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

By July 23, 2020August 3rd, 2020IFAE

The Institute for High Energy Physics (IFAE), a BIST centre, is part of the the Sloan Digital Sky Survey (SDSS) collaboration, which recently published 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.

Our knowledge on the Universe includes both the ancient and recent history of its expansion. There are large voids corresponding to 11,000 million years between both periods however. For five years, scientists working on the Sloan Digital Sky Survey (SDSS ), which has created the most detailed three-dimensional maps of the Universe ever made, have worked to discover what happened during that period of time and used the information to make one of the most important advances in the cosmology of the last decade.

The new results come out of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), an international collaboration that forms part of the SDSS. More than one hundred astrophysicists form part of the collaboration, including Andreu Font Ribera of the Institute for High Energy Physics (IFAE), a BIST centre, who has played an important role in the analysis of the project. Héctor Gil Marín from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and Santiago Ávila from the Autonomous University of Madrid also played important parts in the project, among other international groups.

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 studies that have already been done on the radiation of the cosmic microwave background (CMB), and the measurements of elements that were created after the Big Bang, we know what the Universe was like at its very 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 to merge 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%.

The map is the result of more than twenty years of efforts to map the Universe with the telescope from the Alfred P. Sloan Foundation. The cosmic history it reveals shows the expansion of the Universe started accelerating about 6,000 million years ago, and it has increased since then. This accelerated expansion may be due to a mysterious entity in the Universe called dark matter, which is consistent with Einstein’s theory of general relativity, but difficult to reconcile with our current knowledge of particle physics.

When combining the observations from eBOSS with studies done 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 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 were published this month, with more than twenty scientific 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 have been reached.

More information can be found on the IFAE website