Dark radiation

Dark radiation is a postulated species of radiation that mediates interactions in the dark sector. That is, just the way photons mediate electromagnetic interactions between particles in the Standard Model (baryonic matter in Cosmology), dark radiation is supposed to mediate interactions between dark matter particles. And, similar to dark matter particles, dark radiation doesn't interact with Standard Model particles. Though there has been no notable evidence for the existence of such a species, it is speculated that the dark sector, which currently is supposed to consist only of dark matter, must be quite rich in variety. This speculation is based on the fact that there is already a high degree of complexity in baryonic matter interactions and the energy density of dark matter is estimated to be about five times as much as that of baryonic matter. Moreover, it has been pointed out recently that the CMB data seem to suggest that the number of effective neutrino degrees of freedom is more than 3.046, which is the standard case for 3 species of neutrinos. This extra degree of freedom could arise from having a non-trivial amount of dark radiation in the universe. One possible candidate for dark radiation is a sterile neutrino.

The energy of dark radiation is proportional to the mass of a black hole. Recent analysis of cosmological data shows hints of the existence of additional relavistic component. Dark radiation is usually released at higher speed from decaying particles. Dark radiation currently represents one of the most accordant anomalies in the Lambda-CDM model.