SDSS reveals 11 billion years of the history of our expanding Universe

July 20, 2020

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 collaboration, including researchers at the Max Planck Institute for Extraterrestrial Physics, was able to obtain the most accurate measurements of the expansion history of our Universe over the widest-ever range of cosmic time.

“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 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 axis indicates the lookback time.

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.

The new results come from the extended Baryon Oscillation Spectroscopic Survey (eBOSS), an international collaboration of more than 100 astrophysicists including researchers at the Max Planck Institute for Extraterrestrial Physics (MPE) 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.

Within the eBOSS team, individual groups focused on different aspects of the analysis with each of these samples requiring careful analysis in order to remove contaminants, and to reveal the patterns of the Universe. 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.

“Quasars provide a unique sample that allows us to bridge the redshift gap between galaxies and the Lyman-alpha forest at the highest redshifts,” says Jiamin Hou, a junior MPE researcher who led the quasar clustering analyses within eBOSS. “With galaxies we can look back over the last few billion years of cosmic history, the quasars take us back about 10 billion years, and finally the Lyman-alpha galaxies allow us to look back to when the universe was less than 2 billion years old.” Studies of the Cosmic Microwave Background then reach even further back, to the infant Universe, just 380 000 years after the Big Bang.

Two-dimensional correlation function for the eBOSS quasar sample. The signal measured, the so-called Baryonic Acoustic Oscillation, is revealed as the ring feature at a characteristic scale of ~100 Mpc/h. In addition, also the so-called redshift space distortion can be identified as the anisotropic clustering towards the centre.

The final map released by SDSS reveals the filaments and voids that define the structure in the Universe. A characteristic of this distribution, the so-called "baryonic acoustic oscillations", is a very subtle signal from the early epochs of the universe, when sound waves got “frozen” after traveling about 500 million light years and imprinted a signal in the distribution of matter. This footprint is visible today in the distribution of galaxies, where it is a little more likely to find pairs of galaxies separated by this scale than at smaller or larger distances. Measurements of the apparent size of this scale allow the scientists to calculate cosmic distances with high precision.

In addition to that, galaxies also have peculiar motions that make their distribution appear anisotropic with respect to the line-of-sight direction, an effect known as “redshift-space distortions”. This characteristic anisotropic pattern allowed the eBOSS team to measure the rate at which cosmic structures grow due to gravitational interactions. “Using quasars, we can constrain the distance measurement to around 3 percent and the measurement of the growth rate of the Universe within 10 percent,” says Hou. 

“In our group, we have had a continuous contribution to the SDSS cosmology programs since almost a decade ago”, says cosmologist Ariel Sánchez, a senior researcher at MPE. “Taken together, these studies allow us to reconstruct the expansion and growth of structure histories of our Universe over a range of cosmic time covering roughly 90% of its age.” Combined with additional information from the Cosmic Microwave Background and supernovae, this allows the scientists to determine several key parameters of our Universe to better than one percent accuracy. The cosmic history that has been revealed by these data 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.

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.

In the coming years, MPE scientists will continue to use SDSS surveys also to chart the large-scale structure of the Universe and constrain cosmological parameters by following up on galaxy clusters and AGN detected by the eROSITA X-ray telescope.

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