Big evidence for “second generation” black hole mergers

Back in 2015, humanity detected our first gravitational wave event.

Now, in 2026, we’re up to 390 confirmed detections.

Most correspond to merging pairs of black holes, some at quite high masses.

This poses a theoretical challenge, because stars shouldn’t form such heavy black holes.

When stars form, they come in a variety of masses: following an initial mass function.

Based on that initial mass, they’ll eventually die.

The most massive stars can undergo core-collapse,

detonate in a pair-instability event,

or directly collapse to form a black hole.

Then, black hole mergers arise from that population.

Our detectors then see them with a distance sensitivity based on mass.

This scenario can’t explain the abundance of high-mass black hole mergers.

However, hierarchical mergers can help.

After first-generation black holes merge, their remnants can merge again.

A clue should arise in their spin-alignment with their orbits.

Two independent studies found large spin misalignments in high mass mergers.

Only low masses are dominated by first-generation mergers.

At higher masses, especially above 40-45 solar masses, later-generation mergers occur.

This “hierarchical” scenario has long been suspected, but is now supported by the best evidence ever.

As measurements and statistics continually improve, so will our understanding.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.

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