ITC Colloquium

Models for black hole formation from stellar evolution robustly predict the existence of
a pair-instability supernova mass gap in the range ~50 to ~120 solar masses. The binary
black holes of LIGO-Virgo's first two observing runs supported this prediction, showing evidence
for a dearth of component black hole masses above 45 solar masses. Meanwhile, among the
30+ new observations from the third observing run, there are several black holes that appear to
sit above the 45 solar mass limit. I will discuss how these unexpectedly massive black holes fit
into our understanding of the binary black hole population. The data are consistent with several
scenarios, including a mass distribution that evolves with redshift and the possibility that the
most massive binary black hole, GW190521, straddles the mass gap, containing an
intermediate-mass black hole heavier than 120 solar masses.