Small-scale primordial fluctuations in the 21 cm Dark Ages signal

Primordial black hole production in the mass range 10-10⁴ M_⊙ is motivated respectively by interpretations of the LIGO/Virgo observations of binary black hole mergers and by their ability to seed intermediate black holes that would account for the presence of supermassive black holes at very high redshift. Their existence would imply a boost in the primordial power spectrum if they were produced by overdensities reentering the horizon and collapsing after single-field inflation. This, together with their associated Poisson fluctuations would cause a boost in the matter power spectrum on small scales. The extra power could become potentially observable in the 21 cm power spectrum on scales around k ∼ 0.1-50 Mpc^-1 with the new generation of filled low-frequency interferometers. We explicitly include the contribution from primordial fluctuations in our prediction of the 21 cm signal that has been previously neglected, by constructing primordial power spectra motivated by single-field models of inflation that would produce extra power on small scales. We find that depending on the mass and abundance of primordial black holes, it is important to include this contribution from the primordial fluctuations, so as not to underestimate the 21 cm signal. Evidently our predictions of detectability, which lack any modelling of foregrounds, are unrealistic, but we hope that they will motivate improved cleaning algorithms that can enable us to access this intriguing corner of primordial black hole-motivated parameter space.