The SDSS map is shown as a rainbow of colors, located within the observable universe (the outer sphere, showing fluctuations in the cosmic microwave background). We are located at the center of this map. The inset for each color-coded section of the map includes an image of a typical galaxy or quasar from that section, and also the signal of the pattern that the eBOSS team measures there. As we look out in distance, we look back in time. So, the location of these signals reveals the expansion rate of the universe at different times in cosmic history.
Image: Anand Raichoor (EPFL), Ashley Ross (Ohio State University) and SDSS
A comprehensive analysis of the largest three-dimensional map of the universe ever created was released today (July 19) by the Sloan Digital Sky Survey (SDSS), an ongoing project that includes Penn State scientists. The new results fill in the most significant gaps in our exploration of the universe’s history and were produced by the extended Baryon Oscillation Spectroscopic Survey (eBOSS), which is one of the SDSS’s component surveys.
“This description of the cosmic distribution of over two million galaxies and quasars, covering nearly the entire history of the universe, is the product of years of effort by a large international team of scientists,” said Donald Schneider, a member of the eBOSS collaboration and distinguished professor and head of Penn State’s Department of Astronomy and Astrophysics. Schneider is the scientific and technical publications manager for the SDSS and has been a member of the project since its inception.
“The map and its analysis not only expand our understanding of the history and the structure of our universe but also provide a foundation that can facilitate new discoveries,” said Schneider.
The observations were acquired at the Apache Point Observatory in New Mexico.
“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,” said 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.”
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,” said 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 1% accuracy.
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% lower than the value found by tracking the 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,” 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.”
There is no broadly accepted explanation for this discrepancy in measured expansion rates, but one exciting possibility is that a previously unknown form of matter or energy from the early universe might have left a trace on our history.
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 11 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 in order to remove contaminants and reveal the patterns of the universe.
In total, the eBOSS team made public the results from more than 20 scientific papers today. Those papers describe, in more than 500 pages, the team’s analyses of the latest eBOSS data, marking the completion of the key goals of the survey.
The SDSS is far from completing its mission to map the universe. Karen Masters, 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, concentrating on mapping our galaxy and nearby universe, and the flickers and flares of distant black holes.”
