According to Albert Einstein’s theory, the Universe is deformed by matter, like a large, flexible sheet. These deformations, caused by the gravity of celestial bodies, are called “gravitational wells”. When light passes through this irregular frame, its path is bent by these wells, similar to the effect of a glass lens. However, in this case, it is gravity, not the glass, that bends the light. This phenomenon is known as “gravitational lensing”.

Observing it provides insights into the components, history and expansion of the Universe. His first measurement, made during a solar eclipse in 1919, confirmed Einstein’s theory, which predicted a deflection of light twice that predicted by Isaac Newton. This difference stems from Einstein’s introduction of a key new element: the warping of time, in addition to the warping of space, to achieve the exact curvature of light.

Theory vs. data

Are these equations still valid at the edge of the Universe? This question is being explored by many scientists who seek to quantify the density of matter in the cosmos and understand the acceleration of its expansion. Using data from the Dark Energy Survey — a project to map the shapes of hundreds of millions of galaxies — a team from the universities of Geneva (UNIGE) and Toulouse III — Paul Sabatier offers new insights.

“Until now, Dark Energy Survey 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 of time and space, allowing us to compare our findings with Einstein’s predictions,” says Camille Bonvin, associate professor at the Department of Theoretical Physics at the UNIGE Faculty of Sciences, who conducted the research.

A slight discrepancy

The Dark Energy Survey data allows scientists to look deep into space, and therefore far into the past. The Franco-Swiss team analyzed 100 million galaxies at four different times in the history of the Universe: 3.5, 5, 6 and 7 billion years ago. These measurements revealed how gravity wells have evolved over time, covering more than half of the history of the cosmos.

“We found that in the distant past — 6 and 7 billion years ago — the depth of the wells lines up well with Einstein’s predictions. However, closer to today, 3.5 and 5 billion years ago, they are slightly shallower than Einstein predicted,” reveals Isaac Tutusaus, assistant astronomer at the Research Institute for Astrophysics and Planetology (IRAP/OMP ) of Toulouse III University — Paul Sabatier and lead author of the study.

Also during this period, closer to today, the expansion of the Universe began to accelerate. Therefore, the answer to two phenomena — the acceleration of the Universe and the slower growth of gravitational wells — may be the same: gravity might operate under different large-scale physical laws than those predicted by Einstein.

Are you challenging Einstein?

”Our results show that Einstein’s predictions have a 3-sigma mismatch with measurements. In the language of physics, such an incompatibility threshold piques our interest and calls for further investigation. But this incompatibility is not great enough, at this stage, to invalidate Einstein’s theory. For this to happen, we would need to reach a threshold of 5 sigma. Therefore, it is essential to have more precise measurements to confirm or deny these initial results and to find out if this theory remains valid in our Universe, at very large distances”, emphasizes Nastassia Grimm, postdoctoral researcher in the Department of Theoretical Physics from UNIGE and co-author of the study.

The team is preparing to analyze new data from the Euclid space telescope, launched a year ago. As Euclid observes the Universe from space, his measurements of gravitational lensing will be significantly more accurate. In addition, it expects to observe about 1.5 billion galaxies during the six-year mission. This will enable more precise measurements of space-time distortions, allowing us to look further back in time and ultimately test Einstein’s equations.