Beyond Inflation: A Cyclic Universe Scenario

TL;DR

The paper proposes a cyclic universe model that explains the universe's homogeneity and perturbations without high energy inflation, addressing key issues of the inflationary paradigm by using a long-lived attractor state.

Contribution

It introduces a cyclic cosmology that reproduces inflation's successes without requiring high energy inflation, and discusses progress in understanding the passage through the cosmic singularity.

Findings

A cyclic universe can generate scale-invariant perturbations during low energy acceleration.

Progress has been made in linearized gravity to address the singularity issue.

The model offers an alternative to inflation that avoids some of its conceptual problems.

Abstract

Inflation has been the leading early universe scenario for two decades, and has become an accepted element of the successful `cosmic concordance' model. However, there are many puzzling features of the resulting theory. It requires both high energy and low energy inflation, with energy densities differing by a hundred orders of magnitude. The questions of why the universe started out undergoing high energy inflation, and why it will end up in low energy inflation, are unanswered. Rather than resort to anthropic arguments, we have developed an alternative cosmology, the Cyclic universe, in which the universe exists in a very long-lived attractor state determined by the laws of physics. The model shares inflation's phenomenological successes without requiring an epoch of high energy inflation. Instead, the universe is made homogeneous and flat, and scale-invariant adiabatic perturbations are generated during an epoch of low energy acceleration like that seen today, but preceding the last big bang. Unlike inflation, the model requires low energy acceleration in order for a periodic attractor state to exist. The key challenge facing the scenario is that of passing through the cosmic singularity at t=0. Substantial progress has been made at the level of linearised gravity, which is reviewed here. The challenge of extending this to nonlinear gravity and string theory remains.