Gravitational Wave Memory: From SMBHs to PBHs

Gravitational-wave (GW) memory leaves a lasting imprint on spacetime — a permanent offset in the GW strain that persists after a burst passes, producing a tiny but measurable displacement of test masses. In this talk, I will explore the nonlinear GW memory from binary black hole mergers and the prospects for detecting it from supermassive black hole (SMBH) coalescences with LISA. I will highlight the distinctive features of the memory signal and show how it appears in interferometric data after Time-Delay Interferometry (TDI) is applied. Thanks to its low-frequency character, GW memory allows interferometers to catch out-of-band mergers, offering a unique window onto very light primordial black hole (PBH) binaries beyond the reach of current ground-based detectors. I will conclude by comparing the detectability of both the dominant oscillatory and memory components across current and next-generation interferometers, including proposed high-frequency detectors.