Understand
When large overdensities gravitationally collapse in the early universe, they lead to primordial black holes (PBH).
- Depending on the exact model of inflation leading to necessary large perturbations at scales much smaller than scales probed at the Cosmic Microwave Background (CMB) surveys, PBHs of masses $\lesssim$$10^3 M_{\odot}$ can be formed sometime between the end of inflation and nucleosynthesis.
- However, the lack of a direct probe for the exact expansion history of the universe in this duration introduces uncertainties in the PBH formation process.
- The presence of alternate cosmological evolution for some duration after inflation affects the relation between (i) PBH mass and the scale of the collapsing overdensity; and (ii) PBH abundance and amplitude of the overdensities.