Evidence for suppression of structure growth in the concordance cosmological model

Abstract

We present evidence for a suppressed growth rate of large-scale structure during the dark-energy dominated era. Modeling the growth rate of perturbations with the ``growth index'' $γ$, we find that current cosmological data strongly prefer a higher growth index than the value $γ=0.55$ predicted by general relativity in a flat $Λ$CDM cosmology. Both the cosmic microwave background data from Planck and the large-scale structure data from weak lensing, galaxy clustering, and cosmic velocities separately favor growth suppression. When combined, they yield $γ=0.633^{+0.025}_{-0.024}$, excluding $γ=0.55$ at a statistical significance of 3.7$σ$. The combination of $fσ_8$ and Planck measurements prefers an even higher growth index of $γ=0.639^{+0.024}_{-0.025}$, corresponding to a 4.2$σ$-tension with the concordance model. In Planck data, the suppressed growth rate offsets the preference for nonzero curvature and fits the data equally well as the latter model. A higher $γ$ leads to a higher matter fluctuation amplitude $S_8$ inferred from galaxy clustering and weak lensing measurements, and a lower $S_8$ from Planck data, effectively resolving the $S_8$ tension.