A Step in Understanding the S8 Tension

Melissa Joseph, Daniel Aloni, Martin Schmaltz, Eashwar N. Sivarajan, Neal Weiner

Submitted on 7 July 2022


Models of dark sectors with a mass threshold can have important cosmological signatures. If, in the era prior to recombination, a relativistic species becomes non-relativistic and is then depopulated in equilibrium, there can be measurable impacts on the CMB as the entropy is transferred to lighter relativistic particles. In particular, if this ``step'' occurs near z20,000, the model can naturally accommodate larger values of H0. 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 non-relativistic and depleted. This can naturally lead to suppressed power at scales inside the sound horizon before the step, while leaving conventional CDM signatures for power outside the sound horizon. We study these effects and show such models can naturally provide lower values of S8 than scenarios without a step. This suggests these models may provide an interesting framework to address the S8 tension, both in concert with the H0 tension and without.


Subjects: Astrophysics - Cosmology and Nongalactic Astrophysics; High Energy Physics - Phenomenology