Formation and Abundance of Late Forming Primordial Black Holes as Dark Matter

Understand

We propose a novel mechanism where Primordial Black Hole (PBH) dark matter is formed much later in the history of the universe between the epoch of Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) photon decoupling.

• In our setup, one does not need to modify the scale-invariant inflationary power spectra; instead, a late phase transition in strongly interacting fermion-scalar fluid (which naturally occurs around red-shift $ 10^6 \leq \, z_{\scriptscriptstyle T} \, \leq 10^8$ ) creates an instability in the density perturbation as sound speed turns imaginary.
• As a result, the dark matter perturbation grows exponentially in sub-Compton scales.
• This follows the immediate formation of early dense dark matter halo, which finally evolves into PBH due to cooling through scalar radiation.