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.
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.
The results are the outcome of 20 years of observations and were contributed to by past and present members of the University of Portsmouth’s Institute of Cosmology and Gravitation (ICG).
Dr Julian Bautista is a research fellow at the ICG and the lead data scientist for eBOSS. He said: “Thanks to these maps, my colleagues and I were able to observe and study how fast the Universe was expanding and how fast structures formed.”
Dr Bautista was responsible for processing the observations from the telescope into clean three-dimensional maps of the Universe for use by the whole collaboration. He is also the main author of one of the publications describing the state-of-the-art measurement of the expansion rate of the Universe and the growth rate of their structures with the red galaxy sample.
Before eBOSS, it was as if we had this movie of the Universe but with most of it missing. We had some images of the beginning and we could see how the story ends, but now, thanks to eBOSS, we can finally see most of the Universe's tale and how dark energy was one of the main actors in it.
Dr Julian Bautista, Research fellow and lead data scientist for eBOSS
Professor Kyle Dawson, of the University of Utah, led the team announcing today’s results. He said: “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. 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.”
Scientists know what the Universe looked like in its infancy, thanks to the thousands of experts 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. They 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.
The final map is shown in the below image. 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 per cent 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 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.
Dr Bautista said: “We think this acceleration is due to an invisible component of the Universe we call ‘dark energy’, but we don’t understand well how dark energy behaves. It could just be a cosmological constant as suggested by Einstein, but that is extremely difficult to reconcile with theories of particle physics.
“Before eBOSS, it was as if we had this movie of the Universe but with most of it missing. We had some images of the beginning and we could see how the story ends, but now, thanks to eBOSS, we can finally see most of the Universe's tale and how dark energy was one of the main actors in it.”
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 per cent lower than the value found from distances to very 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.
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.
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.
Professor Adam Amara, the newly appointed Director of the ICG who starts next month, said: “These are very exciting results coming from decades of developments from the SDSS team. It is wonderful to see my cosmology colleagues at the ICG continuing to play leading roles in these important international collaborations.”