Axiverse cosmology and the energy scale of inflation
  Ultra-light axions ($m_a\lesssim 10^{-18}$eV), motivated by string theory,
can be a powerful probe of the energy scale of inflation. In contrast to
heavier axions the isocurvature modes in the ultra-light axions can coexist
with observable gravitational waves. Here it is shown that large scale
structure constraints severely limit the parameter space for axion mass,
density fraction and isocurvature amplitude. It is also shown that radically
different CMB observables for the ultra-light axion isocurvature mode
additionally reduce this space. The results of a new, accurate and efficient
method to calculate this isocurvature power spectrum are presented, and can be
used to constrain ultra-light axions and inflation.
