How old are LIGO’s black holes?

Abstract: The time it takes for two black holes to merge due to gravitational radiation, known as the coalescence time, scales with their initial separation. I will derive the so-called Peters formula for the gravitational coalescence time and discuss the resulting predictions for binary black hole delay times. Although the gravitational-wave sources we have observed so far merged at around z < 1, the delay time distribution allows us to infer their formation redshifts, which in some cases are likely to be past z > 6.