Multiple Inflation, Cosmic String Networks and the String Landscape

TL;DR

The paper investigates a two-step inflation scenario motivated by the string landscape, where a brief high-scale phase is followed by a longer low-scale phase. It derives general constraints tying the required numbers of e-foldings in both epochs to the inter-inflationary equation of state and energy scales, showing that a string-dominated interlude can reduce the high-scale inflation needed. A simple toy multi-field model demonstrates how multiple inflationary bouts can arise and interact with subsequent matter-dominated eras, and the authors discuss the possible emergence of a cosmic-string network after brane-antibrane inflation as a natural interlude. Overall, the work shows that two-step inflation can be cosmologically viable within string theory and yields distinct observational implications, including mode reprocessing and potential tensor signals, depending on the interlude's dynamics.

Abstract

Motivated by the string landscape we examine scenarios for which inflation is a two-step process, with a comparatively short inflationary epoch near the string scale and a longer period at a much lower energy (like the TeV scale). We quantify the number of $e$-foldings of inflation which are required to yield successful inflation within this picture. The constraints are very sensitive to the equation of state during the epoch between the two inflationary periods, as the extra-horizon modes can come back inside the horizon and become reprocessed. We find that the number of $e$-foldings during the first inflationary epoch can be as small as 12, but only if the inter-inflationary period is dominated by a network of cosmic strings (such as might be produced if the initial inflationary period is due to the brane-antibrane mechanism). In this case a further 20 $e$-foldings of inflation would be required at lower energies to solve the late universe's flatness and horizon problems.