  Theoretical models and observations suggest that primordial Stellar Black
Holes (Pop-III-BHs) were prolifically formed in HMXBs, which are powerful
relativistic jet sources of synchrotron radiation called Microquasars (MQs).
  Large populations of BH-HMXB-MQs at cosmic dawn produce a smooth synchrotron
cosmic radio background (CRB) that could account for the excess amplitude of
atomic hydrogen absorption at z~17, recently reported by EDGES.
  BH-HMXB-MQs at cosmic dawn precede supernovae, neutron stars and dust.
BH-HMXB-MQs promptly inject hard X-rays and relativistic jets into the IGM,
which overtake the slower expanding HII regions ionized by progenitor Pop-III
stars, heating and partially ionizing the IGM over larger distance scales.
  BH-HMXBs are channels for the formation of Binary-Black-Holes (BBHs). The
large masses of BBHs detected by gravitational waves, relative to the masses of
BHs detected by X-rays, and the high rates of BBH-mergers, are consistent with
high formation rates of BH-HMXBs and BBHs in the early universe.
