Eliminating the LIGO bounds on primordial black hole dark matter

TL;DR

This paper revises constraints on primordial black holes as dark matter candidates by accounting for their evolving mass in an expanding universe, suggesting they could still constitute dark matter despite previous limits.

Contribution

It introduces a time-dependent mass model for PBHs in an expanding universe, altering binary formation and merger rate calculations, thus reopening the viability of PBHs as dark matter.

Findings

PBHs have a time-dependent mass in an expanding universe.

Black hole binaries form and merge faster than previously estimated.

LIGO-mass PBHs remain a viable dark matter candidate.

Abstract

Primordial black holes (PBHs) in the mass range $(30$--$100)~M_{\odot}$ are interesting candidates for dark matter, as they sit in a narrow window between microlensing and cosmic microwave background constraints. There are however tight constraints from the binary merger rate observed by the LIGO and Virgo experiments. In deriving these constraints, PBHs were treated as point Schwarzschild masses, while the more careful analysis in an expanding universe we present here, leads to a time-dependent mass. This implies a stricter set of conditions for a black hole binary to form and means that black holes coalesce much more quickly than was previously calculated, namely well before the LIGO/Virgo's observed mergers. The observed binaries are those coalescing within galactic halos, with a merger rate consistent with data. This reopens the possibility for dark matter in the form of LIGO-mass PBHs.