In the busy hearts of globular star clusters, black holes may pair up and merge over and over again. That process could produce especially heavy black holes. And upgraded detectors should be able to “hear” those mergers.
Globular clusters are balls of hundreds of thousands of stars. Most of the clusters are more than 10 billion years old. Their most massive stars burned out a long time ago, leaving behind black holes. Many of them settle into the cluster’s core — perhaps thousands of them.
A team of researchers recently used a supercomputer to simulate how these black holes interact. In particular, they looked at the effects of general relativity — Albert Einstein’s theory of gravity. The researchers found that the black holes can become quite chummy.
As two black holes pass near each other, they emit bursts of gravitational waves — ripples in space and time. That causes them to lose a bit of their speed, so they settle into orbit around each other. As they orbit, they continue to emit gravitational waves, causing them to spiral together and merge.
The merged pair could be shot out of the cluster. If not, then a merged black hole can pair up with another black hole, leading to another merger — a process that can repeat multiple times.
Scientists are upgrading the leading gravitational-wave detectors. The new capabilities should allow them to “hear” the mergers of these larger black holes, in the hearts of ancient star clusters.