Can accreting primordial black holes explain the excess radio background?

TL;DR

The paper evaluates whether accreting primordial black holes can explain the excess radio background and finds that their ultraviolet emission would conflict with cosmological observations, making this explanation unlikely.

Contribution

It critically assesses the primordial black hole hypothesis for the radio excess, highlighting conflicts with cosmological constraints.

Findings

Ultraviolet emission from these black holes would ionize the universe at high redshift.

This ionization would eliminate the expected 21 cm absorption signature.

The model conflicts with existing CMB anisotropy and spectral distortion limits.

Abstract

The excess radio background seen at $\simeq 0.1-10\,{\rm GHz}$ has stimulated much scientific debate in the past years. Recently, it was pointed out that the soft photon emission from accreting primordial black holes may be able to explain this signal. We show that the expected ultraviolet photon emission from these accreting black holes would ionize the universe completely at $z>6$ and thus wash out the 21 cm absorption signature at $z\simeq$ 20 as well as be in tension with existing cosmic microwave background anisotropy and average spectral distortion limits. We discuss possible augmentations of the model; however, it seems that an explanation of radio excess by accreting primordial black holes is not well-justified.