It’s been said that maps codify the miracle of existence. Often referred to as the “cosmic genome” project, the new Sloan Digital Sky Survey is the largest three-dimensional map of the universe ever created, filling in the most significant gaps in 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, about the results of the analysis of The Sloan Digital Sky Survey (SDSS) released today.
“For five years,” Dawson added, “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.”
11 Billion Years of Cosmic Time
The new results come from the extended Baryon Oscillation Spectroscopic Survey (eBOSS), one of the SDSS’s component surveys that collated a three-dimensional map of distant galaxies, enabling the most precise measurements yet of dark energy and the acceleration of the expansion of the Universe, and to study the early, formative years of the cosmos. 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 have spent five years collecting measurements of 1.2 million galaxies over one quarter of the sky to map out the structure of the Universe over a volume of 650 billion cubic light years,” said astronomer Jeremy Tinker, about the latest map that extends about twice as far as the previous sky survey, providing unprecedented insight into the evolution of the Universe by examining the distribution of galaxy clusters.
“We know,” reports the Sloan team,” 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 –a multi-institution group of astronomers who since 2000 have searched the skies with the 100-inch, wide-angle optical telescope at the Apache Point Observatory in New Mexico.”
“Inconsistent” –Hubble Constant Measurements
The Hubble Constant measurements from SDSS and other surveys are inconsistent with the measurements from nearby galaxies, which find a value close to 74 in these units – as opposed to 68 for the SDSS. Only with the data taken from SDSS and other experiments in the last decade has it been possible to reveal this discrepancy.
“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 Dark Energy Enigma
The 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.
The busy SDSS map below 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. (Anand Raichoor (EPFL), Ashley Ross (Ohio State University) and the SDSS Collaboration)
“I have absolutely no clue what dark energy is. Dark energy appears strong enough to push the entire universe – yet its source is unknown, its location is unknown and its physics are highly speculative,” said Noble Prize winning physicist Adam Riess in an interview with The Atlantic about the mysterious force — beyond the four already known, gravitational, electromagnetic, and the strong and weak nuclear–that is causing the universe to expand at an accelerating rate– that may be a hidden ‘fifth’ force that acts on matter with no evidence of its existence.
“The discovery of dark energy has greatly changed how we think about the laws of nature,” said Edward Witten, one of the world’s leading theoretical physicist at the Institute for Advanced Study in Princeton, N.J. who has been compared to Newton and Einstein.
Cracks in the Picture
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.”
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.
In total, the eBOSS team made the results from more than 20 scientific papers public 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.
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 in order to remove contaminants, and reveal the patterns of the universe.
“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,” says Mueller. “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.”
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.
The Daily Galaxy, Max Goldberg, via Sloan Digital Sky Survey
Image credit top of page: Each dot in this SDSS picture indicates the position of a galaxy 6 billion years into the past. The image covers about 1/20th of the sky, a slice of the Universe 6 billion light-years wide, 4.5 billion light-years high, and 500 million light-years thick. Color indicates distance from Earth, ranging from yellow on the near side of the slice to purple on the far side. Galaxies are highly clustered, revealing superclusters and voids whose presence is seeded in the first fraction of a second after the Big Bang. This image contains 48,741 galaxies, about 3% of the full survey dataset. Grey patches are small regions without survey data. Daniel Eisenstein and the SDSS-III collaboration