Natural inflation with pseudo Nambu-Goldstone bosons

We show that a pseudo Nambu-Goldstone boson, with a potential of the form V(φ)=${\mathrm{\Lambda }}^4$[1±cos(φ/f)], can naturally give rise to an epoch of inflation in the early Universe. Successful inflation can be achieved if f∼${\mathit{m}}_{\mathrm{Pl}}$ and Λ∼${\mathit{m}}_{\mathrm{GUT}}$. Such mass scales arise in particle-physics models with a gauge group that becomes strongly interacting at a scale ∼Λ, e.g., as can happen in superstring theories. The density fluctuation spectrum is non-scale-invariant, with extra power on large length scales.