Nearly Scale-Invariant Spectrum of Adiabatic Fluctuations May be from a Very Slowly Expanding Phase of the Universe

TL;DR

This paper proposes a novel cosmological model involving a very slowly expanding universe with phantom matter, which can produce a nearly scale-invariant spectrum of adiabatic fluctuations with a blue tilt, differing from traditional inflationary models.

Contribution

It introduces a new scenario with slow expansion and increasing Hubble parameter that can generate nearly scale-invariant perturbations, expanding the possibilities beyond inflation and ekpyrotic models.

Findings

Produces nearly scale-invariant spectrum with blue tilt

Offers an alternative to inflation and ekpyrotic/cyclic scenarios

Highlights uncertainties in perturbation propagation during transition

Abstract

In this paper we construct an expanding phase with phantom matter, in which the scale factor expands very slowly but the Hubble parameter increases gradually, and assume that this expanding phase could be matched to our late observational cosmology by the proper mechanism. We obtain the nearly scale-invariant spectrum of adiabatic fluctuations in this scenario, different from the simplest inflation and usual ekpyrotic/cyclic scenario, the tilt of nearly scale-invariant spectrum in this scenario is blue. Although there exists an uncertainty surrounding the way in which the perturbations propagate through the transition in our scenario, which is dependent on the detail of possible "bounce" physics, compared with inflation and ekpyrotic/cyclic scenario, our work may provide another feasible cosmological scenario generating the nearly scale-invariant perturbation spectrum.