How does a dark compact object ringdown?

Abstract

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.

Tidal response and near-horizon boundary conditions for spinning exotic compact objects

“…The mixing of these two types of modes essentially indicates the breaking of isospectrality, as pointed out by Ref. [67]. We will get back to this point later.…”

“…As we will see later, the fact that reflection at the ECO surface turns the ingoing ðl; −m; −ω Ã Þ SN components into outgoing ðl; m; ωÞ SN components leads to the breaking of isospectrality, which has also been pointed out by Maggio et al [67]; here we take the further step of relating these coefficients to the tidal response of the ECO.…”

“…In Sec. V, we apply our method to obtain the echo waveform as well as the quasinormal modes (QNMs) of the ECOs, showing that even-and oddparity waves will generate different echoes, and generalizing the breaking of QNM isospectrality found by Maggio et al [67] to the spinning case. In Sec.…”

“…It has been proposed that the reflectivity of ECOs can be modeled by altering the impedance of the ECO surface [48,54,67]. In this paper, we generalize this point of view to ECOs with nonzero spins.…”

Structure and stability of traversable thin-shell wormholes in Palatini f(R) gravity

“…To conclude, the results obtained in this paper prove that the modifications in the junction conditions when considering fðRÞ gravity in the Palatini formulation yield nontrivial novelties in the analysis of traversable thin-shell wormholes as compared to their GR or metric fðRÞ counterparts. Moreover, this puts forward the new possibilities offered by these theories in any scenarios requiring a matching between two regions, such as in cosmic strings, domain walls, and stellar models, or within the membrane paradigm [55], topics which are of both astrophysical and cosmological interest. We hope to further report on these issues soon.…”