Quintessential Cosmological Tensions

TL;DR

This paper investigates whether a quintessence model with the Albrecht-Skordis potential can simultaneously alleviate the H_0 and σ_8 cosmological tensions by modifying early universe dynamics, but finds it less favored than ΛCDM based on data fit.

Contribution

It introduces a single-parameter quintessence model that can partially address key cosmological tensions, highlighting limitations in fitting all observational data.

Findings

H_0 increased by ~1σ compared to ΛCDM

σ_8 decreased by ~1σ compared to ΛCDM

Model disfavored by Planck and BAO data with Δχ² ≈ +6

Abstract

Several cosmological tensions have emerged in light of recent data, most notably in the inferences of the parameters $H_0$ and $\sigma_8$. We explore the possibility of alleviating both these tensions {\it simultaneously} by means of the Albrecht-Skordis ``quintessence'' potential. The field can reduce the size of the sound horizon $r_s^*$ while concurrently suppressing the power in matter density fluctuations before it comes to dominate the energy density budget today. Interestingly, this rich set of dynamics is governed entirely by one free parameter that is of $\mathcal{O}(10)$ in Planck units. We find that the inferred value of $H_0$ can be increased, while that of $\sigma_8$ can be decreased, both by $\approx 1\sigma$ compared to the $\Lambda$CDM case. However, ultimately the model is disfavored by Planck and BAO data alone, compared to the standard $\Lambda$CDM model, with a $\Delta \chi^2 \approx +6$. When including large scale structure and supernova data $\Delta \chi^2 \approx +1$. We note that historically much attention has been focused on preserving the three angular scales $\theta_D$, $\theta_{EQ}$, and $\theta_s^*$ to their $\Lambda$CDM values. Our work presents an example of how, while doing so indeed maintains a relatively good fit to the CMB data for an increased number of ultra-relativistic species, it is a-priori insufficient in maintaining such a fit in more general model spaces.