The Dark Ages — after recombination but before the first stars — are observationally empty. No luminous objects existed yet. But primordial black holes, if they exist, were already there, accreting ambient gas and emitting radiation. Even a tiny fraction of dark matter in the form of primordial black holes would alter the intergalactic medium's radiation field, temperature, and hydrogen spin state.
This paper shows that the accretion rate is modulated by relic supersonic streaming velocities between dark matter and baryons, imprinting velocity acoustic oscillations (VAOs) in the 21 cm power spectrum. Even at PBH fractions as small as 10⁻¹³ of total dark matter (for 200 solar mass PBHs), the oscillations produce wiggles with 30% relative amplitude in the power spectrum. The signature has two distinct stages — Lyman-α scattering dominance followed by X-ray heating dominance — reflecting the PBH properties and their interaction with the medium.
The detection threshold matters: wiggles at redshift ~20 are accessible to SKA-low, while redshift ~40 requires a lunar-surface interferometer. The 21 cm signal from the Dark Ages could detect primordial black holes at abundances twelve orders of magnitude below what gravitational lensing or other methods can reach. The emptiest epoch in cosmic history becomes the most sensitive detector.