The dynamics of primordial black-hole formation

We examine numerically the formation of small black holes from primordial density fluctuations in a radiation-dominated spatially flat Friedmann-Robertson-Walker spacetime. Large amplitude fluctuations might be expected to form black holes, while smaller fluctuations will be washed out by the expansion of the universe. We have studied the interface between these two types of behaviour. Unlike earlier studies which suggested that there was no lower limit to the mass of a black hole, this work suggests that there is a minimum mass for a primordial black hole of the order of one ten thousandth of the mass contained within the horizon. We discuss the implications for critical collapse studies.