Astrophysicists fill in 11 billion years of the universe's expansion history

The Sloan Digital Sky Survey (SDSS) released today 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 today's 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."

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. 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.

We know what the universe looked like in its infancy, thanks to the thousands of scientists from around the world who have measured the relative amounts of elements created soon after the Big Bang, and who have studied the Cosmic Microwave Background. We also know its expansion history over the last few billion years from galaxy maps and distance measurements, including those from previous phases of the SDSS.

"Taken together, detailed analyses of the eBOSS map and the earlier SDSS experiments have now provided the most accurate expansion history measurements over the widest-ever range of cosmic time," says Will Percival of the University of Waterloo, eBOSS's Survey Scientist. "These studies allow us to connect all these measurements into a complete story of the expansion of the universe."

A close look at the map reveals the filaments and voids that define the structure in the universe, starting from the time when the universe was only about 300,000 years old. From this map, researchers measure patterns in the distribution of galaxies, which give several key parameters of our universe to better than one percent accuracy. The signals of these patterns are shown in the insets in the image.

This map represents the combined effort of more than 20 years of mapping the universe using the Sloan Foundation telescope. The cosmic history that has been revealed in this 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.

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 of the University of Oxford, who led the analysis to interpret the results from the full SDSS sample. "These newest maps from eBOSS show it more clearly than ever before."