Our knowledge of the universe includes both its ancient history and the recent history of its expansion, but between both periods there were gaps in a period corresponding to 11,000 million years.
They obtain the most accurate image of the night sky that has allowed the discovery of 500 million astronomical objects
For five years, scientists at Sloan Digital Sky Survey (SDSS), a large study of multicolored and multispectral images of the universe captured from a telescope in New Mexico (USA), have worked to find out what happened during that period.
The information obtained has allowed us to achieve one of the most important advances in cosmology in the last decade: an exhaustive analysis of the largest three-dimensional map of the universe ever created, which fills those most significant gaps in the exploration of the history of the cosmos.
The new results are the result of one of the SDSS programs, the international collaboration Extended Baryon Oscillation Spectroscopic Survey (eBOSS), in which more than a hundred astrophysicists participate, three of them Spanish.
At the core of the new breakthroughs are detailed measurements of more than two million galaxies and quasars, spanning 11 billion years of cosmic time.
Thanks to the study of the cosmic microwave background radiation (CMB) and the measurements of the relative amounts of the elements created shortly after the Big Bang, we know what the universe was like in its infancy. We also know the history of its expansion in the last billion years from maps of galaxies and measurements of the distances between them, including those made in earlier phases of the SDSS.
“But the eBOSS analysis and previous SDSS experiments show the history of the expansion of the Universe over the longest period of time studied so far,” he states. Héctor Gil Marín, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB).
This researcher, awarded a scholarship by the “la Caixa” Foundation to carry out his post-doctorate at this center, has led the analysis of these galaxy maps, measuring the rate of expansion and growth of the structures of the universe from 6 billion years ago.
These measurements help to unite the early physics with the late one, allowing to generate a complete image of the expansion of the cosmos over time. A look at the map obtained reveals filaments and voids that define the structure of the universe from the time it was only about 300,000 years old.
Using this map, the researchers look for patterns in the distribution of galaxies, which provide information on various key parameters of the universe, which eBOSS has been able to measure with a accuracy greater than 1%, showing the signals of the patterns through images.
This map is the result of more than 20 years of efforts to map the universe through the Alfred P. Sloan Foundation’s telescope in New Mexico.
Cosmic history reveals that expansion began to accelerate about 6 billion years ago, and that it has continued to increase since then. This accelerated expansion seems to be due to a mysterious invisible component: the dark energy, which is consistent with the Einstein’s general theory of relativity but extremely difficult to reconcile with our current knowledge of particle physics.
By combining the observations made by eBOSS with the childhood studies of the cosmos, the researchers have obtained an image with some incompatibilities. Measuring the current rate of expansion of the universe (known as the Hubble constant) is approximately 10% less than the value found when measuring the rate of expansion using the distance to nearby galaxies.
“The high precision of the data makes it very unlikely that this mismatch is due to chance,” he points out. Andreu Font Ribera, a researcher at the Institute of High Energy Physics (IFAE) in Barcelona, who has led the interpretation of the results. “The large variety of eBOSS data leads to the same conclusion in several independent ways,” he says.
There is no widely accepted explanation for this discrepancy in the rates of expansion rates, but an exciting possibility is that a previously unknown form of matter or energy from the early universe would have left a mark on the expansion that we observe today.
More than 20 simultaneous papers
All these results have come to light today with the publication of more than 20 scientific articles in ArXiv, documents that describe, over more than 500 pages, the analysis of the latest eBOSS data. With this milestone the key objectives of the study are met.
Different groups of the eBOSS team, located in universities around the world, have focused on different aspects of the analysis. Researchers have used massive, red galaxies to obtain the part of the map dated to 6 billion years ago. For farther distances, they have used younger blue galaxies.
Finally, they have used quasars — bright galaxies that light up as a consequence of matter absorbed by a supermassive black hole at their core — to obtain the map of the universe from 11 billion years ago and earlier. To reveal the patterns of the Universe, a very careful analysis of each measurement has been made, with the aim of eliminating possible contaminants.
“We have measured the statistical properties of these galaxy maps and have deduced the rate at which the Universe expands over time,” he explains. Santiago Avila, a scientist from the Autonomous University of Madrid (UAM), who has developed new methods to simulate galaxy maps by computer, such as those observed in this study.
Ávila adds: “In combination with additional data from the cosmic microwave background and observations of supernovae, we have deduced that the geometric curvature of the universe is, in fact, flat, and we have also measured the local expansion rate with an accuracy greater than 1% “.
Following the SDSS path, work is already underway on the next generation of telescopes that will take over from eBOSS. It will start 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 located at the Kitt Peak National Observatory (Arizona, United States).
At the same time, the European Space Agency plans for 2022 the launch of the Euclid satellite, equipped with a unique telescope that will provide a complementary view of the Universe. These instruments, both with Spanish participation, will provide data with a precision never seen before, a fact that will allow us to solve the enigma of dark energy and the discordance between the expansion rate of the local and primitive Universe. Or perhaps, they will reveal more surprises.