geodesic -- is a generic feature of gravitational perturbations of such configurations. Already at the linear level, such modes become unstable if the object rotates sufficiently fast to develop an ergoregion. Finally, we conjecture that the long-lived modes become unstable under fragmentation via a Dyson-Chandrasekhar-Fermi mechanism at the nonlinear level. Depending on the structure of the star, it is also possible that nonlinearities lead to the formation of small black holes close to the stable light ring. Our results suggest that the mere observation of a light ring is a strong evidence for the existence of black holes.

View Paper: https://www.semanticscholar.org/paper/92e05d35734b765eea0804170bd83d2f7a99f8c5

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Citations in the corpus

Imaging a boson star at the Galactic center (2015, methodology): “FHV is grateful to Enrico Barausse for providing [40] and to Carlos Herdeiro, Helgi Runarsson and Thomas Sotiriou for stimulating discussions at a mini-workshop on scalar fields and gravitation in Meudon.”
Superradiance: Energy Extraction, Black-Hole Bombs and Implications for Astrophysics and Particle Physics (2015, background): “In an alternative scenario, nonlinear interactions over the ultralong lifetime of the unstable modes may lead to the formation of small BHs close to the stable light ring [396].”
Tests for the existence of black holes through gravitational wave echoes (2017, background): “Unfortunately, the effect of viscosity is practically unknown [31], and so are the timescales involved in putative dissipation mechanisms that might quench this instabil-”
The imitation game: Proca stars that can mimic the Schwarzschild shadow (2021, background): “Such stable LRs have been argued to lead to a non-perturbative instability [59–61].”
Superradiance (2015, methodology): “imit; these modes are very slowly damped and can become unstable when rotation is included. This has been rst discussed in the eikonal limit [394] and it has been recently put on a rmer basis in Ref. [399]. Such long-lived modes exist in ultracompact spacetimes which possess a light ring (i.e. an un”
Slowly decaying waves on spherically symmetric spacetimes and ultracompact neutron stars (2014, result): “Finally, we note that the paper [34], which appeared after a preprint of this work, studied ultracompact stars numerically and found close agreement with the results presented here.”
Approximate Universal Relations for Neutron Stars and Quark Stars (2016, background): “On the other hand, very compact, horizonless objects with a light ring may be nonlinearly unstable [361, 362].”