Published: 8 November 2026
A tenth of a second with enormous weight
Detected on 23 November 2023, GW231123 lasted only about a tenth of a second in the sensitive band of the two LIGO detectors. Statistical tests place the chance of noise mimicking the event at less than once in ten thousand years.
The heaviest binary so far
The inferred black holes weighed roughly 137 and 103 times the Sun. Accounting for uncertainty, their combined mass was probably 190 to 265 solar masses. The remnant, likely 182 to 251 solar masses, belongs to the elusive intermediate-mass class.
Inside the stellar mass gap
Standard models predict that pair-instability processes should make black holes between about 60 and 130 solar masses rare. The lighter component has an estimated 83 percent probability of lying in this gap, while the heavier component has a smaller but nonzero chance.
A merger built from earlier mergers?
One explanation is hierarchical growth: black holes merge, remain inside a dense nuclear cluster or active galactic nucleus, and later merge again. That pathway naturally produces high masses and spins, but it requires environments capable of retaining the remnants.
The waveform is also a challenge
Both objects appear to have spun almost as fast as black holes theoretically can. Extreme spin, possible orbital eccentricity and the signal's brevity push current waveform models to their limits. The ringdown agrees strongly with general relativity, while subtle features remain under study.
Discussion question
Would one more mass-gap merger convince you that hierarchical black-hole growth is common, or would several independent environmental clues still be necessary?
Sources and further reading
LIGO Scientific Collaboration — GW231123: The most massive black hole binary detected through gravitational waves ↗Share your answer
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