Background of radio photons from primordial black holes
  We compute the isotropic radiation background due to Hawking emission from
primordial black holes (PBHs), and examine if this background is a viable
option in explaining the excess radiowave background observed by the ARCADE2
and LWA1 experiments at $\lesssim 1\,$GHz. We find that even under the extreme
assumption that all of the dark matter is in the form of PBHs, the radio
brightness temperature induced by Hawking evaporation of PBHs is
$\mathcal{O}(10^{-46})\,$K, highly subdominant compared to the cosmic microwave
background. The main reason for this is that for PBHs in the mass range
$\sim10^{12}$-$10^{14}\,$kg, which can be constrained by Hawking emission, the
spectrum peaks at $10^7$ to $10^5\,$eV. As the Hawking spectrum is power law
suppressed towards lower energies, negligible flux of $\mu$eV photons is
obtained. The peak of the Hawking spectrum shifts to lower energies for higher
masses, but the number density is low and so is the specific intensity. Because
Hawking emission from PBHs is thus unable to explain the observed excess radio
background, we also consider the alternative possibility of radio emission from
gas accretion onto supermassive PBHs. These PBHs can readily produce strong
radio emission that could easily explain the ARCADE2/LWA1 excess.
