Can we use gravitational waves to trace where black holes formed?

Next-generation gravitational-wave detectors will see black-hole mergers from the universe's earliest epochs, allowing us to reconstruct where and when their progenitors formed. The authors introduce a statistical method to identify the "target redshift"—the epoch when most stars had a particular metallicity—and show how merger-rate variations between then and now can confirm or rule out competing stellar-evolution models.