Can accreting primordial black holes explain the excess radio background?

Understand

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.