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Scientists test the famous physicist’s predictions by calculating the distortion of time and space.
Why is the expansion of the Universe accelerating? Even 25 years after its discovery, this remains one of science’s most profound mysteries. Unraveling it requires scrutinizing the fundamental laws of physics, including Albert Einstein’s general relativity. Researchers from the University of Geneva (UNIGE) and Toulouse III – Paul Sabatier recently analyzed data from the Dark Energy Survey to compare Einstein’s predictions with observed cosmic phenomena. They identified a slight discrepancy that changes across different eras of the Universe’s history.
Published in Nature Communications, these findings raise questions about the ability of Einstein’s theories to fully explain the behavior of the Universe on the largest scales.
Einstein’s Equations Collide With the Mysteries of the Universe
Albert Einstein’s theory suggests that the Universe is warped by matter, much like a flexible sheet bends under a heavy object. These distortions, created by the gravitational pull of massive celestial bodies, are called “gravitational wells.” When light travels through this uneven landscape, its path bends as it passes through these wells, similar to how a glass lens redirects light. But here, it’s gravity—not glass—that bends the light. This effect is known as “gravitational lensing.”
Observing gravitational lensing helps scientists understand the composition, history, and expansion of the Universe. The first measurement of this effect, taken during a solar eclipse in 1919, confirmed Einstein’s prediction of light deflection—twice as large as what Isaac Newton’s theory suggested. This discrepancy arose because Einstein introduced a groundbreaking concept: that time, as well as space, is distorted by gravity, creating the precise curvature that bends light.
Validating Universal Theories Through Modern Data
Are these equations still valid at the edge of the Universe? This question is being explored by many scientists seeking to quantify the density of matter in the cosmos and to understand the acceleration of its expansion. Using data from the Dark Energy Survey—a project mapping the shapes of hundreds of millions of galaxies—a team from the universities of Geneva (UNIGE) and Toulouse III – Paul Sabatier is providing 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 this data to directly measure the distortion of time and space, enabling us to compare our findings with Einstein’s predictions,” says Camille Bonvin, associate professor in the Department of Theoretical Physics at the UNIGE Faculty of Science, who led the research.
A Slight Discrepancy
The Dark Energy Survey data allow scientists to look deep into space and, therefore, far into the past. The French-Swiss team analyzed 100 million galaxies at four different points in the Universe’s history: 3.5, 5, 6, and 7 billion years ago. These measurements revealed how gravitational wells have evolved over time, covering more than half of the cosmos’s history.
“We discovered that in the distant past — 6 and 7 billion years ago — the depth of the wells aligns well with Einstein’s predictions. However, closer to today, 3.5 and 5 billion years ago, they are slightly shallower than predicted by Einstein,” reveals Isaac Tutusaus, assistant astronomer at the Institute of Research in Astrophysics and Planetology (IRAP/OMP) at Université Toulouse III – Paul Sabatier and the study’s lead author.
It is also during this period, closer to today, that 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 could operate under different physical laws at large scales than those predicted by Einstein.
Challenging Einstein?
“Our results show that Einstein’s predictions have an incompatibility of 3 sigma with measurements. In the language of physics, such an incompatibility threshold arouses our interest and calls for further investigations. But this incompatibility is not large enough, at this stage, to invalidate Einstein’s theory. For that to happen, we would need to reach a threshold of 5 sigma. It is therefore essential to have more precise measurements to confirm or refute these initial results, and to find out whether this theory remains valid in our Universe, at very large distances,” emphasizes Nastassia Grimm, postdoctoral researcher in the Department of Theoretical Physics at 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, its measurements of gravitational lensing will be significantly more precise. Additionally, it is expected to observe about 1.5 billion galaxies within the six years of the mission. This will enable more accurate measurements of space-time distortions, allowing us to look further back in time and ultimately test Einstein’s equations.
Reference: “Measurement of the Weyl potential evolution from the first three years of dark energy survey data” by Isaac Tutusaus, Camille Bonvin and Nastassia Grimm, 11 November 2024, Nature Communications.
DOI: 10.1038/s41467-024-53363-6
