An Alternative to Inflation

TL;DR

This paper challenges the necessity of inflation for explaining the universe's large-scale structure, proposing that alternative non-inflationary models can produce similar density perturbation spectra under certain initial conditions.

Contribution

It introduces a non-inflationary framework that can replicate inflation's predictions for density perturbations without requiring slow roll conditions.

Findings

Non-inflationary models can produce the same density perturbation spectrum as inflation.

The mechanism for deviations from homogeneity operates independently of inflation.

Initial conditions of modes can lead to inflation-like structures without inflation.

Abstract

Inflationary models are generally credited with explaining the large scale homogeneity, isotropy, and flatness of our universe as well as accounting for the origin of structure (i.e., the deviations from exact homogeneity) in our universe. We argue that the explanations provided by inflation for the homogeneity, isotropy, and flatness of our universe are not satisfactory, and that a proper explanation of these features will require a much deeper understanding of the initial state of our universe. On the other hand, inflationary models are spectacularly successful in providing an explanation of the deviations from homogeneity. We point out here that the fundamental mechanism responsible for providing deviations from homogeneity -- namely, the evolutionary behavior of quantum modes with wavelength larger than the Hubble radius -- will operate whether or not inflation itself occurs. However, if inflation did not occur, one must directly confront the issue of the initial state of modes whose wavelength was larger than the Hubble radius at the time at which they were "born". Under some simple hypotheses concerning the "birth time" and initial state of these modes (but without any "fine tuning"), it is shown that non-inflationary fluid models in the extremely early universe would result in the same density perturbation spectrum and amplitude as inflationary models, although there would be no "slow roll" enhancement of the scalar modes.