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 Absolute Radiometer for Cosmology, Astrophysics and Diffuse Emission (ARCADE2) and Long Wavelength Array (LWA1) experiments at less than or similar to 1 GHz. We find that even under the extreme assumption that all of the dark matter is in the form of PBH5, the radio brightness temperature induced by Hawking evaporation of PBHs is O(10(-)(46)) K, highly subdominant compared to the cosmic microwave background. The main reason for this is that for PBH5 in the mass range similar to 10(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 on to supermassive PBHs. These PBHs can readily produce strong radio emission that could easily explain the ARCADE2/LWA1 excess.

Automated summary

In this study, we computed the isotropic radiation background from Hawking emission of primordial black holes (PBHs) and found that even under the extreme assumption that all dark matter is in the form of PBHs, the resulting radio brightness temperature is highly subdominant compared to the observed excess radio background. We also considered the alternative possibility of radio emission from gas accretion onto supermassive PBHs.