Let Effective Field Theory of Inflation flow: stochastic generation of models with red/blue tensor tilt

TL;DR

This paper extends the Flow Equations method to Effective Field Theory of inflation, analyzing a large set of models to predict tensor tilt and their compatibility with observational bounds, highlighting the prevalence of blue or red spectra.

Contribution

It introduces a numerical framework for studying inflationary models within EFT, exploring tensor spectral tilt predictions and their observational implications.

Findings

Majority of models with ta > 0 are below current tensor-to-scalar ratio bounds.

Most EFT models with ta > 0 have red tilt, while NEC-violating models tend to have blue tilt.

Blue tensor tilt models could enhance gravitational wave detection prospects.

Abstract

We extend the method of Flow Equations to the Effective Field Theory framework of inflation, in order to investigate the observable predictions of a very broad class of inflationary models. Focusing our attention on the gravitational-wave sector, we derive a general expression for the consistency relation for effective models and provide a numerical implementation which allows to study how the generated models populate the $(r,nt)$ plane. We analyse $5 \times 10^{4}$ realizations of inflationary scenarios that respect the Null-Energy Condition ($\epsilon > 0$) and $5 \times 10^{4}$ realizations that violate it ($\epsilon > 0$). In both cases, 90% of the viable models are below the most recent upper bound on the tensor-to-scalar ratio from Planck and BICEP2/Keck Array BK15 data: $r_{0.002} < 0.056$ at 95 % CL. We find that general EFT inflationary models with $\epsilon > 0$ are typically characterized by $n_{t} < 0$, whereas the vast majority of NEC-violating models presents a blue-tilted spectrum ($n_{t} > 0$). Since a blue tensor spectral index implies more power on small scales, this result is of considerable interest in view of a possible direct detection of the primordial gravitational-wave background.