Towards hybrid inflation with $n_s=1$ in light of Hubble tension and primordial gravitational waves

TL;DR

This paper proposes a mechanism within slow-roll inflation models to achieve a scale-invariant spectrum ($n_s=1$), potentially resolving the Hubble tension and reviving previously ruled-out models like chaotic inflation, with testable predictions for future CMB experiments.

Contribution

It introduces a novel way to reach $n_s=1$ during inflation via waterfall instability, connecting Hubble tension resolution with inflation model viability.

Findings

Achieves $n_s=1$ in slow-roll models through waterfall instability.

Revives chaotic $$ inflation with early dark energy.

Predicts testable B-mode polarization signals in upcoming experiments.

Abstract

Recently, it has been found that complete resolution of the Hubble tension might point to a scale-invariant Harrison-Zeldovich spectrum of primordial scalar perturbation, i.e. $n_s=1$ for $H_0\sim 73$km/s/Mpc. We show that for well-known slow-roll models, if inflation ends by a waterfall instability with respect to another field in the field space while inflaton is still at a deep slow-roll region, $n_s$ can be lifted to $n_s= 1$. A surprise of our result is that with pre-recombination early dark energy, chaotic $\phi^2$ inflation, ruled out by Planck+BICEP/Keck in standard $\Lambda$CDM, can be revived, which is now well within testable region of upcoming cosmic microwave background B-mode experiments.