Multibrane Inflation and Dynamical Flattening of the Inflaton Potential

We investigate the problem of fine tuning of the potential in the KKLMMT warped flux compactification scenario for brane-antibrane inflation in Type IIB string theory. We argue for the importance of an additional parameter psi_0 (approximated as zero by KKLMMT), namely the position of the antibrane, relative to the equilibrium position of the brane in the absence of the antibrane. We show that for a range of values of a particular combination of the Kahler modulus, warp factor, and psi_0, the inflaton potential can be sufficiently flat. We point out a novel mechanism for dynamically achieving flatness within this part of parameter space: the presence of multiple mobile branes can lead to a potential which initially has a metastable local minimum, but gradually becomes flat as some of the branes tunnel out. Eventually the local minimum disappears and the remaining branes slowly roll together, with assisted inflation further enhancing the effective flatness of the potential. With the addition of Kahler and superpotential corrections, this mechanism can completely remove the fine tuning problem of brane inflation, within large regions of parameter space. The model can be falsified if future cosmic microwave background observations confirm the hint of a large running spectral index.