Nonthermal neutrino-like hot dark matter in light of the S8 tension

Abstract

The Λ CDM prediction of S 8 ≡ σ 8 ( Ω m / 0.3 ) 0.5 —where σ 8 is the root mean square of matter fluctuations on an 8

h − 1

Mpc scale—once calibrated on Planck cosmic microwave background data is 2 − 3 σ lower than its direct estimate by a number of weak lensing surveys. In this paper, we explore the possibility that the “ S 8 tension” is due to a fractional contribution of nonthermal hot dark matter (HDM) to the energy density of the Universe leading to a power suppression at small scales in the matter power spectrum. Any HDM model can be characterized by its effective mass m eff sp and its contribution to the relativistic degrees of freedom at cosmic microwave background decoupling Δ N eff . Taking the specific example of a sterile particle produced from the decay of the inflaton during an early matter-dominated era, we find that the tension can be reduced below 2 σ from Planck data only, but it does not favor a nonzero { m eff sp , Δ N eff } . In combination with a measurement of S 8 from KiDS 1000 + BOSS + 2 dfLenS , the S 8 tension would hint at the existence of a particle of mass m eff sp ≃ 0.6 7 + 0.26 − 0.48

eV with a contribution to Δ N eff ≃ 0.06 ± 0.05 . However, Pantheon and BOSS BAO / f σ 8 data restricts the particle mass to m eff sp ≃ 0.4 8 + 0.17 − 0.36 and contribution to Δ N eff ≃ 0.04 6 + 0.004 − 0.031 . We discuss implications of our results for other canonical nonthermal HDM models—the Dodelson-Widrow model and a hidden sector model of a thermal sterile particle with a different temperature. We report competitive results on such hidden sector temperature that might have interesting implications for particle physics model building, in particular connecting the S 8 tension to the longstanding short baseline oscillation anomaly.