Constraining Primordial Black Holes with the EDGES 21-cm Absorption
  Signal
  The EDGES experiment has recently measured an anomalous global 21-cm spectrum
due to hydrogen absorptions at redshifts of about $z\sim 17$. Model
independently, the unusually low temperature of baryons probed by this
observable sets strong constraints on any physical process that transfers
energy into the baryonic environment at such redshifts. Here we make use of the
21-cm spectrum to derive bounds on the energy injection due to a possible
population of ${\cal O}(1-100) M_\odot$ primordial black holes, which induce a
wide spectrum of radiation during the accretion of the surrounding gas. After
calculating the total radiative intensity of a primordial black hole
population, we estimate the amount of heat and ionisations produced in the
baryonic gas and compute the resulting thermal history of the Universe with a
modified version of RECFAST code. Finally, by imposing that the temperature of
the gas at $z\sim 17$ does not exceed the indications of EDGES, we constrain
the possible abundance of primordial black holes. Depending on uncertainties
related to the accretion model, we find that ${\cal O}(10) M_\odot$ primordial
black holes can only contribute to a fraction $f_{\rm PBH}<(1-10^{-3})$ of the
total dark matter abundance.
