LIGO and primordial black holes

LIGO has opened a new window onto the Universe, by detecting the gravitational waves from merging black holes and neutron stars. Black holes can be the remnant of dead stars, but they might also have formed during the first second of the Universe. So called “primordial black holes” could constitute the dark matter, form seeds for supermassive black holes and provide information about inflation on scales too small to probe by other methods. If LIGO has detected any primordial black holes, I will show that the reduction in pressure during the QCD phase transition predicts an enhancement number of solar mass primordial black holes. Such low mass black holes cannot form via collapsing stars and are hence a smoking gun for a primordial origin of some black holes.