Primordial black hole isocurvature modes from non-Gaussianity

Abstract

Primordial black holes (PBHs) are black holes that might have formed in high density regions in the early universe. The presence of local-type non-Gaussianity can lead to large-scale fluctuations in the PBH formation rate. If PBHs make up a non-negligible fraction of dark matter, these fluctuations can appear as isocurvature modes, and be used to constrain the amplitude of non-Gaussianity. We build upon the results of previous work by extending the calculation to include peaks theory and making use of the compaction C for the formation criteria, accounting for non-linearities between C and the curvature perturbation ζ. For quadratic models of non-Gaussianity, our updated calculation gives constraints that are largely unaltered compared to those previously found, while for cubic models the constraints worsen significantly. In case all of the DM is made up of PBHs, the parameters of non-Gaussianity are −2.9 • 10 −4 < f < 3.8 • 10 −4 and −1.5 • 10 −3 < g < 1.9 • 10 −3 for quadratic and cubic models respectively.

Spectral distortions from acoustic dissipation with non-Gaussian (or not) perturbations

“…It was however pointed out in [58][59][60] that any significant mode-mode coupling between the large scales observed in CMB maps and the small scales potentially responsible for PBH formation is excluded by Planck unless the fraction of PBHs in dark matter is very small. Indeed, such a coupling would be effectively interpreted as very large CDM isocurvature perturbations on CMB scales, and would ruin the agreement with the observed CMB temperature and polarisation spectrum.…”