eBOSS concentrated on the observation of galaxies and quasars, in a range of distances (redshifts) left completely unexplored by other three-dimensional maps of large-scale structure in the Universe. In filling this gap, eBOSS created the largest volume survey of the Universe to date, publicly released in July 2020. The figure to the left shows the region mapped by the eBOSS project. This region corresponds to the epoch when the Universe was transitioning from deceleration because of gravity, to the current epoch of acceleration due to Dark Energy.
Will Percival was Survey Scientist for the project and several students and postdocs working in the WCA led components of the science resulting from this project.
In July 2022, the eBOSS (extended Baryon Oscillation Spectroscopic Survey) team formally announced its final measurements, labelled Data Release 16. eBOSS is the last dedicated galaxy redshift survey to be produced as part of the Sloan Digital Sky Survey (SDSS), and represents the culmination of 20 years of effort, that has produced a series of galaxy surveys that now span a period of 11 billion years of cosmic history. Combining observations from eBOSS with studies of the universe in its infancy from the CMB reveals cracks in the standard cosmological model. In particular, the eBOSS team’s measurement of the current rate of expansion of the universe (the “Hubble Constant”) is about 10 percent lower than the value found from distances to nearby galaxies.
Within the eBOSS team, individual groups at universities around the world focused on different aspects of the analysis. To create the part of the map dating back six billion years, the team used large, red galaxies. Farther out, they used younger, blue galaxies. Finally, to map the universe eleven billion years in the past and more, they used quasars, which are bright galaxies lit up by material falling onto a central supermassive black hole. Each of these samples required careful analysis to remove contaminants and reveal the patterns of the universe. Faizan Mohammad, a postdoctoral researcher at the WCA, led one such analysis. “The hardware used to make the observations itself leaves an imprint on the map,” said Mohammad. “It has been a lot of hard work to understand this imprint and remove it from the data, enabling robust measurements to be made about the universe.” eBOSS, and SDSS more generally, leaves the puzzle of dark energy, and the mismatch of local and early universe expansion rate, as a legacy to future projects – such as the Dark Energy Spectroscopic Instrument (DESI) and Euclid, a European Space Agency satellite mission.
23 papers were published on arXiv together on July 20 2022, and a press release was sent out describing the work aimed at news outlets across the world. As Survey Scientist for the latest SDSS survey Will Percival played a big part in the coordination of these results, and the press interest that followed. In particular, he gave three interviews on primetime Canadian TV networks (including the CBC National) describing the research, and a similar number of radio interviews. The press release was picked up by many news outlets (CNN, CBC, …) across the world and news articles were written about this work. In addition, four YouTube videos were released describing the work and providing a “fly-through” of the Universe, including two produced by the Perimeter Institute. Collective views currently stand at over one million for these videos.