The baryon acoustic oscillation signal is one of the few imprints from the Big Bang we can still see in the universe. They form from initial density perturbations in the early universe expanding out like sound waves whilst the universe was small and hot enough that matter acted like a fluid. These ripples froze out when the universe expanded enough that pressure waves could no longer travel in space, and we can still see these ripples in the universe today! They are imprinted in an excess of galaxies separated at a size which is a combination of this ripple radius and the expansion of the universe. We can calculate the size of the ripple, and so mapping out how big the separation is in the modern universe tells us how much it has expanded in the last 13.8 billion years!

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