Primordial black holes in the era of gravitational-wave astronomy

Primordial black holes (PBHs), one of the oldest and least speculative candidates for the elusive dark matter, attracted a renewed interest recently as various studies investigate  whether the LIGO/Virgo interferometers has observed the fraction of the dark matter comprised of PBHs. To address correctly this question one must consider a realistic description of primordial black holes that incorporates the effects of cosmological expansion prior to the PBH decoupling from the Hubble flow. Adopting the Thakurta metric, as a more adequate description of PBHs in the early universe, we argue that the merger rate constraints on the abundance of dark matter PBHs are evaded entirely, hence reopening the possibility of LIGO mass PBH dark matter.

Among the different mechanisms leading to PBH we will consider two: inflationary fluctuations and the gravitational collapse of cosmic strings. The former is accompanied by a scalar induced gravitational wave background. Performing a Bayesian search of such background in the data from Advanced LIGO and Virgo’s first, second and third observing runs, we find no evidence for such a background, and place 95% confidence level upper limits on the integrated power of the curvature power spectrum peak. We then show that LIGO and Virgo, at design sensitivity, and the Einstein Telescope can compete with the constraints related to the abundance of the formed PBHs.

The latter mechanism leads to PBHs that are highly spinning and boosted to ultrarelativistic velocities. Such PBHs populate a unique region of the black hole mass-spin parameter space, and are therefore a "smoking gun" observational signature of cosmic strings. We derive new constraints on the cosmic string tension from the evaporation of cusp-collapse PBHs, and update existing constraints on the string tension from gravitational-wave searches.