Where is the dark radiation?

Where is the dark radiation?

Observations show that the energy density of our universe is dominated by dark energy, and was dominated by dark matter at the epoch of Matter domination. Therefore, it is quite surprising that the epoch of radiation domination was dominated by SM physics. Moreover, the constituents of dark radiation, namely light particles, are generic in QFT. Famous examples such as goldstone bosons, chiral fermions, and gauge bosons are common in many extensions of the SM. Therefore, from both the cosmology and particle physics points of view, one expects to see some form of dark radiation. Although we have experimental evidence that dark radiation cannot be the dominant component of radiation energy density, current data still permits the existence of a significant dark component. However, this situation is expected to change dramatically in the next two decades. Future CMB and Large Scale Structure experiments will have a sub-percent sensitivity to a component of dark radiation. Interestingly, current cosmological anomalies, especially the ~5? Hubble tension, might be our first hints of such an exotic type of new physics. In this talk, I will review the topic and present some previous and ongoing works about dark radiation.