  Future CMB experiments have the potential to probe the density of
relativistic species at the sub-percent level. Sensitivity at this level allows
light thermal relics to be detected up to arbitrarily high decoupling
temperatures. Conversely, the absence of a detection would require extra light
species never to have been in equilibrium with the Standard Model. In this
paper, we exploit this feature to demonstrate the sensitivity of future
cosmological observations to the couplings of axions to all of the Standard
Model degrees of freedom. In many cases, the constraints achievable from
cosmology will surpass existing bounds from laboratory experiments and
astrophysical observations by orders of magnitude.
