Quantum modifications to the Schwarzschild ringdown spectrum

In anticipation of a successful theory of quantum gravity, there has been much interest in predicting whether and how such a theory could appreciably influence environments that we can interact with or otherwise measure. The ringdown of a black hole in the aftermath of a violent event such as a merger is thus seen as a potential candidate for revealing the signatures of quantum gravity due to our ability to measure the gravitational waves that emerge. Nevertheless, it is not so far clear how the effects of quantum gravity should change the classical picture of the ringdown, as the discretised energy spectrum of the black hole clashes with the notion of a horizon that absorbs all infalling modes of the metric and matter fields. In this work we propose that the quantum black hole dissipates the energy it cannot absorb with the emission of soft gravitons carrying the energy difference between an infalling particle’s energy and the quantum transition energy. We find that the ringdown spectrum is consequently enriched with low frequency components, and particularly that there exists a weak low frequency flux that persists for a timescale far longer than the ringdown itself, due to the leaking cavity properties of the surrounding spacetime.