We show that the implications of the generalized uncertainty principle (GUP) in the black hole physics are consistent with the predictions of the corpuscul

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.

In the context of ghost-free, infinite derivative gravity, we provide a quantum mechanical framework in which we can describe astrophysical objects devoid of curvature singularity and event horizon. In order to avoid ghosts and singularity, the gravitational interaction has to be nonlocal; therefore, we call these objects nonlocal stars. Quantum mechanically a nonlocal star is a self-gravitational bound system of many gravitons interacting nonlocally. Outside the nonlocal star the spacetime is well described by the Schwarzschild metric, while inside we have a nonvacuum spacetime metric which tends to be conformally flat at the origin. Remarkably, in the most compact scenario the radius of a nonlocal star is of the same order of the Buchdahl limit, therefore slightly larger than the Schwarzschild radius, such that there can exist a photosphere. These objects live longer than a Schwarzschild blackhole and they are very good absorbers, due to the fact that the number of available states is larger than that of a blackhole. As a result nonlocal stars can not only be excellent blackhole mimickers, but can also be considered as dark matter candidates. In particular, nonlocal stars with masses below $10^{14}\mathrm{g}$ can be made stable compared to the age of the Universe.