Illustrating the consequences of a misuse of $\sigma_8$ in cosmology

TL;DR

This paper highlights the biases caused by using the parameter _8 in cosmology, demonstrating that switching to the h-independent _{12} can reduce biases and clarify the interpretation of growth and Hubble tensions.

Contribution

The paper illustrates the biases introduced by _8 and advocates for adopting _{12} to improve cosmological model comparisons and tension analyses.

Findings

Biases from _8 are significant in models with high H_0.

Using _{12} reduces bias and clarifies tension assessments.

Transitioning to _{12} can simplify resolving the Hubble tension.

Abstract

The parameter $\sigma_8$ is the rms mass fluctuations on a scale of $R_8=8h^{-1}$ Mpc and is used to quantify the amplitude of matter fluctuations at linear scales. However, the dependence of $R_8$ on $h$ complicates direct comparisons of $\sigma_8$ values obtained under different assumptions about $H_0$, since $\sigma_8$ in such cases characterizes the amount of structure at different physical scales. This issue arises both when comparing $\sigma_8$ values from fitting analyses of cosmological models with differing $H_0$ posteriors, and when contrasting constraints from galaxy clustering experiments with different priors on $H_0$. As first noted by Ariel G. S\'anchez in PRD 102, 123511 (2020), quantifying the growth tension using $\sigma_8$ can introduce substantial biases and couple the growth and Hubble tensions in an intricate way. To address these challenges, S\'anchez proposed an alternative parameter, $\sigma_{12}$, defined as the rms mass fluctuations at $12$ Mpc, which is independent of $h$. Although S\'anchez's work was published five years ago and other authors have since highlighted the limitations of $\sigma_8$, much of the cosmological community -- including large collaborations -- continues to rely on this parameter rather than adopting $\sigma_{12}$, seemingly due only to historical considerations. In this work, we illustrate the biases introduced by $\sigma_8$ through some clear examples, aiming to motivate the community to transition from $\sigma_8$ to $\sigma_{12}$. We show that the bias found in models with large values of $H_0$ is more prominent, artificially complicating the search for a model that can efficiently resolve the Hubble tension without exacerbating the growth tension inferred from galaxy clustering measurements. We argue that the worsening of the growth tension in these models is much less pronounced than previously thought or may even be nonexistent.