Dynamical dark energy from an ultralight axion

Understand

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 $\Lambda < 0$ is preferred in this model.