Brane-antibrane inflation in orbifold and orientifold models

We analyse the cosmological implications of brane-antibrane systems instring-theoretic orbifold and orientifold models. In a class of realisticmodels, consistency conditions require branes and antibranes to be stuck atdifferent fixed points, and so their mutual attraction generates a potentialfor one of the radii of the underlying torus or the 4D string dilaton. Assumingthat all other moduli have been fixed by string effects, we find that thispotential leads naturally to a period of cosmic inflation with the radion ordilaton field as the inflaton. The slow-roll conditions are satisfied moregenerically than if the branes were free to move within the space. Theappearance of tachyon fields at certain points in moduli space indicates theonset of phase transitions to different non-BPS brane systems, providing waysof ending inflation and reheating the corresponding observable brane universe.In each case we find relations between the inflationary parameters and thestring scale to get the correct spectrum of density perturbations. In someexamples the small numbers required as inputs are no smaller than 0.01, and arethe same small quantities which are required to explain the gauge hierarchy.Comment: 30 pages, 2 figures. Substantial changes on version 1. New cosmological scenarios proposed including the dilaton as the inflaton. Main conclusions unchange