How does a dark compact object ringdown?. (arXiv:2006.14628v1 [gr-qc])

<a href="http://arxiv.org/find/gr-qc/1/au:+Maggio_E/0/1/0/all/0/1">Elisa Maggio</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Buoninfante_L/0/1/0/all/0/1">Luca Buoninfante</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Mazumdar_A/0/1/0/all/0/1">Anupam Mazumdar</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Pani_P/0/1/0/all/0/1">Paolo Pani</a>

A generic feature of nearly out-of-equilibrium dissipative systems is that

they resonate through a set of quasinormal modes. Black holes – the absorbing

objects par excellence – are no exception. When formed in a merger, black holes

vibrate in a process called “ringdown”, which leaves the gravitational-wave

footprint of the event horizon. In some models of quantum gravity which attempt

to solve the information-loss paradox and the singularities of General

Relativity, black holes are replaced by regular, horizonless objects with a

tiny effective reflectivity. Motivated by these scenarios, here we develop a

generic framework to the study of the ringdown of a compact object with various

shades of darkness. By extending the black-hole membrane paradigm, we map the

interior of any compact object in terms of the bulk and shear viscosities of a

fictitious fluid located at the surface, with the black-hole limit being a

single point in a three-dimensional parameter space. We unveil some remarkable

features of the ringdown and some universal properties of the light ring in

this framework. We also identify the region of the parameter space which can be

probed by current and future gravitational-wave detectors. A general feature is

the appearance of mode doublets which are degenerate only in the black-hole

limit. We argue that the merger event GW150914 already imposes a strong lower

bound on the compactness of the merger remnant of approximately 99% of the

black-hole compactness. This places model-independent constraints on black-hole

alternatives such as diffuse “fuzzballs” and nonlocal stars.

A generic feature of nearly out-of-equilibrium dissipative systems is that

they resonate through a set of quasinormal modes. Black holes – the absorbing

objects par excellence – are no exception. When formed in a merger, black holes

vibrate in a process called “ringdown”, which leaves the gravitational-wave

footprint of the event horizon. In some models of quantum gravity which attempt

to solve the information-loss paradox and the singularities of General

Relativity, black holes are replaced by regular, horizonless objects with a

tiny effective reflectivity. Motivated by these scenarios, here we develop a

generic framework to the study of the ringdown of a compact object with various

shades of darkness. By extending the black-hole membrane paradigm, we map the

interior of any compact object in terms of the bulk and shear viscosities of a

fictitious fluid located at the surface, with the black-hole limit being a

single point in a three-dimensional parameter space. We unveil some remarkable

features of the ringdown and some universal properties of the light ring in

this framework. We also identify the region of the parameter space which can be

probed by current and future gravitational-wave detectors. A general feature is

the appearance of mode doublets which are degenerate only in the black-hole

limit. We argue that the merger event GW150914 already imposes a strong lower

bound on the compactness of the merger remnant of approximately 99% of the

black-hole compactness. This places model-independent constraints on black-hole

alternatives such as diffuse “fuzzballs” and nonlocal stars.

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