Waterfall stiff period can generate observable primordial gravitational waves

TL;DR

This paper presents a toy-model where a stiff period following hybrid inflation generates a peak in primordial gravitational waves, potentially observable by future gravitational wave detectors like LISA and Advanced LIGO.

Contribution

It introduces a novel hybrid inflation scenario with a pole in the waterfall field's kinetic term, leading to a stiff period that enhances gravitational wave signals.

Findings

A stiff period with w≈1/2 occurs after hybrid inflation.

A peak in primordial gravitational wave spectrum is predicted.

The gravitational wave signal is detectable by LISA and Advanced LIGO.

Abstract

A toy-model is studied, which considers two flat directions meeting at an enhanced symmetry point such that they realise the usual hybrid inflation mechanism. The kinetic term of the waterfall field features a pole at its Planckian vacuum expectation value (VEV), as with $\alpha$-attractors. Consequently, after the phase transition which terminates hybrid inflation, the waterfall field never rolls to its VEV. Instead, it drives a stiff period, where the barotropic parameter of the Universe $w\approx 1/2$ results in a peak in the spectrum of primordial gravitational waves, which will be observable by the forthcoming LISA mission as well as by Advanced LIGO.