Illustrating the consequences of a misuse of $σ_8$ in cosmology
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| Format: | Preprint |
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2025
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| _version_ | 1866915347547815936 |
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| author | Forconi, Matteo Favale, Arianna Gómez-Valent, Adrià |
| author_facet | Forconi, Matteo Favale, Arianna Gómez-Valent, Adrià |
| contents | The parameter $σ_8$ 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 $σ_8$ values obtained under different assumptions about $H_0$, since $σ_8$ in such cases characterizes the amount of structure at different physical scales. This issue arises both when comparing $σ_8$ 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. Sánchez in PRD 102, 123511 (2020), quantifying the growth tension using $σ_8$ can introduce substantial biases and couple the growth and Hubble tensions in an intricate way. To address these challenges, Sánchez proposed an alternative parameter, $σ_{12}$, defined as the rms mass fluctuations at $12$ Mpc, which is independent of $h$. Although Sánchez's work was published five years ago and other authors have since highlighted the limitations of $σ_8$, much of the cosmological community -- including large collaborations -- continues to rely on this parameter rather than adopting $σ_{12}$, seemingly due only to historical considerations. In this work, we illustrate the biases introduced by $σ_8$ through some clear examples, aiming to motivate the community to transition from $σ_8$ to $σ_{12}$. 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2501_11571 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Illustrating the consequences of a misuse of $σ_8$ in cosmology Forconi, Matteo Favale, Arianna Gómez-Valent, Adrià Cosmology and Nongalactic Astrophysics The parameter $σ_8$ 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 $σ_8$ values obtained under different assumptions about $H_0$, since $σ_8$ in such cases characterizes the amount of structure at different physical scales. This issue arises both when comparing $σ_8$ 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. Sánchez in PRD 102, 123511 (2020), quantifying the growth tension using $σ_8$ can introduce substantial biases and couple the growth and Hubble tensions in an intricate way. To address these challenges, Sánchez proposed an alternative parameter, $σ_{12}$, defined as the rms mass fluctuations at $12$ Mpc, which is independent of $h$. Although Sánchez's work was published five years ago and other authors have since highlighted the limitations of $σ_8$, much of the cosmological community -- including large collaborations -- continues to rely on this parameter rather than adopting $σ_{12}$, seemingly due only to historical considerations. In this work, we illustrate the biases introduced by $σ_8$ through some clear examples, aiming to motivate the community to transition from $σ_8$ to $σ_{12}$. 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. |
| title | Illustrating the consequences of a misuse of $σ_8$ in cosmology |
| topic | Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2501.11571 |