Assisted inflation

TL;DR

This paper shows that in multifield inflation with exponential potentials, the fields naturally evolve toward a scaling solution where their energy densities stay in fixed ratios, enabling inflation even when individual potentials are too steep. The expansion rate is governed by the sum of the slopes, p̃ = ∑ p_i (or mp for identical slopes), and the scaling solution is a late-time attractor. Density perturbations in this setup reproduce the standard single-field form with an effective p̃, yielding a spectral index n closer to 1 as more fields participate. The findings suggest a robust mechanism by which multiple fields can enhance inflationary dynamics and produce near-scale-invariant spectra, with potential relevance for supergravity-inspired models.

Abstract

In inflationary scenarios with more than one scalar field, inflation may proceed even if each of the individual fields has a potential too steep for that field to sustain inflation on its own. We show that scalar fields with exponential potentials evolve so as to act cooperatively to assist inflation, by finding solutions in which the energy densities of the different scalar fields evolve in fixed proportion. Such scaling solutions exist for an arbitrary number of scalar fields, with different slopes for the exponential potentials, and we show that these solutions are the unique late-time attractors for the evolution. We determine the density perturbation spectrum produced by such a period of inflation, and show that with multiple scalar fields the spectrum is closer to the scale-invariant than the spectrum that any of the fields would generate individually.