A Step in Understanding the $S_8$ Tension

Understand

Models of dark sectors with a mass threshold can have important cosmological signatures.

• If, in the era prior to recombination, a relativistic species becomes nonrelativistic and is then depopulated in equilibrium, there can be measurable impacts on the cosmic microwave background as the entropy is transferred to lighter relativistic particles.
• In particular, if this ``step'' occurs near $z\sim 20,000$, the model can naturally accommodate larger values of $H_0$.
• If this stepped radiation is additionally coupled to dark matter, there can be a meaningful impact on the matter power spectrum as dark matter can be coupled via a species that becomes nonrelativistic and depleted.