Beyond the Basics: the Messy Reality of Gravitational Wave Ringdowns
The past decade has completely transformed our understanding of what happens after black holes collide. What we once thought could be well-described by simple ringdowns—neat linear combinations of damped sinusoids (quasi-normal modes)—have turned out to be far richer and messier. Today we recognize ringdowns as an intricate tapestry woven from these quasinormal modes, nonlinear effects, tails, and secular phenomena like gravitational memory. In this talk, I’ll dive into the nonlinear aspects: where the theory stands today and some exciting recent observational claims that we might already be seeing these effects in GW250114. In the second half, I’ll switch gears to the early inspiral phase and tackle a practical question. Post-Newtonian theory is an asymptotic series, so at some point adding higher orders will yield less accurate results. Have we already reached this? By comparing PN to NR, we argue that we can still gain significant improvements by going to higher PN order, but that soon NR and PN will be equally accurate in the early inspiral.