Space-time may have fooled detectors in the largest black-hole merger on record

The biggest black-hole merger ever recorded may have looked larger because a cosmic “magnifying glass” distorted its signal. Astrophysicists are revisiting GW231123, a gravitational-wave event detected by the LIGO and Virgo observatories in 2023.

The original reading indicated that two massive stellar black holes, 103 and 137 times the mass of the Sun, had merged. The collision appeared to create a structure weighing about 225 solar masses, placing it in the intermediate-mass black-hole category.

New analyses suggest that this measurement could have resulted from gravitational lensing. Under Albert Einstein’s theory of general relativity, massive objects can bend and amplify light and gravitational waves passing behind them before those signals reach Earth.

Researchers say an object between Earth and the merging black holes may have magnified the signal. One possibility is another intermediate-mass black hole with a mass about 1,000 times that of the Sun. Another is a dense globular cluster containing thousands of stars, or another massive concentration of matter.

Scientists have not ruled out the original interpretation. The signal could still reflect a gradual, hierarchical sequence of black-hole mergers. As gravitational-wave astronomy develops, better estimates of lensing rates could help researchers map otherwise hidden structures deep in the universe.