Cosmic microwave background limits on accreting primordial black holes

TL;DR

This paper investigates how accreting primordial black holes could affect the cosmic microwave background and uses observational data to constrain their possible contribution to dark matter.

Contribution

It provides a detailed analysis of accretion effects on PBHs and derives new constraints on their mass and abundance from CMB observations.

Findings

Spectral distortions from PBHs are too small for detection by FIRAS and future experiments.

Planck CMB data rules out PBHs >~ 100 solar masses as the main dark matter component.

Accretion effects impose constraints on PBH mass and abundance based on CMB observations.

Abstract

Interest in the idea that primordial black holes (PBHs) might comprise some or all of the dark matter has recently been rekindled following LIGO's first direct detection of a binary-black-hole merger. Here we revisit the effect of accreting PBHs on the cosmic microwave background (CMB) frequency spectrum and angular temperature/polarization power spectra. We compute the accretion rate and luminosity of PBHs, accounting for their suppression by Compton drag and Compton cooling by CMB photons. We estimate the gas temperature near the Schwarzschild radius, and hence the free-free luminosity, accounting for the cooling resulting from collisional ionization when the background gas is mostly neutral. We account approximately for the velocities of PBHs with respect to the background gas. We provide a simple analytic estimate of the efficiency of energy deposition in the plasma. We find that the spectral distortions generated by accreting PBHs are too small to be detected by FIRAS, as well as by future experiments now being considered. We analyze Planck CMB temperature and polarization data and find, under our most conservative hypotheses, and at the order-of-magnitude level, that they rule out PBHs with masses >~ 10^2 M_sun as the dominant component of dark matter.