The Baryon Acoustic Oscillation feature is a key observable in cosmology, corresponding to the maximum distance sound waves of baryonic matter can travel in the early Universe. It leaves a measurable imprint in the clustering of galaxies at late times, which can be used as a "standard ruler". In this double bill talk we will discuss how this feature can be used in combination with other observations to construct a cosmic distance ladder which strongly constrains the expansion history of the Universe.
While the traditional way to build a cosmic distance ladder is to calibrate locally and extend it out to high redshift, the BAO feature allows us to calibrate at z~1000 and extend the ladder inwards. In combination with other observations (standard candles and/or standard clocks) it is even possible to measure the expansion history of the Universe, constrain its spatial curvature, and determine the length of the standard ruler in a way that is completely model independent. In fact this methodology uses only very weak assumptions of symmetry and a smooth expansion history, but does not even need assuming General Relativity. Since the length is set early on in the Universe's history, the conclusions are independent of late-time physics such as Dark Energy properties or late modifications to gravity. Even without GR, we find that the Universe looks very much like the standard LCDM model.