Revisiting Primordial Black Holes Constraints from Ionization History

Abstract

Much attention has been drawn to the recent discoveries by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) of merging intermediate mass black holes. Of particular interest is the possibility that the merger events detected could be evidence of dark matter in the form of primordial black holes (PBHs). It has been argued that the presence of many black holes would effect the thermal and ionization history of the universe via their accretion of matter which would have strong signatures in the Cosmic Microwave Background's (CMB) power spectra evident in the damping of anisotropies and change in low-$l$ polarization power. In general the accretion is quite sensitive to the specific physics involved and the conditions of the early universe. In this work, we take a minimal approach and find constraints on PBHs not including the model dependent effects of nonlinear structure of formation or transition between different accretion models which would work to increase the effect. In addition, we include the relative velocity between dark matter and baryonic matter including the effects of supersonic streaming at high redshift which work to significantly reduce the constraining power. We also examine the constraints on more astrophysically-motivated extended black hole mass functions and discuss how mergers might effect this distribution. We find constraints on PBHs in the range $ \approx 30 M_\odot$, finding that they could not compose more than $10\%$ of the total dark matter content.