Tests for the existence of black holes through gravitational wave echoes

Abstract

The existence of black holes and of spacetime singularities is a fundamental issue in science. Despite this, observations supporting their existence are scarce, and their interpretation unclear. We overview how strong a case for black holes has been made in the last few decades, and how well observations adjust to this paradigm. Unsurprisingly, we conclude that observational proof for black holes is impossible to come by. However, just like Popper's black swan, alternatives can be ruled out or confirmed to exist with a single observation. These observations are within reach. In the next few years and decades, we will enter the era of precision gravitational-wave physics with more sensitive detectors. Just as accelerators require larger and larger energies to probe smaller and smaller scales, more sensitive gravitational-wave detectors will be probing regions closer and closer to the horizon, potentially reaching Planck scales and beyond. What may be there, lurking? 1 arXiv:1707.03021v4 [gr-qc]

Can we observe fuzzballs or firewalls?

“…We will in fact argue that this similarity between fuzzballs and the traditional hole may be evidence for a general 'correspondence principle', which states that the fuzzball mimics the traditional black hole for all but the most delicate quantum observations. 1 Similar observations have also appeared in a nice recent paper [18] which gives a detailed discussion of the observability of black holes.…”

“…It was also noted that the spread of the wavefunction over the space of fuzzballs could give a novel kind of gravitational wave burst. In [18] a detailed analysis was given of different structures near the horizon, and what these structures might imply for the observability of quantum gravity effects. In [51] it was noted that it may be possible to observe the interference of pulsar radiation with the radiation emitted by a black hole, and thereby see signatures of quantum gravity.…”

Lensing and dynamics of ultracompact bosonic stars

“…The correspondence between a BH's natural oscillation frequencies (so called quasinormal modes [5]) and light ring (LR) vibrations [6][7][8], implies that compact objects with a LRhenceforth ultracompact objects (UCOs)-but with no event horizon can mimic the initial part of the ringdown gravitational-wave signal of perturbed BHs. Later parts of the ringdown signal may have signatures of the true nature of the object (through the so called echos [9,10]), but the corresponding lower signal to noise ratio challenges clean detections of this part of the signal, at least in the near future-see [11][12][13][14][15][16] for recent discussions.…”

Inverse spectrum problem for quasi-stationary states

“…With the results that have been presented in the previous sections, we can now generalize the inverse problem to potentials admitting quasi-stationary states. From the expanded form of the generalized Bohr-Sommerfeld rule equation (6) it is known that the real part E 0n is approximatively given by the classical Bohr-Sommerfeld rule equation (7), while its imaginary part E 1n follows from the Gamow formula equation (8). The calculation of E 0n is independent of E 1n , but not the other way round.…”