Scientists test a famous physicist’s predictions by calculating distortions in time and space. Why is the expansion of the universe accelerating? Even 25 years after its discovery, it remains one of science’s deepest mysteries. To solve it, you need to carefully study the fundamental laws of physics, including Albert Einstein’s theory of general relativity. Researchers from the University of Geneva (UNIGE) and Toulouse III – Paul Sabatier recently analyzed data from the study of dark energy to compare Einstein’s predictions with observed cosmic events. They found a small discrepancy that varies at different periods in the history of the universe.

These findings, published in Nature Communications, cast doubt on the ability of Einstein’s theories to fully explain the behavior of the universe at the largest scales.

Einstein’s equations clash with the mysteries of the universe

Albert Einstein’s theory suggests that the universe is distorted by matter, just as a flexible film bends under a heavy object. These distortions created by the gravitational force of huge celestial bodies are called “gravity wells”. As light passes through this rugged landscape, its path bends as it passes through these wells, just as a glass lens redirects light. However, it is gravity, not glass, that bends the light here. This effect is known as “gravitational lensing”.

Gravitational lensing observations help scientists understand the composition, history and expansion of the universe. The first measurement of this effect, made during a solar eclipse in 1919, confirmed Einstein’s prediction that light would be deflected; this was twice as much as predicted by Isaac Newton’s theory. This disagreement arose because Einstein introduced a groundbreaking concept: Time, like space, is distorted by gravity, creating a precise curvature that bends light.

Testing universal theories using modern data

Are these equations still valid at the edge of the universe? This question is being investigated by many scientists who are trying to measure the density of matter in space and understand the acceleration of its expansion. A team from the Universities of Geneva (UNIGE) and Paul Sabatier of Toulouse III provides new insights using data from the Dark Energy Survey, a project that maps the shapes of hundreds of millions of galaxies.

“Until now, dark energy research data have been used to measure the distribution of matter in the universe. In our study, we used these data to directly measure the distortion in time and space, allowing us to compare our findings with Einstein’s predictions,” says Camille, Associate Professor of Theoretical Physics at UNIGE’s Faculty of Natural Sciences, who led the research. Bonvain. .

A minor discrepancy

Data from the Dark Energy Probe allows scientists to look deep into space and therefore into the past. The French-Swiss team analyzed 100 million galaxies at four different points in the history of the universe: 3.5, 5, 6 and 7 billion years ago. These measurements showed how gravity wells evolved over time, spanning more than half of space history.

“We found that in the distant past, that is, between 6 and 7 billion years ago, the depth of the wells coincided with Einstein’s predictions. But closer to the present, between 3.5 and 5 billion years ago, they are slightly smaller than Einstein predicted,” says the University of Toulouse III Astrophysics and Planetology Isaac Tutusaus – Paul Sabatier, assistant astronomer at the Research Institute (IRAP/OMP). and lead author of the study.

Additionally, in this period close to the present day, the expansion of the universe began to accelerate. Therefore, the answer to the two phenomena—the acceleration of the universe and the slower growth of gravity wells—may be the same: At large scales, gravity may operate according to different physical laws than those predicted by Einstein.

Challenging Einstein?

“Our results show that Einstein’s predictions have a 3 sigma discrepancy with measurements. In the language of physics, such a discrepancy threshold intrigues us and requires further investigation. However, this discrepancy is not large enough to refute Einstein’s theory at this stage. 5 sigma is required for this to happen.” “Therefore, it is important to have more precise measurements to confirm or refute these initial results and also to find out whether this theory is valid over very large distances in our Universe.” emphasizes Nastassia Grimm, PhD student at UNIGE’s Theoretical Department. Physical. and co-author of the study.

The team is preparing to analyze new data from the Euclid space telescope, which was launched a year ago. Since Euclid observed the universe from space, gravitational lensing measurements would be much more accurate. In addition, the six-year mission is expected to observe approximately 1.5 billion galaxies. This will allow us to more precisely measure the distortion of space-time, peer deeper into time, and ultimately test Einstein’s equations.