Starting inflation in asymptotically flat spacetimes

A key question in early universe cosmology is whether inflation can be successfully seeded by a generic, localised fluctuation in the inflaton field that probes the inflationary part of the potential. Past simulations have mainly considered periodic spacetimes representing either a closed universe of a specific size or a typical patch of a larger one, and as a result have needed to impose restrictive conditions on the extrinsic and intrinsic curvatures, which are arguably not generic. In this work we consider initial fluctuations in asymptotically flat spacetimes, allowing more general profiles of the intrinsic and extrinsic curvature. Our findings confirm and generalise the result of Goldwirth and Piran that a fluctuation with proper size several times the inflationary scale $(G\rho_{\rm infl})^{-1/2}$ is required for successful inflation. We also discuss inherent restrictions on the initial data, and how imposing a periodic length close to the inflationary scale may bias results.