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. Assuming that the parameters of non-Gaussianity are constant over all scales, 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 $\zeta$. 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 $-2.9\cdot10^{-4}<f<3.8\cdot10^{-4}$ and $-1.5\cdot10^{-3}<g<1.9\cdot10^{-3}$ for quadratic and cubic models respectively.