Gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale

Cardoso, Vitor; Pani, Paolo; Palenzuela, Carlos; Hopper, Seth; Macedo, Caio F. B.

doi:10.1103/PhysRevD.94.084031

Article
Report number	arXiv:1608.08637
Title	Gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale
Related title	Echoes of ECOs: gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale
Author(s)	Cardoso, Vitor (Lisbon, CENTRA ; Lisbon, IST ; Perimeter Inst. Theor. Phys. ; CERN) ; Hopper, Seth (Lisbon, CENTRA ; Lisbon, IST) ; Macedo, Caio F. B. (Lisbon, CENTRA ; Lisbon, IST) ; Palenzuela, Carlos (U. Iles Balears, Palma) ; Pani, Paolo (Rome U. ; INFN, Rome ; Lisbon, CENTRA ; Lisbon, IST)
Publication	2016-10-21
Imprint	30 Aug 2016
Number of pages	13
Note	13 pages, RevTex4. v2: typo in equation 7 corrected, references added, to appear in PRD
In:	Phys. Rev. D 94 (2016) 084031
DOI	10.1103/PhysRevD.94.084031
Subject category	General Relativity and Cosmology
Abstract	Gravitational waves from binary coalescences provide one of the cleanest signatures of the nature of compact objects. It has been recently argued that the post-merger ringdown waveform of exotic ultracompact objects is initially identical to that of a black-hole, and that putative corrections at the horizon scale will appear as secondary pulses after the main burst of radiation. Here we extend this analysis in three important directions: (i) we show that this result applies to a large class of exotic compact objects with a photon sphere for generic orbits in the test-particle limit; (ii) we investigate the late-time ringdown in more detail, showing that it is universally characterized by a modulated and distorted train of "echoes" of the modes of vibration associated with the photon sphere; (iii) we study for the first time equal-mass, head-on collisions of two ultracompact boson stars and compare their gravitational-wave signal to that produced by a pair of black-holes. If the initial objects are compact enough as to mimic a binary black-hole collision up to the merger, the final object exceeds the maximum mass for boson stars and collapses to a black-hole. This suggests that -- in some configurations -- the coalescence of compact boson stars might be almost indistinguishable from that of black-holes. On the other hand, generic configurations display peculiar signatures that can be searched for in gravitational-wave data as smoking guns of exotic compact objects.
Copyright/License	arXiv nonexclusive-distrib. 1.0 publication: © 2016-2025 The Author(s) (License: CC-BY-3.0)

$Qualitative features of the effective potential felt by perturbations of a Schwarzschild BH compared to the case of wormholes~\cite{Cardoso:2016rao} and of star-like ECOs with a regular center~\cite{Cardoso:2014sna}. The precise location of the center of the star is model-dependent and was chosen for visual clarity. The maximum and minimum of the potential corresponds approximately to the location of the unstable and stable PS, and the correspondence is exact in the eikonal limit of large angular number $l$. In the wormhole case, modes can be trapped between the PSs in the two ``universes''. In the star-like case, modes are trapped between the PS and the centrifugal barrier near the center of the star~\cite{1991RSPSA.434..449C,Chandrasekhar:1992ey,Abramowicz:1997qk}. In all cases the potential is of finite height, and the modes leak away, with higher-frequency modes leaking on shorter timescales.$ $Left: A dipolar ($l=1,m=0$) scalar wavepacket scattered off a Schwarzschild BH and off different ECOs with $\ell=10^{-6} M$ ($r_0=2.000001 M$). The right panel shows the late-time behavior of the waveform. The result for a wormhole, a gravastar, and a simple empty shell of matter are qualitatively similar and display a series of ``echoes'' which are modulated in amplitude and distorted in frequency. For this compactness, the delay time in Eq.~\eqref{Deltat} reads $\Delta t\approx 110 M$ for wormholes, $\Delta t\approx 82 M$ for gravastars, and $\Delta t\approx 55 M$ for empty shells, respectively.$ $Left: A dipolar ($l=1,m=0$) scalar wavepacket scattered off a Schwarzschild BH and off different ECOs with $\ell=10^{-6} M$ ($r_0=2.000001 M$). The right panel shows the late-time behavior of the waveform. The result for a wormhole, a gravastar, and a simple empty shell of matter are qualitatively similar and display a series of ``echoes'' which are modulated in amplitude and distorted in frequency. For this compactness, the delay time in Eq.~\eqref{Deltat} reads $\Delta t\approx 110 M$ for wormholes, $\Delta t\approx 82 M$ for gravastars, and $\Delta t\approx 55 M$ for empty shells, respectively.$ Show more plots

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