A closer look at dark photon explanations of the excess radio background

The observed excess radio background has remained a puzzle for over a decade. A recent new physics solution involves dark matter that decays into dark photons in the presence of a thermal dark photon background. The produced non-thermal dark photon spectrum then converts into standard photons around the reionization era, yielding an approximate power-law radio excess with brightness temperature T(nu) similar or equal to upsilon(-2.5) over a wide range of frequencies, nu. This simple power-law model comes intriguingly close to the current data, even if several ingredients are required to make it work. In this paper, we investigate some of the details of this model, showcasing the importance of individual effects. In particular, significant deviation from a power law is present at nu less than or similar to 100 MHz and nu greater than or similar to 1 GHz. These effects result in improving the fit to data compared to a power-law spectrum, and may become testable in future observations. We also highlight independent signatures that can be tested with future cosmic microwave background spectral distortion experiments such as PIXIE. However, there are challenges for the model from the observed radio background anisotropies, as discussed here. We furthermore highlight a possible runaway process due to the finite width of the dark matter decay profile, which suggests that additional work might be required to obtain a viable model.

Automated summary

The observed excess radio background has puzzled scientists for over a decade. A recent new physics solution involves the decay of dark matter into dark photons which then convert into standard photons in the reionization era. This simple power-law model fits the current data closely, although additional work is needed to address challenges and improve the model.