Online Seminar

Our online seminar will take place on June 22 at 5:00 PM (CET), featuring a talk by Paolo Pani (Sapienza University) titled “GreyRing: Novel ringdown tests of general relativity with black hole greybody factors”.

Title:

GreyRing: Novel ringdown tests of general relativity with black hole greybody factors

About the speaker:

Paolo Pani is Professor of Theoretical Physics at Sapienza University of Rome, where he coordinates the gravity theory group, and a Senior Fellow of the Sapienza School for Advanced Studies. He is a recipient of the INFN Fubini Prize and the SIGRAV Prize for his studies on black hole physics, gravitational-wave phenomenology, and their connections to fundamental physics. He is the author of a textbook on General Relativity and of numerous influential review articles, and has co-authored over 200 publications. He is the Principal Investigator of the FIS project ET-NOW and, since 2026, serves as a Chair of the Einstein Telescope Observational Science Board.

Abstract:

Recent work has highlighted the black-hole greybody factor as a robust “no-hair” probe of the spacetime geometry during ringdown, modulating the signal in the frequency domain and remaining stable under small perturbations of the effective potential, unlike quasinormal modes. We show that the greybody factor is directly encoded in the Green’s function governing the linear ringdown response, providing an analytical foundation for previous phenomenological models. Building on this theoretical insight, we introduce GreyRing, a frequency-domain model for the post-merger signal that accurately reproduces numerical-relativity waveforms for comparable-mass, aligned-spin binaries, achieving mismatches of order 1e-6 for the dominant l=m=2 mode, and typically outperforming state-of-the-art time-domain models. We then propose a novel, fully post-merger consistency test of strong gravity: comparing the remnant mass and spin inferred from GreyRing with those from standard black-hole spectroscopy. This approach is agnostic and avoids the need for overtones or early-time ringdown choices. Applied to GW250114, it yields results consistent with the full-signal analysis, with comparable or slightly improved precision. We discuss how greybody-factor waveform models can open a new avenue for precision tests of strong gravity using the ringdown signal.