Brane-Antibrane Inflation in Orbifold and Orientifold Models

TL;DR

This work extends brane–antibrane inflation to realistic string constructions based on orbifold and orientifold compactifications, showing that a light 4D modulus (radion or dilaton) can naturally act as the inflaton. The authors derive the 4D effective action, including the potential from brane tensions and inter-brane interactions, and demonstrate that slow-roll inflation is generically achievable with the modulus fields. Inflation ends via tachyonic phase transitions at the string scale, providing a hybrid-like exit and a reheating channel tied to brane dynamics. Importantly, the required small parameters emerge from underlying brane physics without ad hoc tuning, and the scenario connects inflationary observables to the string scale in a way compatible with intermediate-scale models and the gauge hierarchy.

Abstract

We analyse the cosmological implications of brane-antibrane systems in string-theoretic orbifold and orientifold models. In a class of realistic models, consistency conditions require branes and antibranes to be stuck at different fixed points, and so their mutual attraction generates a potential for one of the radii of the underlying torus or the 4D string dilaton. Assuming that all other moduli have been fixed by string effects, we find that this potential leads naturally to a period of cosmic inflation with the radion or dilaton field as the inflaton. The slow-roll conditions are satisfied more generically than if the branes were free to move within the space. The appearance of tachyon fields at certain points in moduli space indicates the onset of phase transitions to different non-BPS brane systems, providing ways of ending inflation and reheating the corresponding observable brane universe. In each case we find relations between the inflationary parameters and the string scale to get the correct spectrum of density perturbations. In some examples the small numbers required as inputs are no smaller than 0.01, and are the same small quantities which are required to explain the gauge hierarchy.