On natural inflation

We reexamine the model of natural inflation, in which the inflaton potential is flat due to shift symmetries. The original version of the model, where the inflaton is a pseudo--Nambu-Goldstone boson with potential of the form V(φ)=Λ⁴[1±cos(φ/f)], is studied in light of recent data. We find that the model is alive and well. Successful inflation as well as data from the Wilkinson Microwave Anisotropy Probe require f>0.6mPl (where mPl=1.22×10¹⁹ GeV) and Λ~mGUT (where mGUT~10¹⁶ GeV), scales which can be accommodated in particle physics models. The detectability of tensor modes from natural inflation in upcoming microwave background experiments is discussed. We find that natural inflation predicts a tensor/scalar ratio within reach of future observations.