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.
