# Post-merger chirps from binary black holes as probes of the final black-hole horizon

## TL;DR

This paper links observable post-merger gravitational wave features to geometrical horizon structures in binary black hole mergers, offering new insights into extreme gravity through numerical relativity simulations.

## Contribution

It demonstrates a direct connection between horizon geometrical features and gravitational wave signals in binary black hole mergers, providing a new probe of black hole dynamics.

## Key findings

- Correlation between horizon 'cusp'-like defects and gravitational wave chirps
- Post-merger chirp features can be used to study black hole horizons
- Potential for future gravitational wave observations to test these predictions

## Abstract

The merger of a binary black hole gives birth to a highly distorted final black hole. The gravitational radiation emitted as this black hole relaxes presents us with the unique opportunity to probe extreme gravity and its connection with the dynamics of the black hole horizon. Using numerical relativity simulations, we demonstrate a connection between a concrete observable feature in the gravitational waves and geometrical features on the dynamical apparent horizon of the final black hole. Specifically, we show how the line-of-sight passage of a "cusp"-like defect on the horizon of the final black hole correlates with "chirp"-like frequency peaks in the post-merger gravitational-waves. These post-merger chirps should be observed and analyzed as the sensitivity of LIGO and Virgo increases and as future generation detectors, such as LISA and the Einstein Telescope, become operational.