Scale invariance without inflation?

C Armend Riz-Pic N,Eugene A Lim

doi:10.1088/1475-7516/2003/12/002

Abstract

We propose a new alternative mechanism to seed a scale invariant spectrum of primordial density perturbations that does not rely on inflation. In our scenario, a perfect fluid dominates the early stages of an expanding, non-inflating universe. Because the speed of sound of the fluid decays, perturbations are left frozen behind the sound horizon, with a spectral index that depends on the fluid equation of state. We explore here a toy model that realizes this idea. Although the model can explain an adiabatic, Gaussian, scale invariant spectrum of primordial perturbations, it turns out that in its simplest form it cannot account for the observed amplitude of the primordial density perturbations.

Highlights

During the last decade, several experiments have probed the nature of the primeval perturbations that gave rise to the anisotropies observed in the universe [1]
In this paper we propose an alternative seeding mechanism in an expanding universe that does not require inflation
The particular model we discuss can successfully seed a scale invariant spectrum of density perturbations, it turns out that it cannot account for the observed amplitude

Summary

INTRODUCTION

Several experiments have probed the nature of the primeval perturbations that gave rise to the anisotropies observed in the universe [1] The results of these measurements are consistent with an adiabatic, Gaussian and nearly scale invariant primordial spectrum of perturbations, as predicted by the simplest models of inflation [2]. It is fair to say though, that the only presently known, widely accepted way to seed a scale invariant spectrum of perturbations in an expanding universe requires a stage of de Sitter inflation. The particular model we discuss can successfully seed a scale invariant spectrum of density perturbations, it turns out that it cannot account for the observed amplitude. The only way to escape this conclusion is to assume that between seeding and observation the amplitude of the seeded spectrum is boosted to the observed value by a different mechanism

FORMALISM

DECAYING SOUND SPEED

A SIMPLE MODEL

Observational constraints

A Microscopic Description

CONCLUSIONS