Light rings as observational evidence for event horizons

Long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects

Vitor Cardoso, Luís C B Crispino, Caio F B Macedo, Hirotada Okawa, Paolo Pani

Research output: Contribution to journalArticle

74 Citations (Scopus)

Abstract

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.

Original languageEnglish
JournalPhysical Review D - Particles, Fields, Gravitation and Cosmology
Volume90
Issue number4
DOIs
Publication statusPublished - 2014 Aug 29
Externally publishedYes

Fingerprint

event horizon
rings
stars
gravitational waves
horizon
electromagnetic radiation
fragmentation
nonlinearity
perturbation
configurations

ASJC Scopus subject areas

  • Nuclear and High Energy Physics

Cite this

Light rings as observational evidence for event horizons : Long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects. / Cardoso, Vitor; Crispino, Luís C B; Macedo, Caio F B; Okawa, Hirotada; Pani, Paolo.

In: Physical Review D - Particles, Fields, Gravitation and Cosmology, Vol. 90, No. 4, 29.08.2014.

Research output: Contribution to journalArticle

@article{cbaa713aad1e405781b1ea8e87545d1e,
title = "Light rings as observational evidence for event horizons: Long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects",
abstract = "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.",
author = "Vitor Cardoso and Crispino, {Lu{\'i}s C B} and Macedo, {Caio F B} and Hirotada Okawa and Paolo Pani",
year = "2014",
month = "8",
day = "29",
doi = "10.1103/PhysRevD.90.044069",
language = "English",
volume = "90",
journal = "Physical review D: Particles and fields",
issn = "0556-2821",
publisher = "American Institute of Physics Publising LLC",
number = "4",

}

TY - JOUR

T1 - Light rings as observational evidence for event horizons

T2 - Long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects

AU - Cardoso, Vitor

AU - Crispino, Luís C B

AU - Macedo, Caio F B

AU - Okawa, Hirotada

AU - Pani, Paolo

PY - 2014/8/29

Y1 - 2014/8/29

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=84940344862&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84940344862&partnerID=8YFLogxK

U2 - 10.1103/PhysRevD.90.044069

DO - 10.1103/PhysRevD.90.044069

M3 - Article

VL - 90

JO - Physical review D: Particles and fields

JF - Physical review D: Particles and fields

SN - 0556-2821

IS - 4

ER -