Brane inflation is attractive

We study the phase space of initial conditions for brane inflation, and find that including the effects of the Dirac-Born-Infeld (DBI) kinetic term dramatically improves previous estimates on the amount of fine-tuning of initial conditions necessary for inflation, even for models dominated by slow roll. Two effects turn out to be important for the phase space analysis: restrictions on the total available phase space due to UV effects in brane inflation, and the extension of the inflationary attractor to the DBI inflationary regime. We compare the amount of initial conditions fine-tuning required for a brane-inflation model and its standard field theory counterpart and find that brane inflation decreases the required tuning by several orders of magnitude.