The slow expansion with nonminimal derivative coupling and its conformal dual

TL;DR

This paper proposes a model where primordial gravitational waves with a scale-invariant spectrum can originate from a nearly Minkowski space with asymptotic-past free gravity, using nonminimal derivative coupling.

Contribution

It introduces a novel slowly expanding background model with nonminimal derivative coupling that is conformally dual to matter contraction, avoiding anisotropy issues.

Findings

Primordial gravitational waves can emerge from a nearly Minkowski space.

The model's tensor perturbations are dominated by growing modes.

The background is conformally dual to matter contraction without anisotropy problems.

Abstract

We show that the primordial gravitational wave with scale-invariant spectrum might emerge from a nearly Minkowski space, in which the gravity is asymptotic-past free. We illustrate it with a model, in which the derivative of background scalar field nonminimally couples to gravity. We also show that since here the tensor perturbation is dominated by its growing mode, mathematically our slowly expanding background is conformally dual to the matter contraction, but there is no the anisotropy problem.