Arbitrating the S8 discrepancy with growth rate measurements from redshift-space distortions

Within the Lambda cold dark matter (Lambda CDM) model, measurements from recent cosmic microwave background (CMB) and weak lensing (WL) surveys have uncovered a similar to 3 sigma disagreement in the inferred value of the parameter S-8 = sigma(8)root Omega(m)/0.3, quantifying the amplitude of late-time matter fluctuations. Before questioning whether the S-8 discrepancy calls for new physics, it is important to assess the view of measurements other than CMB and WL ones on the discrepancy. Here, we examine the role of measurements of the growth rate f(z) in arbitrating the S-8 discrepancy, considering measurements of f sigma(8)(z) from redshift-space distortions (RSDs). Our baseline analysis combines RSD measurements with geometrical measurements from baryon acoustic oscillations (BAO) and Type Ia Supernovae (SNeIa), given the key role of the latter in constraining Omega(m). From this combination and within the Lambda CDM model, we find S-8 = 0.762(-0.025)(+0.030), and quantify the agreement between RSD + BAO + SNeIa and Planck to be at the 2.2 sigma level: the mild disagreement is therefore compatible with a statistical fluctuation. We discuss combinations of RSD measurements with other data sets, including the E-G statistic. This combination increases the discrepancy with Planck, but we deem it significantly less robust. Our earlier results are stable against an extension where we allow the dark energy equation of state w to vary. We conclude that, from the point of view of combined growth rate and geometrical measurements, there are hints, but no strong evidence yet, for the Planck Lambda CDM cosmology overpredicting the amplitude of matter fluctuations at redshifts z less than or similar to 1. From this perspective, it might therefore still be premature to claim the need for new physics from the S-8 discrepancy.

Automated summary

The article discusses the discrepancy of the S-8 parameter within the Lambda cold dark matter model, and notes that there is not yet strong evidence from combined growth rate and geometrical measurements to support the overprediction of matter fluctuations at redshifts z less than or equal to 1 by Planck Lambda CDM cosmology.