Astrophysicists Fill In 11 Billion Years of Our Universe's Expansion History
Scientists have analysed the largest three-dimensional map of the Universe ever created to understand what happened during a history gap of 11 billion years in the middle of its expansion

The analyses of this galaxy map has allowed to measure the expansion rate and growth of structure of the Universe 6500 millions of years ago 

A researcher from the Institute of Space Studies of Catalonia (IEEC) at the Institute of Cosmos Sciences (ICCUB) and two other Spanish scientists have played an important role in these results

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.

We know both the ancient history of the Universe and its recent expansion history fairly well, but there was a troublesome 11-billion-year gap in its mid-history. For five years, scientists from SDSS have worked to fill in that gap, and have used that information to provide some of the most substantial advances in cosmology in the last decade.  

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. Three Spanish scientists have played an important role in the analysis presented today: Hector Gil-Marín, from the Institute of Space Studies of Catalonia (IEEC) at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB), Andreu Font-Ribera, from the Institut de Física d’Altes Energies (IFAE) and Santiago Ávila, from the Instituto de Física Teórica (IFT, UAM-CSIC). 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 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. 

"The analyses of eBOSS and their previous SDSS experiments have probed the expansion history of the Universe over the broadest-ever range of cosmic time" says Héctor Gil-Marín, Junior Leader La Caixa Fellow from ICCUB. Gil-Marín has led the analyses of these galaxy maps, measuring the expansion rate and growth of structure of the Universe 6500 millions of years ago. These measurements help us to bring together the late and early-time physics, composing a full picture of the expansion of the Universe across time.  

The final map is presented in the image below. 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. From this map, researchers measured patterns in the distribution of galaxies, which provide several key parameters of our Universe to better than one percent accuracy. 

PR_Image2: SDSS map
Caption: The SDSS map is shown as a rainbow of colors, located within the observable Universe. 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. The location of these signals reveals the expansion rate of the Universe at different times in cosmic history. 
Credit: Anand Raichoor (EPFL), Ashley Ross (Ohio State University) and SDSS.

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 ten 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”, says Andreu Font-Ribera, researcher from IFAE in Barcelona, who led the interpretation of these results. “The rich variety of eBOSS data gives us multiple independent ways to draw the same conclusion.”

There is not a widely 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.

These results have seen the light today with the publication of more than twenty science articles in ArXiv, documents that describe, over more than five hundred pages, the analysis of the latest data in eBOSS. With this summit, the key objectives of the study are reached. 

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. 

"We have measured the statistical properties of these galaxy maps and inferred the rate at which the Universe expanded over time", says Santiago Ávila, researcher from IFT, who developed a method to simulate the galaxy maps. "In combination with additional data from the cosmic microwave background and supernovae observations we have inferred that the geometrical curvature of the Universe is actually flat, and also have measured the local expansion rate with better than one percent precision". 

The 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.

Following the path of SDSS, a next generation of galaxy surveys is already on its way. Starting later this year, the Dark Energy Spectroscopic Instrument (DESI) will observe 10 times more galaxies and quasars than eBOSS, using a new instrument located at Kitt Peak (Arizona, USA). Meanwhile, the European Space Agency is planning for 2022 the launch of the Euclid satellite, equipped with a unique telescope to offer a complementary picture of the Universe. These instruments, both with Spanish participation, will provide data with unprecedented precision that will allow us to know more about the history and contents of the Universe.   

Link to results: Acknowledgments:

Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is

SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatório Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.


PR_Image1: SDSS Telescope
Caption: SDSS Telescope. The Sloan Digital Sky Survey (SDSS) is a huge project dedicated to mapping as much of the Universe as is visible from Earth using this dedicated 2.5-metre optical telescope.
Credit: David Kirkby and SDSS
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The Institute of Space Studies of Catalonia (IEEC  — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of over 20 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centers, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.  

IEEC is a private non-profit foundation, governed by a Board of Trustees composed of Generalitat de Catalunya and four other institutions that each have a research unit, which together constitute the core of IEEC R&D activity: the University of Barcelona (UB) with the research unit ICCUB — Institute of Cosmos Sciences; the Autonomous University of Barcelona (UAB) with the research unit CERES — Center of Space Studies and Research; the Polytechnic University of Catalonia (UPC) with the research unit CTE — Research Group in Space Sciences and Technologies; the Spanish Research Council (CSIC) with the research unit ICE — Institute of Space Sciences. IEEC is integrated in the CERCA network (Centres de Recerca de Catalunya).


IEEC Communication Office
Barcelona, Spain

Ana Montaner Pizà

Institute of Cosmos Sciences of Universitat de Barcelona (ICCUB)
Barcelona, Spain

Héctor Gil Marin 
Attached Documents
Generalitat de CatalunyaUniversitat de BarcelonaUniversitat Autònoma de BarcelonaUniversitat Politècnica de CatalunyaConsejo Superior de Investigaciones CientíficasCentres de Recerca de Catalunya