The Sloan Digital Sky Survey (SDSS) has published a groundbreaking analysis of the cosmos. Reported in 23 papers, the work measured more than 2 million galaxies and quasars, producing the largest three-dimensional map of the universe yet.

Although previous observations had mapped the (relatively) closest galaxies to the Milky Way and looked at the Cosmic Microwave Background (CMB), with the latest one published last week, there was a gap in the data.

“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,” team leader Kyle Dawson, from the University of Utah, said in a statement. “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 map is our best understanding so far of the expansion rate of the universe since the Big Bang. The work confirms that the universe's expansion began to speed up about 6 billion years ago, getting faster and faster ever since. This expansion is believed to be caused by a hypothetical substance known as dark energy.

“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,” added 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.”

The expansion rate of the universe is given by a parameter known as the Hubble Constant. In this work, the team estimated that the constant has a value of 68 kilometers per second per megaparsec (a megaparsec is equivalent to 3.26 million light-years). This means that if two galaxies are 1 megaparsec apart, they would appear to be moving away from each other at a speed of 68 kilometers (42 miles) per second.

This value is consistent with measurements from the CMB but it disagrees with measurements of closer galaxies, which produce a rate of about 74 kilometers (46 miles) per second per megaparsec. This tension in cosmology is a problem still in need of a solution.

“Only with maps like ours can you actually say for sure that there is a mismatch in the Hubble Constant,” said 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.”

The achievement is incredible and more is likely to come.

“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,” Karen Masters of Haverford College, spokesperson for the current phase of SDSS, concluded.