Staggered multi-field inflation

Abstract

We investigate multi-field inflationary scenarios with fields that drop out of the model in astaggered fashion. This feature is natural in certain multi-field inflationary setups withinstring theory; for instance, it can manifest itself when fields are related to tachyonsthat condense, or inter-brane distances that become meaningless when branesannihilate. Considering a separable potential, and promoting the number of fields to asmooth time dependent function, we derive the formalism to deal with thesemodels at the background and perturbed level, providing general expressionsfor the scalar spectral index and the running. We recover known results of e.g. adynamically relaxing cosmological constant in the appropriate limits. We furthershow that isocurvature perturbations are suppressed during inflation, and soperturbations are adiabatic and nearly Gaussian. The resulting setup might beinterpreted as a novel type of warm inflation, readily implemented within stringtheory and without many of the shortcomings associated with warm inflation.To exemplify the applicability of the formalism we consider three concretemodels: assisted inflation with exponential potentials as a simple toy model (agraceful exit becomes possible), inflation from multiple tachyons (a constantdecay rate of the number of fields and negligible slow roll contributions turnsout to be in good agreement with observations) and inflation from multipleM5-braneswithin M-theory (a narrow stacking of branes yields a consistent scenario).