Primordial black holes and the velocity acoustic oscillations features in 21 cm signals from the cosmic Dark Ages

TL;DR

This paper explores how primordial black holes influence velocity acoustic oscillations in 21 cm signals during the cosmic Dark Ages, proposing VAOs as a novel probe for PBHs even at very low fractions.

Contribution

It demonstrates that PBHs can induce detectable VAOs in 21 cm signals, providing a new method to probe PBHs during the Dark Ages.

Findings

PBHs can generate VAOs wiggles with up to 30% amplitude in the 21 cm power spectrum.

VAOs features appear at redshifts around 20 and 40 depending on PBH fraction.

Upcoming SKA-low and lunar interferometers could detect these VAOs signals.

Abstract

Astrophysical luminous objects such as the first stars have not yet formed in the Dark Ages. However, primordial black holes (PBHs) always exist throughout cosmic history since the inflation epoch. During the Dark Ages, PBHs may accrete the ambient gas and release radiation like astrophysical luminous objects, change the cosmic radiation field, the thermal status of the intergalactic medium (IGM), and the hydrogen spin temperature. The accretion rate is modulated by the relic supersonic relative streaming velocities between dark matter (DM) and baryons, imprinting Velocity Acoustic Oscillations (VAOs) features in the 21 cm power spectrum. Such VAOs features could be a promising probe for detecting the PBHs in Dark Ages. We find that even if PBHs comprise only a small fraction of DM, they can generate VAOs wiggles with a relative amplitude up to about 30% in Dark Ages. For example, for PBHs with a mass at recombination of 200 solar masses and mass fraction in the total DM f_PBH,rec around 1e-13 at the recombination era, VAOs features appear at redshift around 20; if f_PBH,rec is around 3e-10, then VAOs features could appear as early as redshift around 40. Moreover, the redshift evolution of the VAOs features exhibits clearly separated stages dominated by inhomogeneous Ly-alpha scattering, and inhomogeneous X-ray heating, respectively. It reflects the characteristics of PBHs (mass and fraction in total DM) and their interactions with the IGM. We also estimate that, the VAOs wiggles at redshift around 20 are detectable for the upcoming SKA-low AA*, while wiggles at redshift around 40 are detectable for an hypothetic lunar surface-based interferometer array in the future.