Evading the BBN bound with a soft stiff period

TL;DR

This paper explores a modified hybrid inflation model that introduces a soft stiff period to enhance primordial gravitational wave signals without violating Big Bang Nucleosynthesis constraints, making future GW detection feasible.

Contribution

It proposes a new inflationary scenario where the stiff period is softened, allowing observable gravitational waves while maintaining BBN consistency.

Findings

The model produces a distinctive gravitational wave spectrum.

The stiff period can be softened to avoid BBN constraints.

Potential observability of primordial GWs in upcoming experiments.

Abstract

Cosmic inflation is the leading theory to explain early Universe history and structure formation. Non-oscillatory inflation is a class of models which can naturally introduce a post-inflationary stiff period of the Universe's evolution which boosts the signal of primordial gravitational waves (GWs), making it possible to observe them in forthcoming GW experiments. However, this pushes the GW energy density high enough to destabilise the process of Big Bang Nucleosynthesis (BBN). This problem can be overcome by "softening" the stiff period, so that the field is gradually tending towards freefall from a frozen start. Here, we consider a modified hybrid inflation model where the stiff period is driven by the waterfall field, allowing the barotropic parameter of the Universe to vary, so that it does not violate the BBN constraint but produces a characteristic gravitational wave spectrum soon to be observable.