$S_8$ increases with effective redshift in $\Lambda$CDM cosmology

Abstract

Hubble constant $H_0$ and weighted amplitude of matter fluctuations $S_8$ determinations are biased to higher and lower values, respectively, in the late Universe with respect to early Universe values inferred by the Planck collaboration within flat $\Lambda$CDM cosmology. If these anomalies are physical, i.e. not due to systematics, they naively suggest that $H_0$ decreases and $S_8$ increases with effective redshift. Here, subjecting matter density today $\Omega_{m}$ to a prior, corresponding to a combination of Planck CMB and BAO data, we perform a consistency test of the Planck-$\Lambda$CDM cosmology and show that $S_8$ determinations from $f \sigma_8(z)$ constraints increase with effective redshift. Due to the redshift evolution, a $\sim 3 \sigma$ tension in the $S_8$ parameter with Planck at lower redshifts remarkably becomes consistent with Planck within $1 \sigma$ at high redshifts. This provides corroborating support for an $S_8$ discrepancy that is physical in origin. We further confirm that the flat $\Lambda$CDM model is preferred over a theoretically ad hoc model with a jump in $S_8$ at a given redshift. In the absence of the CMB+BAO $\Omega_m$ prior, we find that $> 3 \sigma$ tensions with Planck in low redshift data are ameliorated by shifts in the parameters in high redshift data. Results here and elsewhere suggest that the $\Lambda$CDM cosmological parameters are redshift dependent. Fitting parameters that evolve with redshift is a recognisable hallmark of model breakdown.