Ekpyrotic and Cyclic Cosmology

TL;DR

Ekpyrotic and cyclic cosmologies propose a universe where the big bang is a brane collision, involving phases of slow contraction and expansion, offering alternatives to standard cosmology with distinctive observational predictions.

Contribution

This paper reviews the detailed properties, theoretical embedding, and observational prospects of ekpyrotic and cyclic models, highlighting their potential to resolve cosmological puzzles.

Findings

The models produce a nearly scale-invariant perturbation spectrum.

They generate small-amplitude, blue-spectrum gravitational waves.

Dark energy is reinterpreted as brane attraction leading to cyclic phases.

Abstract

Ekpyrotic and cyclic cosmologies provide theories of the very early and of the very late universe. In these models, the big bang is described as a collision of branes - and thus the big bang is not the beginning of time. Before the big bang, there is an ekpyrotic phase with equation of state w=P/rho >> 1 (where P is the average pressure and rho the average energy density) during which the universe slowly contracts. This phase resolves the standard cosmological puzzles and generates a nearly scale-invariant spectrum of cosmological perturbations containing a significant non-gaussian component. At the same time it produces small-amplitude gravitational waves with a blue spectrum. The dark energy dominating the present-day cosmological evolution is reinterpreted as a small attractive force between our brane and a parallel one. This force eventually induces a new ekpyrotic phase and a new brane collision, leading to the idea of a cyclic universe. This review discusses the detailed properties of these models, their embedding in M-theory and their viability, with an emphasis on open issues and observational signatures.