Inflationary Axion Cosmology Beyond Our Horizon
  In theories of axion dark matter with large axion decay constant, temperature variations in the CMB are extremely sensitive to perturbations in the initial axion field, allowing one to place a lower bound on the total amount of inflation. The most stringent bound comes from axion strings, which for axion decay constant f=10^17 GeV would currently be observable at a distance of 6 x 10^16 light-years, nearly ten million times as far away as our horizon. 
David B. Kaplan, Ann E. Nelson