
pmid: 10016327
We investigate the issue of initial conditions for natural inflation in a homogeneous universe. Unlike many inflationary models, the pseudo Nambu-Goldstone boson nature of the inflaton field provides a natural measure for the phase space of initial conditions. We study the effects of the inflaton kinetic term numerically, and show that it shifts the range of initial field values which lead to successful inflation without altering the size of that range. The fraction of phase space in the successful range is determined by the spontaneous symmetry-breaking scale f and is 0.7, 0.2, and 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ for f=3${\mathit{M}}_{\mathrm{Pl}}$, ${\mathit{M}}_{\mathrm{Pl}}$, and ${\mathit{M}}_{\mathrm{Pl}}$/2 respectively. Natural inflation becomes similar to chaotic inflation for values of f\ensuremath{\gtrsim}${\mathit{M}}_{\mathrm{Pl}}$, and for f\ensuremath{\lesssim}${\mathit{M}}_{\mathrm{Pl}}$ it is more akin to new inflation.
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