Ten years after the first direct detection of gravitational waves, gravitational-wave astronomy has moved from a single spectacular discovery to a rich, population-level science. The updated compact binary merger catalog, GWTC-4, now exceeds two hundred mergers of black holes and neutron stars. Using GWTC-4, we can robustly trace astrophysical trends, including features in the black hole mass spectrum and the distribution of spins and tilt angles, which together point to distinct channels of binary formation and evolution across cosmic time. I will begin with these population-level inferences, highlighting the interesting trends that have emerged with the expanded sample. I will then turn to three flagship events from the first part of the fourth observing run (O4a): GW230529, a merger in which the heavier companion lies in the proposed lower mass gap; GW231123, the most massive binary black hole merger observed to date; and GW250114, the loudest signal yet, enabling unusually precise strong gravity tests and clean ringdown spectroscopy. Taken together, O4a shows a field that has matured into one that still delivers headline events while providing statistically grounded, population-level insights into the lives of compact binaries.
Speaker: Ish Gupta, Northwestern University
Host: Vicky Kalogera