Researchers from the international Dark Energy Survey consortium (DES in the English acronym for Dark Energy Survey) held their biannual meeting, last week (3 to 7), at Unicamp. A collaboration between 25 institutions, from eight different countries, involving hundreds of researchers, DES focuses on two of the main issues in physics of this century: dark energy and dark matter.
20 years ago, interpretations of the dynamics of the universe were shaken by the evidence that the Universe is expanding rapidly. Contrary to what was thought, the force of gravity was not stopping the expansion movement initiated by the Big Bang. There was a negative pressure that had the effect of a force contrary to gravity that not only maintained the movement, but accelerated it. This negative pressure is the main characteristic of so-called dark energy, about which very little is still known. The discovery won the Nobel Prize in Physics in 2011.
Observations, with increasingly advanced equipment, and models that seek to explain interactions in space have also, in recent years, pointed to the existence of more matter in the Universe than we can see. This matter would be composed of particles, whose gravitational force would affect visible matter, but which would not reflect light. This new enigma was called: dark matter. Before dark energy was discovered, dark matter had already surprised scientists for several decades when measurements of the mass of galaxies, made through their rotation, did not match the luminous mass of these same objects. This missing mass has become a true enigma of cosmology.
In an attempt to decipher them, technologies were developed, which, when it is impossible to observe them directly, photograph their surroundings and provide researchers with data about their influence on the visible Universe. The estimate today is that 70% of the known Universe is made up of dark energy, 25% of dark matter and only 5% of everything we know as the visible Universe. “What we see is part of the visible matter, it is only a small fraction of the total amount of matter. But it gives traces of the hidden distribution of dark matter,” explained Richard Kron, DES coordinator and professor at the University of Chicago. “The better the technologies we have available to learn more things, the more we understand how little we know,” he said.
DES collects and analyzes data obtained by the Blanco telescope, installed at the Cerro Tololo Observatory in Chile. Attached to it, a 570 Mpix super camera, records images of 300 million galaxies. “We have made great progress in recent years, especially in the number of objects observed that are very distant. We are increasingly able to observe more in less time. The telescope that is under construction, the LSST (Large Synoptic Survey Telescope), will make it possible, in 1 year, to create a catalog of objects 40 times larger than those generated by DES in five years”, reported Flavia Sobreira, professor at the Gleb Institute of Physics Wataghin (IFGW), which represents Unicamp in the consortium.
As the Brazilian researcher explained, based on the data obtained, models are created to explain how the Universe works. “The model has to obey the data. These are the data that present the true Universe once we understand the systems involved in them. When we discovered that the Universe is expanding rapidly, many models of modified gravitation, of general relativity, were created. From analyzing the data, many of these models have already been eliminated,” she explained. “It may be necessary to modify something in Einstein's general relativity to be able to explain this expansion. This is a possibility that has not been ruled out,” she said.
According to Kron, to date, research points to the approximate amount of dark energy and dark matter and their distribution in the Universe. “We know approximately how much there is and have some information about how it is distributed. Dark energy is distributed on some scales, with some success. We know a few things about this. But we don’t know what it is exactly,” he said.
What can be said with some certainty, according to him, is that the universe would be a very different place without dark matter and dark energy. “In fact, one of the arguments is that we wouldn’t exist if dark matter didn’t exist. It is dark matter that provides the gravitational attraction that enables the formation of galaxies. We need the gravity of dark matter in order for us to exist,” he noted.
The meeting at Unicamp brought together 90 researchers, representing all institutions involved in DES. For a week, they presented and discussed the results of analyzing data from the telescope's third year of capture. “We are in the sixth year of observation. But between observation and the production of science, there is a long period of data processing. Currently, we have an enormous amount of scientific articles, which were the result of the analysis of year 1 of observation. When we analyze the data from year 6, the data set will be much larger and the accuracy of the science produced will be much better”, explained Sobreira.
As Kron described it, “when we say we are going to measure something in cosmology, it means we are trying to reduce the errors and uncertainties in each parameter.” Among the parameters used, the professor points out the expansion of the Universe. “We seek to understand not only how the Universe expands today, how fast galaxies are separating at present time. But also how fast they were moving apart in the past. We call this the expansion history of the Universe. The expansion history of the universe depends on the amount of dark matter and dark energy. We therefore characterize the amount of dark energy and dark matter by taking measurements of the expansion history of the Universe,” he summarized.
Another parameter presented by Kron was the “degree of clumping of matter”. “Throughout history, since the Big Bang, if two parts of the universe are far enough away from each other, they will continue to move apart. This is the expansion of the universe. But if they are close enough, the gravity between them can even cause the two to come together. Therefore, some of the material is moving away and some is moving closer. We call this structure growth. Depending on how the universe is built, which is what we are trying to figure out here, we will have more or less structure growth,” he explained.
Unicamp was included in this collaboration with the incorporation of Flavia Sobreira to the teaching staff in 2016. The researcher has been part of the group since 2011. According to her, Brazilian participation in the project includes several institutions, through the Interinstitutional Laboratory of e-Astronomy (No crew), which makes it financially viable.
The professor highlights the importance of this type of collaboration also for IFGW students. “You have some of the most important people in the area here and you have easy access to them. Any student at the beginning of their scientific career can talk and ask questions to the great experts on the subject. It’s a great opportunity for growth,” she pointed out.
