9 - Baryon acoustic oscillations

Summary

Introduction

Whilst often phrased in terms of discovering the nature of dark energy, cosmology in the twenty-first century might also aptly be described as ‘the distance revolution’. With new knowledge of the extragalactic distance ladder we are, for the first time, beginning to accurately probe the expansion history of the cosmos beyond the local universe, at redshifts z > 0.1. While standard candles - most notably Type Ia supernovae (SN Ia) - kicked off the distance revolution, it is clear that standard rulers, and the baryon acoustic oscillations (BAO) in particular, will play an important role in the coming revolution.

Here we review the theoretical, observational and statistical aspects of the BAO as standard rulers and examine the impact BAO will have on our understanding of dark energy, the distance and expansion ladder.

9.1.1 A brief history of standard rulers and the BAO

Let us start by putting the BAO in context. The idea of a standard ruler is one familiar from everyday life. We judge the distance of an object of known length (such as a person) by its angular size. The further away it is, the smaller it appears. The same idea applies in cosmology, with one major complication: space can be curved. This is similar to trying to judge the distance of our known object through a smooth lens of unknown curvature. Now when it appears small, we are no longer sure it is because it is far away.