Avoiding the blue spectrum and the fine-tuning of initial conditions in hybrid inflation

TL;DR

This paper investigates hybrid inflation by studying exact two-field dynamics, showing that successful inflation does not require excessive fine-tuning and can naturally produce a red-tilted spectrum across various models.

Contribution

It demonstrates that successful hybrid inflation is more robust than previously thought, with broader initial conditions and consistent spectral tilt across multiple models.

Findings

Approximately 15% of initial conditions lead to successful inflation within Planckian bounds.

Successful inflation can occur outside the traditional inflationary valley in continuous patterns.

Hybrid inflation models can produce a red-tilted spectrum due to slow-roll violations.

Abstract

Hybrid inflation faces two well-known problems: the blue spectrum of the non-supersymmetric version of the model and the fine-tuning of the initial conditions of the fields leading to sufficient inflation to account for the standard cosmological problems. They are investigated by studying the exact two-fields dynamics instead of assuming slow-roll. When the field values are restricted to be less than the reduced Planck mass, a non-negligible part of the initial condition space (around 15% depending on potential parameters) leads to successful inflation. Most of it is located outside the usual inflationary valley and organized in continuous patterns instead of being isolated as previously found. Their existence is explained and their properties are studied. This shows that no excessive fine-tuning is required for successful hybrid inflation. Moreover, by extending the initial condition space to planckian-like or super-planckian values, inflation becomes generically sufficiently long and can produce a red-tilted scalar power spectrum due to slow-roll violations. The robustness of these properties is confirmed by conducting our analysis on three other models of hybrid-type inflation in various framework: "smooth" and "shifted" inflation in SUSY and SUGRA, and "radion assisted" gauge inflation. A high percentage of successful inflation for smooth hybrid inflation (up to 80%) is observed.