Tests for the existence of horizons 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?

Figures (2)

• Figure 1: Schematic classification of dark compact objects. Their compactness is expressed as the difference between the object radius $r_0$ and the Schwarzschild radius $r_g$. Objects in the same category have similar dynamical properties on a timescale $\tau\sim \frac{r_g}{c}|\log\epsilon|$. The upper axis refers to the time, as measured by distant observers, that light from the photosphere takes to reach the surface $r_0$. Numbers refer to an object of $60M_\odot$ and scale linearly with it mass.
• Figure 2: Ringdown waveforms from black holes (black line) and ClePhOs (red line). We consider objects of $60M_\odot$. For ClePhOs, there is a reflective surface at $r_0=r_g(1+\epsilon)$, $\epsilon=10^{-11}$. The amplitude of the GW signal (proportional to the relative strain of the interferometer's arm induced by the GW) is normalized to its peak value. The initial data describes a quadrupolar Gaussian wavepacket of axial GWs. The inset shows a zoom-in version of the waveform at late times. Note that each subsequent echo has a smaller frequency content.