Updated Constraints on Large Field Hybrid Inflation

TL;DR

This paper reevaluates large field hybrid inflation models considering recent observational data, identifying parameter regimes that align with Planck and BICEP2 results, especially where slow-roll violations influence predictions.

Contribution

It provides a detailed analysis of hybrid inflation dynamics, including slow-roll violations, and identifies parameter spaces compatible with current cosmological observations.

Findings

Certain parameter regimes match Planck and BICEP2 data.

Model predictions can degenerate with quadratic potential in some cases.

Hybrid inflation is favored over other models based on chi-squared analysis.

Abstract

We revisit the status of hybrid inflation in the light of Planck and recent BICEP2 results, taking care of possible transient violations of the slow-roll conditions as the field passes from the large field to the vacuum dominated phase. The usual regime where observable scales exit the Hubble radius in the vacuum dominated phase predicts a blue scalar spectrum, which is ruled out. But whereas assuming slow-roll one expects this regime to be generic, by solving the exact dynamics we identify the parameter space for which the small field phase is naturally avoided due to slow-roll violations at the end of the large field phase. When the number of e-folds generated at small field is negligible, the model predictions are degenerated with those of a quadratic potential. There exists also a transitory case for which the small field phase is sufficiently long to affect importantly the observable predictions. Interestingly, in this case the spectral index and the tensor to scalar ratio agree respectively with the best fit of Planck and BICEP2. This results in a \Delta \chi^2 \simeq 5.0 in favor of hybrid inflation for Planck+BICEP2 (\Delta \chi^2 \simeq 0.9 for Planck only). The last considered regime is when the critical point at which inflation ends is located in the large field phase. It is constrained to be lower than about ten times the reduced Planck mass. The analysis has been conducted with the use of Markov-Chain-Monte-Carlo bayesian method, in a reheating consistent way, and we present the posterior probability distributions for all the model parameters.