CMB and accretion

Primordial black holes (PBHs) of stellar mass or heavier would accrete baryonic gas, which becomes denser and hotter, injecting energetic photons in the cosmological medium soon after cosmic recombination, in the so-called dark ages. The ionisation history of the universe would be altered, an effect cosmic microwave background (CMB) temperature and polarisation anisotropies is sensitive to. The magnitude of the effect depends on the abundance and mass distribution of the PBHs, as well as on still uncertain aspects of accretion luminosity. We review the current understanding of this field, with an emphasis on the peculiarities of the phenomenon in the cosmological context, and present the existing constraints on the PBH abundances.
Comment: 19 pages+refs, 3 figures. Invited chapter to the book "Primordial Black Holes'', Springer 2024, ed. Chris Byrnes, Gabriele Franciolini, Tomohiro Harada, Paolo Pani, Misao Sasaki