Getting information from nothing - using voids to make cosmological measurements

A group of astronomers led by Dr Sesh Nadathur at the University of Portsmouth, and including WCA Director Will Percival, have spent the last 3 years studying large structures in the distribution of galaxies in the Universe to provide the most precise tests of dark energy and cosmic expansion yet. The new method is based on a combination of the statistics of voids – large expanding bubbles of space containing very few galaxies – and the faint imprint of sound waves in the very early Universe, known as baryon acoustic oscillations (BAO), that can be seen in the distribution of galaxies. This provides much more precise direct measurements of dark energy than from the previous best method based on exploding massive stars, known as supernovae. The new results confirm the model of a cosmological constant dark energy and spatially flat Universe to unprecedented accuracy, and strongly disfavour recent hints of positive spatial curvature inferred from observations of the cosmic microwave background. The study also provides a new precise measurement of the Hubble parameter describing the current rate of expansion of the Universe, the value of which is the subject of intense debate. They found tentative evidence that nearby voids and BAO at redshift z<2 favour the high Hubble rate seen from other low redshift methods, while high-redshift BAO favour a lower rate consistent with that inferred from the cosmic microwave background. The full publication, which was recently accepted for publication in Physical Review Letters, is available for download at: https://arxiv.org/pdf/2001.11044.pdf

Image: A section of the three-dimensional map constructed by the Baryon Oscillation Spectroscopic Survey (BOSS) used in this analysis. The rectangle on the far left shows a patch of the sky from which BOSS observed spectra of 120,000 galaxies (roughly 10% of the total survey). These spectra were used to create a 3D map of the Universe, shown in the centre and left images. Brighter regions in this map correspond to the regions of the Universe with more galaxies from which voids and BAO were measured. Image credit: Jeremy Tinker and the SDSS-III collaboration