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We analyze the onset of inflation for a simple single-field model when thesystem begins with significant inhomogeneities on length-scales shorter thanthe initial Hubble radius. We incorporate certain nonlinear interactions amongthe coupled degrees of freedom by using the nonperturbative Hartreeapproximation. Consistent with recent, more computationally intensivenumerical-relativity studies, we find inflation to be robust for large-fieldmodels, even when the system begins with significant structure on sub-Hubblescales. We consider the space of initial conditions $(\varphi (t_0),\dot{\varphi} (t_0))$, where $\varphi$ is the vacuum expectation value of thequantized field that drives inflation. Although some regions of $(\varphi(t_0), \dot{\varphi} (t_0))$ that would have yielded sufficient inflation inthe absence of inhomogeneities fail to do so when backreaction frominhomogeneities is incorporated, an equal volume of such regions succeeds inproducing sufficient inflation which did not do so in the absence ofinhomogeneities. For large-field models, in other words, the total volume ofthe space of initial conditions $(\varphi (t_0), \dot{\varphi} (t_0))$ thatyields sufficient inflation is {\it conserved} when we incorporate nonlinearbackreaction from inhomogeneities, compared to the case in whichinhomogeneities are neglected.

A Bumpy Start to a Smooth Ride: Onset of Inflation amid Backreaction from Inhomogeneities