Elsevier

Astroparticle Physics

Volume 131, September 2021, 102604
Astroparticle Physics

Snowmass 2021 - Letter of Interest
Cosmology intertwined III: fσ8 and S8

https://doi.org/10.1016/j.astropartphys.2021.102604Get rights and content

Abstract

The standard Λ Cold Dark Matter cosmological model provides a wonderful fit to current cosmological data, but a few statistically significant tensions and anomalies were found in the latest data analyses. While these anomalies could be due to the presence of systematic errors in the experiments, they could also indicate the need for new physics beyond the standard model. In this Letter of Interest we focus on the tension between Planck data and weak lensing measurements and redshift surveys, in the value of the matter energy density Ωm and the amplitude σ8 (or the growth rate fσ8) of cosmic structure. We list a few promising models for solving this tension, and discuss the importance of trying to fit multiple cosmological datasets with complete physical models, rather than fitting individual datasets with a few handpicked theoretical parameters.

Section snippets

The S8 tension

The standard Λ Cold Dark Matter (ΛCDM) cosmological model provides an excellent fit to current cosmological data. However, some statistically-significant tensions in cosmological parameter estimates emerged between the Planck experiment, measuring the Cosmic Microwave Background (CMB) anisotropies, and other low-redshift cosmological probes. In addition to the long standing Hubble constant H0 disagreement, a tension of the Planck data with weak lensing measurements and redshift surveys has been

Conjoined history problem

The H0 disagreement is correlated with the σ8 problem. Indeed the solutions proposed to alleviate the former, are exacerbating the S8 problem and, consequently, the σ8 tension between the CMB and the more direct measurements, such as galaxy clusters using the Sunyaev-Zel’dovich effect [26], [27], [28], i.e. measuring the number of clusters of a certain mass M over a range of redshift. For example, late time transitions that predict a higher H0 value, if they match the CMB data, also prefer a

Solutions

There are many papers investigating the S8 tension [22], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], but the solutions proposed are not enough to make all the cosmological data mutually consistent [69], [70], [71]. We can distinguish the following categories of solutions:

  • Axion monodromy inflation [57].

  • Extended parameter spaces [22], [38], [39], [41]

Future

In the near future, we expect percent-level measurements of the expansion and growth history from a large range of experiments, i.e. using maps of the Universe obtained by the Euclid satellite, measuring the peculiar motions of galaxies using Type Ia supernovae from LSST [84], [85], mapping out RSD with DESI and 4MOST, or using voids [36]. An important role will be played by the SKA telescopes performing BAO surveys and measuring weak gravitational lensing using 21 cm intensity mapping [86],

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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