ASTRONOMY COLLOQUIUM

The first three observing runs of the LIGO, Virgo, and KAGRA gravitational wave detectors produced a wealth of "firsts," including the first observation of multiple "ringdown" modes from a black hole---the remnant of the first-ever binary black hole merger, GW150914.  Just like an excited atom, a recently-formed black hole is expected to emit a characteristic spectrum of gravitational radiation as it settles down to its equilibrium state---in general relativity, a Kerr spacetime.  "Black hole spectroscopy" thus provides the opportunity to directly probe these extreme spacetimes and test fundamental gravitational theories, just as atomic spectroscopy provided an ideal testbed for the new quantum mechanics of the early 20th century.  I will review the 50+ year effort toward black hole spectroscopy, including the detection of multiple ringdown modes in GW150914 and hints in GW190521 and describe the near future of this field as LIGO, Virgo, and KAGRA's fourth observing run continues (150 detection candidates so far!).  Along the way, I will review the rapidly changing landscape of gravitational wave astronomy, and describe some of the other exciting recent firsts in this new field, particularly the possibility of using binary black hole mergers for intrinsically-calibrated distance measurements to trace cosmic expansion.