The sky distribution of the BOSS massive galaxies (Image: Michael Blanton and the SDSS-III Collaboration)

Sound waves that rang out in the early universe sculpted its structure. The best measurement yet of their imprint on galaxies is a boon to dark-energy studies.

About 30,000 years after the big bang, matter collapsed around dense seeds of dark matter. Outward pressure from photons caused the collapsing matter to rebound, creating acoustic waves, like ripples in a pond. These ripples expanded until the universe cooled to a certain temperature, stalling the waves. More matter existed at the centres and edges of these ripples, and therefore should have led to more galaxies there.

This is exactly what the Baryon Oscillation Spectroscopic Survey (BOSS) sees in a study of 265,000 galaxies. When the BOSS team measured the distance between pairs of galaxies between 4.5 billion and 6.3 billion light years from Earth, they found an excess of galaxy pairs separated by 500 million light years (arxiv.org/abs/1203.6594).

That is the expected radius of the sound waves – if the universe’s expansion has been accelerated in line with the leading model of dark energy. Called the cosmological constant, the model suggests that the amount of energy in a given volume of space does not vary with time.