The naturalness of vanishing black hole response

Black holes in 4D General Relativity have the remarkable property that they lack any form of static linear response. At length scales much larger than the Schwarzschild radius, where black holes can be described as point particles, their linear response is captured by an infinite number of Wilson coefficients in the worldline action. From this perspective, the absence of static linear response would appear to pose a naturalness problem for the point-particle effective theory. In this talk, I will discuss a recently discovered network of hidden symmetries of the action for black hole perturbations. Some of these symmetries act non-trivially on the long-distance variables in the point-particle effective theory, and force the linear response coefficients to vanish, thus solving the naturalness problem.

Black hole spectroscopy with the next generation of gravitational wave detectors

Black hole spectroscopy is the measurement of the quasi normal mode spectrum in the gravitational waves emitted by a perturbed black hole. Detecting multiple excited modes provides an excellent opportunity to test the dynamics of general relativity in the strong field regime. Third generation detectors will lead to a large increase in the ringdown signal-to-noise ratio (SNR), which will allow us to identify routinely subdominant excited modes besides the dominant one. We use a simulated catalog of merging stellar mass binary black holes to investigate the prospects of black hole spectroscopy with ground based third generation detectors. We study the cumulative distribution in the ringdown SNR and quantify the precision with which the quasi normal modes will be measured during the operational time of the detectors. Overall, we find that third generation detectors will be able to detect several thousands of ringdown events per year, with O(10) golden events per year with SNR larger than 100. Next, we find that about a thousand events per year will constrain deviations from general relativity in the spectrum with better than 10 per cent precision in the individual events. Finally, we discuss the prospects for constraining deviations from general relativity when stacking together the population of the detected events.

Geometric Resolution of Schwarzschild Horizon

I will present a new technique to construct non-extremal solitons in gravity with compact dimensions. They correspond to smooth horizonless geometries induced by topology in spacetime and supported by electromagnetic flux, which characterize coherent states in quantum gravity. I will present various types of solutions contained in the ansatz such as topological stars and bubble bag ends. Then I will focus on neutral topological solitons that are indistinguishable from Schwarzschild black holes with a scalar field, called Schwarzschild scalarwall, but replace the horizon at r=2M with a smooth bubbling geometry. This is the first example of such a geometric transition in string theory for astrophysically relevant black holes. I will conclude by discussing some of their gravitational signatures