Phys. Rev. D (in the press); preprint at https://arxiv.org/abs/1612.07738

Primordial black holes could be the oldest inhabitants of the Universe. Born from density fluctuations shortly after the Big Bang, they are important pieces of the cosmological puzzle and could also account for dark matter. And although the light primordial black holes would have long evaporated, heavier ones could still be lurking around. Various astrophysical observations have put limits on different segments of the allowed mass range. Steven Clark and colleagues used the most recent Planck cosmic microwave background (CMB) data combined with extragalactic gamma-ray background information to constrain the abundance of primordial black holes with masses 1015–1017 g: about as heavy as the 67P/Churyumov–Gerasimenko comet, made famous by the Rosetta mission.

The Hawking radiation emitted by primordial black holes since the beginning of the Universe is expected to have left a mark in the CMB, which would manifest itself as a damping of the anisotropies. The analysis of the CMB data combined with bounds from the extragalactic gamma-ray background puts a strong constraint on primordial black holes in this mass range — ruling them out as a dark matter component.