- The Sloan Digital Sky Survey provides a comprehensive analysis of the largest three-dimensional map of the Universe ever created.
- The international study had the participation of the Institute of Cosmos Sciences of the University of Barcelona, the Institute of High Energy Physics and the Autonomous University of Madrid.
- The study fills som of the most significant gaps in the exploration of cosmos history.
The Universe, ancient and new
The expansion of the universe is a phenomenon that [results/becomes] an inherent property of the system as a whole, in accordance with the general theory of relativity. This effect results in spatial objects not bound together by gravitational forces to move apart from each other, something that happens at ever-increasing velocities, and with speeds that appear proportional to the distance at which those cosmic objects are located.
Our knowledge of the history of Universe expansion includes both ancient and recent times, but there still remained a signficant void corresponding to about 11,000 million years between both of those known periods. For five years, scientists from the Sloan Digital Sky Survey (SDSS) worked to discover what happened during that less known period of time, and used the information to obtain one of the most important advances in cosmology of the last decade.
The results came from one of the 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: 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 featured detailed measurements of more than two million galaxies and quasars. Together, they cover the mentioned lesser-known 11,000 million years of cosmic time.
A more complete vision of the history of the Universe
Thanks to the study of the radiation of the cosmic microwave background (CMB), and measures related to the elements that were created after the Big Bang, we know how the Universe was like at the beginning of time. 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 the SDSS.
“The eBoss analysis and the previous experiments in the Sloan Digital Sky Survey 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 developments, 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 the map, researchers look for patterns in the distribution of galaxies, which provide information on key parameters of the Universe, which eBOSS could measure with a precision of over 1%.
The SDSS map displays as a colour rainbow located within the observable Universe (the external sphere, which shows fluctuations in the microwave cosmic background). Planet Earth is located at the centre of the map. In the map, each color-code box section at the right includes the image of a galaxy or quasar typical of that section, and the signal of the pattern the eBOSS team measured there. The farther away from Earth one looks in the map, the earlier in time one is looking. Therefore, the location of the signals reveals the rate of expansion of the Universe at different times of the cosmic history.
The map is the result of more than twenty years of efforts to map the Universe through the telescope of 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 ever since. This accelerated expansion may be due to a mysterious component in the Universe, called dark matter, which is consistent with Einstein’s general relativity theory, but is difficult to conciliate with current knowledge of particle physics.
A surprising finding
When combining the observations of eBOSS with studies of the early Universe, researchers noticed certain unexpected incompatibilities. The measurement of the current rate of expansion of the Universe (Hubble’s constant) was found to be about 10% less compared to the value that is found when one measures the rate of expansion using the distance to near galaxies.
“The high precision of the data makes it unlikely for this mismatch to result from chance”, notes Andreu Font Ribera, IFAE researcher in Barcelona, who led the interpretation of the 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 form of matter or energy of the early Universe could have left a mark in the expansion we observe now.
Task division – and further projects
The results have yet seen the light with the publication of more than twenty science articles in ArXiv, documents that describe, over more than five hundred pages, the analysis of data in eBOSS. 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 farther away galaxies, they used younger blue galaxies. Lastly, they used quasars –lightning galaxies that lighten as a consequence of the matter absorbed by 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 that the geometrical curve of the Universe is in fact, plain, and we measured the rate of local expansion with a precision of over 1%”.
Following the path of SDSS, researchers are working on the next generation of telescopes to build on top of the findings of eBOSS. For instance, the Dark Energy Spectroscopic Instrument (DESI), 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 with the participation of Spanish institutions, will provide data with a precision never seen so far, enabling to solve the enigma of dark matter and the discordance between the rate of expansion of the local and early Universe. Or, perhaps… it will uncover more surprises.
SDSS Map by Anand Raichoor (EPFL), Ashley Ross (Ohio State University) and the SDSS Collaboration was kindly provided by ICCUB and is licensed via a Creative Commons Attribution 4.0 International (CC BY 4.0) license.
ULAS J1120+0641 quasar artist representation was downloaded from Wikimedia Commons and is licensed via a Creative Commons Attribution 4.0 International (CC BY 4.0) license.