Dynamical dark energy from an ultralight axion

Recently the Dark Energy Survey (DES) Collaboration presented evidence that the equation of state $w$ of the dark energy is varying, or $w\simeq -0.948$ if it is constant. In either case, the dark energy cannot be due to a cosmological constant alone. Here we study an ultralight axion (or axion-like particle) with mass $m_{\phi }\simeq 2\times {10}^{-33}$ eV that has properties that can explain the new $w$ measurement. In particular, $w\ge -1$ and a negative cosmological constant $\mathrm{\Lambda }<0$ is preferred in this model. We also present a simple formula for $w$ for the model to ease data fitting.