More varieties of hybrid inflation

TL;DR

The paper investigates inverted hybrid inflation, a variant where the inflaton rolls away from the origin, which can produce a red-tilted spectrum with negligible tensor modes. It then develops several generalizations, including higher-order and mutated hybrid inflation forms, and derives analytic relations for the spectral index and perturbation normalization with discussion of supersymmetric realizations and end-of-inflation mechanisms. The main finding is that inverted hybrid inflation can yield $n<1$, while other variants tend to $n$ near or below 1, making the measurement of the spectral index $n$ a powerful discriminator among hybrid inflation models. The work provides a framework to connect potential shapes, end-of-inflation dynamics, and observational signatures in the CMB and large-scale structure.

Abstract

It is pointed out that hybrid inflation can be implemented with the inflaton field rolling away from the origin instead of towards it. This `inverted' hybrid inflation has a spectral index $ n < 1 $, in contrast with ordinary hybrid inflation which has $ n \gtrsim 1 $, so a measured value of $n$ substantially different from 1 would distinguish the two. Other generalisations of hybrid inflation are also considered.