Astrophysicists already have a fairly accurate understanding of how the universe ages. And a new international collaboration, by means of which the most precise map of the structure of the universe has been made, has only confirmed with more force the data that were had until now. 26% of the universe is made up of mysterious dark matter, as well as 70% dark energy , which is causing its accelerated expansion. This is the conclusion of the Dark Energy Survey (DES), which has presented its results during the meeting of the American Physical Society Division of Particles and Fields at the Fermi National Accelerator Laboratory, near Chicago (USA).
The new data, combined with the measurement of microwave background waves, have made it possible to predict the structure of the universe with an accuracy never seen before. The measurement of microwave background waves was, so far, the most accurate test of cosmological models, carried out by the Planck space mission of the European Space Agency (ESA), a faint glow in the sky emitted three hundred and eighty thousand years later of the Big Bang.
Image: map of dark matter made from gravitational lens measurements of 26 million galaxies in the Dark Energy Survey. Red regions have more dark matter than average, and blue regions less dark matter./Chihway Chang / University of Chicago / DES collaboration.
“For the first time, the accuracy of key cosmological parameters emerging from a galaxy measurement is comparable to those derived from measurements of the cosmic microwave background,” in the words of one of the founders of the Dark Energy Survey, Risa Wechsler .
How was the measurement carried out?
Scientists have analyzed the light from 26 million galaxies to study how structures in the universe have changed in the last seven billion years , which is half the age of the universe. The data was taken with DECam, a 570 megapixel camera connected to the 4-meter Víctor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. The scientists took advantage of the phenomenon whereby distant images of galaxies are slightly distorted by the gravity of other foreground galaxies, an effect known as weak gravitational lensing . This analysis led to the largest map ever constructed of the distribution of matter, regular and dark, in the universe, as well as its evolution over time.
“With a margin of error of less than 5% , the combined results of Planck and DES are consistent with the standard model of cosmology,” in Wechsler’s words.
The standard model of cosmology , called Lambda-CDM, includes two key ingredients. The first is cold dark matter (CDM), an invisible form of matter that is five times more prevalent than regular matter, and which clumps together and is at the heart of the formation of structures such as galaxies and galaxy clusters. Lambda, the cosmological constant , describes the accelerating expansion of the universe, driven by an unknown force called dark energy. Dark matter has never been directly detected. For its part, dark energy is even more mysterious , and it is not known if it is actually a constant or changes over time.
Dark matter and energy shape the structure of the universe
In the first four hundred thousand years after the Big Bang, the universe was filled with incandescent gas , the light of which survives to this day. The Planck map of the cosmic microwave background radiation gives us a snapshot of the universe at that time. Since then, on the one hand, the gravity of dark matter has attracted mass and caused structures to form in the universe over time. On the other, dark energy, with its repulsive effect, has been fighting the attraction of matter. Using the Planck map as a starting point, cosmologists can accurately calculate how this battle between dark matter and energy has played out over some fourteen billion years.
“While Planck looked at the structure of the universe very early, DES has measured structures that evolved much later,” says Daniel Gruen of the Kavli Institute for Astrophysics and Cosmology of Particles (KIPAC). “The growth of these structures from the earliest ages of the universe to today matches what our models predict, showing that we can describe cosmic evolution very well.”
