Updated cosmological constraints on axion dark energy with DESI

TL;DR

This work constrains an axion dark-energy model using DESI DR1 alongside CMB and SN data, deriving the background evolution in a three-variable dynamical system and performing a full Bayesian analysis to estimate the axion parameters $m_a$ and $f_a$. The authors find tight, Bayesian-constrained values around $\log(m_a c^2/\mathrm{eV}) \simeq -32.60 \pm 0.12$ and $\log(f_a/M_{Pl}) \simeq -0.28 \pm 0.19$, with the present-day equation-of-state parameters $w_0$ and $w'_0$ indicating dynamical dark energy that is somewhat preferred over a cosmological constant but not decisively favored over $\Lambda$ in Bayesian terms. Closed contours in the physical $(m_a,f_a)$ space are reported for the first time, and while the data marginally favor dynamical dark energy over $\Lambda$, the $w_0w_a$ parametrization generally provides the strongest Bayesian support. The results underscore the axion dark-energy scenario as a well-motivated model whose parameters may be better constrained with future DESI data, potentially improving discrimination between dynamical dark energy scenarios.

Abstract

We present updated constraints on the parameters of an axion dark energy model, for which we took into account the properties of its characteristic potential and its full cosmological evolution. We show that the values of the axion parameters appear sufficiently constrained by the data, including the latest DESI DR1, and are consistent with the theoretical expectations of a field mass $m_a$ in the ultralight regime $\log (m_a c^2/~\mathrm{eV}) \simeq -32.60~\pm~0.12$, and an effective energy scale $f_a$ close to the reduced Planck energy $\log (f_a/~ M_\mathrm{Pl}) \simeq -0.28~\pm~0.19$. Our results also support the idea of dynamical dark energy, although Bayesian evidence still favors the phenomenological dark energy model $w_0w_a$ over the axion dark energy, with the Bayes factor indicating moderate and weak strength of the evidence, respectively, when the models are compared to the cosmological constant $Λ$. However, the results suggest that axion dark energy remains a well-motivated model and its parameters may be better constrained if the upcoming DESI data show further evidence for dynamical dark energy.

Figures (1)

• Figure 1: Left: 95% credible regions of the present values of the equation of state $w_0$ and its logarithmic derivative $w^\prime_0$. The solid contours correspond to the axion field, while the line contours correspond to the $w_0w_a$ model. The colors indicate the different datasets described in the text. Right: 95% CL of the axion parameters $m_a$ and $f_a$, both on logarithmic scale and with the same colors and datasets. Also shown on the background in both panels are the prior distributions of the corresponding parameters.