The refractive index of relic gravitons

TL;DR

This paper investigates how the evolving refractive index of relic gravitational waves influences their spectrum, with implications for detection and constraints from cosmological observations.

Contribution

It introduces a model linking the refractive index evolution to the gravitational wave spectrum and discusses observational bounds and detection prospects.

Findings

Refractive index evolution produces a blue-tilted gravitational wave spectrum.

Current cosmological constraints limit the refractive index variation.

Future detectors could observe signals within the allowed parameter space.

Abstract

The dynamical evolution of the refractive index of the tensor modes of the geometry produces a specific class of power spectra characterized by a blue (i.e. slightly increasing) slope which is directly determined by the competition of the slow-roll parameter and of the rate of variation of the refractive index. Throughout the conventional stages of the inflationary and post-inflationary evolution, the microwave background anisotropies measurements, the pulsar timing limits and the big-bang nucleosythesis constraints set stringent bounds on the refractive index and on its rate of variation. Within the physically allowed region of the parameter space the cosmic background of relic gravitons leads to a potentially large signal for the ground based detectors (in their advanced version) and for the proposed space-borne interferometers. Conversely, the lack of direct detection of the signal will set a qualitatively new bound on the dynamical variation of the refractive index.