Propagating speed of primordial gravitational waves and inflation

TL;DR

This paper explores how a decreasing speed of primordial gravitational waves during inflation can produce a strongly blue-tilted spectrum, potentially enhancing detectability in current and future gravitational wave observatories.

Contribution

It introduces a scenario where the GW propagation speed diminishes during inflation, resulting in a blue spectrum without ghost instability, and links it to superinflation via disformal duality.

Findings

The primordial GW spectrum can be strongly blue with tilt 0<n_T≤1.

Such a scenario can boost stochastic GW background signals at LIGO/Virgo frequencies.

The model avoids ghost instability despite the blue tilt.

Abstract

We show that if the propagating speed of gravitational waves (GWs) gradually diminishes during inflation, the power spectrum of primordial GWs will be strongly blue, while that of the primordial scalar perturbation may be unaffected. We also illustrate that such a scenario is actually a disformal dual to the superinflation, but it does not have the ghost instability. The blue tilt obtained is $0<n_T\lesssim 1$, which may significantly boost the stochastic GWs background at the frequency band of Advanced LIGO/Virgo, as well as the space-based detectors.