Coupled quintessence with a $\Lambda$CDM background: removing the $\sigma_8$ tension

TL;DR

This paper demonstrates that coupled quintessence with a ΛCDM background can resolve the σ8 tension by balancing scalar force effects and dark matter abundance, fitting current data and predicting tight future constraints.

Contribution

It shows that coupled quintessence models can simultaneously fit CMB and large-scale structure data, addressing the σ8 tension with specific parameter estimates.

Findings

Coupled quintessence reduces the σ8 tension in ΛCDM.

Best-fit coupling magnitude is around 0.079.

Future surveys will tightly constrain the coupling and σ8 values.

Abstract

A well-known problem of the $\Lambda$CDM model is the tension between the relatively high level of clustering, as quantified by the parameter $\sigma_8$, found in cosmic microwave background experiments and the smaller one obtained from large-scale observations in the late Universe. In this paper we show that coupled quintessence, i.e. a single dark energy scalar field conformally coupled to dark matter through a constant coupling, can solve this problem if the background is taken to be identical to the $\Lambda$CDM one. We show that two competing effects arise. On one hand, the additional scalar force is attractive, and is therefore expected to increase the clustering. On the other, in order to obtain the same background as $\Lambda$CDM, coupled quintessence must have a smaller amount of dark matter near the present epoch. We show that the second effect is dominating today and leads to an overall slower growth. Comparing to redshift distortion data, we find that coupled quintessence with $\Lambda$CDM background solves the tension between early and late clustering. We find for the coupling $\beta$ and for $\sigma_8$ the best fit values $|\beta| = 0.079^{+ 0.059}_{- 0.067}$ and $\sigma_8 = 0.818^{+0.115}_{-0.088}$. These values also fit the lensing data from the KiDS-450 survey. We also estimate that the future missions SKA and Euclid will constrain $\beta$ with an error of $\pm\, 1.5\times10^{-3}$ and for $\sigma_8$ of $\pm \,1.8\times10^{-3}$ at $1\sigma$ level.