Brian D. Metzger

On November 23, 2023, the LIGO, Virgo, and KAGRA detectors caught a gravitational wave from the merger of two black holes with masses of 137 and 103 solar masses. Both fall into the pair-instability mass gap, the range from 69 to 139 solar masses where ordinary stellar evolution leaves behind nothing but expanding gas.

Some explosions in space last only a few days, but at their peak they outshine entire galaxies. These are LFBOTs — rapid blue transients. They are rare and have been a mystery until now. Astronomers studied 11 such events.

A black hole rips apart a star, and we see a dazzling flare. Then, a couple of years later, when everyone has forgotten about it, radio telescopes suddenly pick up a new signal, like an echo from the past.

Detectors pick up the “ripples” of gravitational waves from mergers of black holes or neutron stars — like an echo from the kitchen where a cosmic dish was being prepared. Astronomers are trying to reconstruct the recipe: how two massive stars turned into an exotic pair and collided.

LIGO and Virgo detectors catch ripples in spacetime, but some signals force us to rewrite the scripts of stellar death. GW190814 — the merger of a black hole of 23 solar masses with a compact object of 2.6 solar masses — is a discordant chord in the symphony of standard models.