Propagating speed of primordial gravitational waves and inflation

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 1, which may significantly boost the stochastic GWs background at the frequency band of Advanced LIGO/Virgo, as well as the space-based detectors.

The propagating speed of relic gravitational waves and their refractive index during inflation

“…tilted towards high frequencies. After the analysis of [13] the same idea has been pursued in two similar papers [14,15]. The first goal of the present analysis is a specific discussion of the relation between the approaches of Refs.…”

“…The first goal of the present analysis is a specific discussion of the relation between the approaches of Refs. [13][14][15] which ultimately coincide since they are related by a conformal rescaling involving the refractive index. We shall then explore a more general approach encompassing the ones discussed previously and analyze the actual phenomenological constraints together with the associated detectability prospects.…”

“…An effective index of refraction may also arise in modified theories of gravity violating the equivalence principle [13][14][15]. In the model-independent perspective pursued in Ref.…”

“…In the model-independent perspective pursued in Ref. [13] and subsequently followed in [14,15] (see, in particular, the second paper) the pivotal parameter of the discussion will therefore be the rate of variation of the refractive index in units of the Hubble rate, conventionally denoted by α and defined by: α = n Ḣ n , = −Ḣ H 2 …”

Realization of blue spectrum in generalized Galileon super-inflation models

“…Although the recent observational data are unable to determine the spectrum of tensor perturbation in CMB polarization, the possibility of being blue-tilted for inflationary perturbations is corresponding to the negative values of ǫ. This condition implies violation of the null energy condition that is concluded fromḢ > 0 [6,7,8].…”