ACT collaboration unveils new map of dark matter distribution in the Universe

85% of the matter in the Universe is made up of mysterious 'dark matter' that is extremely difficult to detect. It emits no electromagnetic radiation and interacts little or not at all with ordinary matter. However, the presence of dark matter can be measured by its gravitational effect.

To track it down, a team of 160 scientists built the ACT telescope in the Chilean Andes. The instrument, one of the highest in the world at an altitude of 5 200 m, observes, with its 6 m diameter mirror and 6 000 ultra-precise sensors, bolometers cooled to 0.1K, the light emanating from the dawn of the formation of the Universe, when it was only 380 000 years old: a radiation known as the 'cosmic microwave background'.

During its 14-billion-year journey to Earth, light from the Cosmic Microwave Background is deflected by the gravitational forces exerted by all the structures in the Universe. This effect, called gravitational lensing, can be measured and allows the mass distribution in the Universe to be reconstructed. After several years of data collection and careful analysis, the collaboration has obtained a very detailed map of the dark matter distribution for about 25% of the sky's surface. The image shows a Universe made up of innumerable zones of varying mass richness, typically of the order of a few hundred million light years in size.

In line with the prediction of the standard model

"Remarkably, the size of the structures in the Universe and their growth over 14 billion years of evolution match exactly what is predicted by our standard model of cosmology based on Einstein's theory of gravity," says Mathew Madhavacheril, assistant professor in the Department of Physics and Astronomy at the University of Pennsylvania.

These new measurements are of fundamental importance to a debate that is currently stirring the cosmology community. Recent measurements of the gravitational lensing of galaxies (another way of measuring the mass distribution in the Universe) have produced results suggesting that the amplitude of the structures is not entirely consistent with the prediction of the Standard Model of Cosmology and have raised concerns about the validity of the model," says Blake Sherwin, Professor of Cosmology at the University of Cambridge. However, the latest results from the ACT data have accurately assessed that the vast lumps of matter seen in this image are exactly the right size."

To guard against possible confirmation bias, the analysis of the ACT data was conducted "blind", with the results only being compared with the theoretical predictions once the analysis was complete. "After years of complex analysis, I was shocked to see such perfect agreement between our measurements and the underlying theory," enthused Frank Qu, a PhD student at the University of Cambridge, who was the first author of the paper presenting the measurement and who first compared the data with the prediction of the Standard Model of Cosmology.

Understanding the evolution of structures in the Universe

"The measurements of the cosmic microwave background combined with those of the upcoming galaxy surveys will clarify our understanding of the evolution of structure in the Universe," adds Suzanne Staggs, professor of physics at Princeton University, who leads the ACT collaboration. These galaxy surveys will be carried out by the Vera-C Rubin Observatory, which will begin observations in 2024, and by the European space mission Euclid, scheduled for launch in July. These instruments will measure the position and shape of billions of galaxies, the same ones that are the source of the gravitational lensing of the cosmic microwave background.

The ACT telescope stopped taking data in September 2022, after 15 years of observations. It will be replaced by the Simons Observatory and CMB-S4 projects, which will make observations from a set of new telescopes. These instruments will be able to measure the cosmic microwave background with an accuracy an order of magnitude better than that of the ACT telescope.

The contribution to ACT of the CMB team of the Laboratoire de Physique des Deux Infinis Irène Joliot Curie (IJCLab) is made by Adrien La Posta (doctoral student), Thibaut Louis (CNRS research fellow) and Xavier Garrido (lecturer at the Université Paris Saclay). They played a major role in the production of this map of the distribution of matter in the Universe. In addition to their involvement in the scientific interpretation of the ACT data, they are responsible for the characterisation of possible systematic instrumental effects in the telescope measurements, as well as the absolute calibration of the data. "This contribution, at the heart of the data analysis, is based on an internationally recognised and appreciated expertise, which will allow them to play a major role in the interpretation of the Simons Observatory and CMB-S4 data", explains Sophie Henrot-Versillé, director of the A2C1 division of IJCLab.

IN2P3 laboratories are involved in the construction and scientific operation of the telescopes that will carry out future large-scale galaxy surveys, the Vera-C Rubin Observatory and the Euclid satellite. They also contribute to the preparation of the next generation of cosmic microwave background observatories, Simons Observatory and CMB-S4.

Text adapted from the English release, written by Nathi Magubane.

• 1Astroparticles, Astrophysics and Cosmology division