Reconstructing Early Primordial Black Hole Domination from Gravitational-Wave Backgrounds

Abstract

Primordial Black Holes (PBHs) with masses below $\mathcal{O}(10^9)$g occupy an interesting region of parameter space that is largely inaccessible to conventional observations. Despite evaporating before Big Bang Nucleosynthesis (BBN), these PBHs can naturally generate a period of early matter domination in the early Universe. A primordial gravitational-wave background (GWB) provides a window onto this otherwise inaccessible regime, since the modified expansion history leaves a characteristic spectral feature associated with the onset and end of PBH domination. The locations of these characteristic frequencies can be used to reconstruct the underlying PBH parameters, in particular the PBH mass. For a freely propagating GWB, the location of these frequencies additionally allow the initial PBH abundance to be determined. We derive simple numerical relations that map these characteristic frequencies directly onto the PBH mass and initial abundance. Future GW experiments span a vast frequency range, providing sensitivity to PBH masses from the BBN bound of $\mathcal{O}(10^9)\,\mathrm{g}$ down to the lower bound of $\mathcal{O}(10)\,\mathrm{g}$. We find that the nanohertz signal reported by NANOGrav, if primordial in origin, is already probing PBH masses in the range $4$--$90\,\mathrm{Mg}$. GW observations therefore offer access to a vast region of PBH parameter space that is otherwise beyond the reach of current experiments.