Flavor Violating Axions in the Early Universe
  Flavor violating axion couplings can be in action before recombination, and
they can fill the early universe with an additional radiation component.
Working within a model-independent framework, we consider an effective field
theory for the axion field and quantify axion production. Current cosmological
data exclude already a fraction of the available parameter space, and the
bounds will improve significantly with future CMB-S4 surveys. Remarkably, we
find that future cosmological bounds will be comparable or even stronger than
the ones obtained in our terrestrial laboratories.
