Cosmological Constraints on Light (but Massive) Relics

Understand

Many scenarios of physics beyond the standard model predict new light, weakly coupled degrees of freedom, populated in the early universe and remaining as cosmic relics today.

• Due to their high abundances, these relics can significantly affect the evolution of the universe.
• For instance, massless relics produce a shift $\Delta N_{\rm eff}$ to the cosmic expectation of the effective number of active neutrinos.
• Massive relics, on the other hand, additionally become part of the cosmological dark matter in the later universe, though their light nature allows them to freely stream out of potential wells.