A mass distribution trick clears the path for ultralight primordial black holes

Primordial black holes lighter than a billion grams could have reheated the early universe, but a predicted gravitational-wave 'Poltergeist' signal seemed to rule this out. Accounting for the inevitable spread in black hole masses — a tail predicted by general relativity itself — smooths out the reheating process and suppresses that signal by orders of magnitude. The ultralight PBH window, long thought closed, is open again.