Signatures of primordial black holes as seeds of supermassive black
  holes
  It is broadly accepted that Supermassive Black Holes (SMBHs) are located in
the centers of most massive galaxies, although there is still no convincing
scenario for the origin of their massive seeds. It has been suggested that
primordial black holes (PBHs) of masses $\gtrsim 10^{2} M_\odot$ may provide
such seeds, which would grow to become SMBHs. We suggest an observational test
to constrain this hypothesis: gas accretion around PBHs during the cosmic dark
ages powers the emission of high energy photons which would modify the spin
temperature as measured by 21cm Intensity Mapping (IM) observations. We model
and compute their contribution to the standard sky-averaged signal and power
spectrum of 21cm IM, accounting for its substructure and angular dependence for
the first time. If PBHs exist, the sky-averaged 21cm IM signal in absorption
would be higher, while we expect an increase in the power spectrum for
$\ell~\gtrsim 10^2-10^3$. We also forecast PBH detectability and measurement
errors in the abundance and Eddington ratios for different fiducial parameter
configurations for various future experiments, ranging from SKA to a futuristic
radio array on the dark side of the Moon. While the SKA could provide a
detection, only a more ambitious experiment would provide accurate
measurements.
