Astrophysicists measure the expansion of the Universe across 11,000 million years

July 20, 2020

The Sloan Digital Sky Survey (SDSS) publishes a comprehensive analysis of the largest three-dimensional map of the Universe ever created, which fills the most significant voids of our exploration on the history of cosmos.
eBOSS map

Our knowledge on the Universe includes both the ancient and recent history of its expansion, but there were voids corresponding to 11,000 million years between both periods. For five years, scientists from SDSS have worked to discover what happened during that period of time, and used the information to get one of the most important advances in the cosmology of the last decade. The new results come from one of programmes in SDSS, the international collaboration Extended Baryon Oscillation Spectroscopic Survey (eBOSS), in which more than a hundred astrophysicists take part. Three Spanish researchers played an important role in the analysis that was presented today: Héctor Gil Marín, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB); Andreu Font Ribera, from the Institute of High Energy Physics (IFAE), and Santiago Ávila, from the Autonomous University of Madrid. The new results feature the detailed measurements of more than two million galaxies and quasars, which cover 11,000 million years of cosmic time. Thanks to the study of the radiation of the cosmic microwave background (CMB), and the measures of the quantity relate to the elements that were created after the Big Bang, we know how the Universe was like at the beginning. We also know the history of the expansion of the Universe over billions of years, thanks to the maps of galaxies and measurements of the distances between them, including those in phases prior to SDSS.

“The eBoss analysis and the previous experiments in SDSS show the history of the expansion of the Universe over the largest amount of time studied so far”, notes Héctor Gil Marín, from ICCUB. The researcher has led the analysis of these galaxy maps, measuring the expansion rhythm and the growth of structures of the Universe from 6,000 million years ago. These measurements help merging the early and late physics, which generates a complete image of the expansion of the Universe over time. The obtained map shows filaments and voids that define the structure of the Universe from the moment it was only 300,000 years old. With this map, researchers look for patterns in the distribution of galaxies, which provide information on these key parameters of the Universe, which eBOSS could measure with a precision over 1%.

eBOSS map
The SDSS map is shown as a colourful rainbow, located in the observable Universe (external sphere, which shows fluctuations in the microwave cosmic background). We are in the center of the map. The box for each color-code section of the map includes the image of a galaxy or quasar typical of that section, and the signal of the pattern the eBOSS team measures there. When looking in the distance, we look back in time. Therefore, the location of these signals reveals the rate of expansion of the Universe at different times of the cosmic history. Credit: Anand Raichoor (EPFL), Ashley Ross (Ohio State University) and SDSS

The map is the result of more than twenty years of efforts to map the Universe through the telescope from the Alfred P. Sloan Foundation. The cosmic history it reveals shows that the expansion of the Universe started accelerating about 6,000 million years ago, and it has increased since then. This accelerated expansion may be so due to a mysterious compound in the Universe, called dark matter, which is consistent with Einstein’s general relativity theory, but difficult to conciliate with our current knowledge of particle physics. When combining the observations from eBOSS with studies on the early Universe, researchers obtained an image with some incompatibilities. The measurement of the current rate of expansion of the Universe (Hubble’s constant) is about 10% less compared to the value found when measuring the rate of expansion using the distance to near galaxies. “The high precision of data makes it unlikely for this mismatch to result from chance”, notes Andreu Font Ribera, IFAE researcher in Barcelona, who led the interpretation of results. “The great variety of data in eBOSS leads to the same conclusion in several ways”, he adds.

There is not a widely accepted explanation for this discrepancy in the measures of expansion rates, but an interesting possibility is that a previously unknown way of matter or energy of the early Universe would have left a mark in the expansion we observe now. 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. The different groups in the eBoss team, located in universities worldwide, have focused on different aspects of the analysis. Researchers have analysed red and massive galaxies to obtain the part of the map dating from 6,000 million years ago. For further galaxies, they used younger blue galaxies. Last, they used quasars –lightning galaxies that lighten as a consequence of the absorbed matter through a supermassive blackhole in its nucleus– to obtain the map of the Universe from 11,000 million years ago and previous periods of time. To reveal the patterns of the Universe, they conducted an analysis of every measurement, in order to rule out potential pollutants. “We measured the statistical properties of these maps of galaxies and deduced the rate at which the Universe expands over time”, says Santiago Ávila, from the Autonomous University of Madrid (UAM), who carried out new methods to simulate computer galaxy maps like the ones in this study. Ávila adds that “in combination with additional data from the microwave cosmic background and observations of supernovas, we estimated the geometrical curve of the Universe is in fact, plain, and we measured the rate of local expansion with a precision over 1%”. Following the path of SDSS, researchers are already working on the next generation of telescopes to reveal eBOSS. It will begin at the end of the year with the Dark Energy Spectroscopic Instrument (DESI), which will observe ten times more galaxies and quasars than eBOSS thanks to a new instrument in the Kitt Peak National Observatory (Arizona, United States). At the same time, the European Space Agency plans the launch of the Euclid satellite by 2022. This is the satellite with a unique telescope to provide a complementary view of the Universe. These instruments, which count on participation from Spain, will provide data with a precision that has never been seen so far, which enables us to solve the enigma of the dark matter and discordance between the rate of expansion of the local and early Universe. Or, perhaps, they will reveal more surprises.

SDSS telescope
SDSS Telescope at Dusk Credit: SDSS. David Kirkby


Funds for Sloan Digital Sky Survey IV have been provided by the Alfred P. Sloan Foundation, the Office of Science of the Department of Energy of the United States and participating institutions. SDSS recognizes the support and resources of the Center for High Performance Computing of the University of Utah. The SDSS website is www.sdss.org . SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration, which includes the Participation Group of Brazil, the Carnegie Institution for Science, The Carnegie Mellon University, the Chile Participation Group, the France Participation Group, the Harvard-Smithsonian Center for Astrophysics, the Canary Islands Institute of Astrophysics, John Hopkins University, Kavly Institute for the Physics and Mathematics of the Universe (IPMU)/Tokyo University, Korea Participation Group, National Laboratory Lawrence Berkeley, the Leibniz Institute for Astrophysics Potsdam (AIP), the Max Planck Institute for Astronomy (MPIA Heidelberg), Max Planck Institute for Astrophysics (MPA Garching), Max Plank Institute for Physics (MPE), the National Astronomic Observatories in China, the State University of New Mexico, New York University, University of Notre Dame, the National Observatory of Brazil/MCTI Ohio State University, Pennsylvania State University, the Astronomical Observatory of Shanghai, the United Kingdom Participation Group, the National Autonomous University of Mexico, University of Arizona, University of Colorado Boulder, University of Oxford, Portsmouth University, University of Utah, University of Virginia, Washington University, University of Wisconsin, Vanderbilt University and Yale University.