Cosmological tests of an axiverse-inspired quintessence field
  Inspired by the string axiverse idea, it has been suggested that the recent
transition from decelerated to accelerated cosmic expansion is driven by an
axion-like quintessence field with a sub-Planckian decay constant. The scenario
requires that the axion field be rather near the maximum of its potential, but
is less finely tuned than other explanations of cosmic acceleration. The model
is parametrized by an axion decay constant $f$, the axion mass $m$, and an
initial misalignment angle $|\theta_i|$ which is close to $\pi$. In order to
determine the $m$ and $\theta_{i}$ values consistent with observations, these
parameters are mapped onto observables: the Hubble parameter $H(z)$ at and
angular diameter distance $d_{A}(z)$ to redshift $z= 0.57$, as well as the
angular sound horizon of the cosmic microwave background (CMB). Measurements of
the baryon acoustic oscillation (BAO) scale at $z\simeq 0.57$ by the BOSS
survey and Planck measurements of CMB temperature anisotropies are then used to
probe the $\left\{m,f,\theta_i\right\}$ parameter space. With current data, CMB
constraints are the most powerful, allowing a fraction of only $\sim 0.2$ of
the parameter-space volume. Measurements of the BAO scale made using the
SPHEREx or SKA experiments could go further, observationally distinguishing all
but $\sim 10^{-2}$ or $\sim 10^{-5}$ of the parameter-space volume (allowed by
simple priors) from the $\Lambda$CDM model.
