Nonthermal neutrino-like hot dark matter in light of the S-8 tension

The ACDM prediction of S-8 equivalent to sigma(8)(Omega(m)/0.3)(0.5)-where sigma(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 sigma 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 meffsp and its contribution to the relativistic degrees of freedom at cosmic microwave background decoupling Delta 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 from Planck data only, but it does not favor a nonzero {m(sp)(eff) sp ; Delta N-eff}. In combination with a measurement of S8 from KiDS1000 + BOSS + 2dfLenS, the S8 tension would hint at the existence of a particle of mass m(sp)(eff) 0.67+0.26 0.48 eV with a contribution to Delta N-eff 0.06 0.05. However, Pantheon and BOSS BAO=f 8 data restricts the particle mass to m(eff) sp 0.48+0.17 0.36 and contribution to Delta N-eff 0.046+0.004 0.031 . We discuss implications of our results for other canonical nonthermal HDM modelsthe 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 S8 tension to the longstanding short baseline oscillation anomaly.