Understanding our Universe

17 July 2020

The Sloan Digital Sky Survey (SDSS) has released a comprehensive analysis of the largest three-dimensional map of the Universe ever created, filling in the most significant gaps in our possible exploration of its history.

‘We know both the ancient history of the Universe and its recent expansion history fairly well, but there’s a troublesome gap in the middle 11 billion years,’ says cosmologist Kyle Dawson of the University of Utah, who leads the team announcing the results. ‘For five years, we have worked to fill in that gap, and we are using that information to provide some of the most substantial advances in cosmology in the last decade.’

Interpreting detailed data

The new results come from the extended Baryon Oscillation Spectroscopic Survey (eBOSS), an international collaboration of more than 100 astrophysicists that is one of the SDSS’s component surveys. Eva-Maria Mueller from Oxford’s Department of Physics was one of the team and led the analysis to interpret the results from the full SDSS sample. At the heart of the new results are detailed measurements of more than two million galaxies and quasars covering 11 billion years of cosmic time.

SDSS map_400.jpg

The map represents the combined effort of more than 20 years of mapping the Universe using the Sloan Foundation telescope, with operations support over the last five years from the Department of Energy, the Sloan Foundation, and partnering institutions. The cosmic history that has been revealed in the map shows that about six billion years ago, the expansion of the Universe began to accelerate, and has continued to get faster and faster ever since. This accelerated expansion seems to be due to a mysterious invisible component of the Universe called ‘dark energy’, consistent with Einstein’s General Theory of Relativity but extremely difficult to reconcile with our current understanding of particle physics.

Questioning the rate of expansion

Combining observations from eBOSS with studies of the Universe in its infancy reveals cracks in this picture of the Universe. 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. The high precision of the eBOSS data means that it is highly unlikely that this mismatch is due to chance, and the rich variety of eBOSS data gives us multiple independent ways to draw the same conclusion.

‘Only with maps like ours can you actually say for sure that there is a mismatch in the Hubble Constant,’ says Eva-Maria Mueller. ‘These newest maps from eBOSS show it more clearly than ever before. By combining SDSS data with additional data from the Cosmic Microwave Background, supernovae, and other programs, we can simultaneously measure many fundamental properties of the Universe. The SDSS data cover such a large swath of cosmic time that they provide the biggest advances of any probe to measure the geometrical curvature of the Universe, finding it to be flat. They also allow measurements of the local expansion rate to better than one percent.’

eBOSSPressRelease_350.jpg

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. In the next decade, future surveys may resolve the conundrum, or perhaps, will reveal more surprises.

Meanwhile, with continued support from the Alfred P. Sloan Foundation and institutional members, the SDSS is nowhere near done with its mission to map the Universe. Karen Masters of Haverford College, Spokesperson for the current phase of SDSS, described her excitement about the next phase. ‘The Sloan Foundation Telescope and its near-twin at Las Campanas Observatory will continue to make astronomical discoveries mapping millions of stars and black holes as they change and evolve over cosmic time.’ The SDSS team is busy building the hardware to start this new phase and is looking forward to the new discoveries of the next 20 years.

Find out more: https://www.sdss.org/press-releases/no-need-to-mind-the-gap/