The mystery of the universe’s accelerated expansion remains one of the great enigmas of modern physics. Twenty-five years ago, scientists discovered that the universe is expanding at an increasingly rapid rate, a phenomenon for which no definitive explanation has yet been found.

Understanding this acceleration involves testing the fundamental laws of physics, particularly Albert Einstein’s theory of general relativity. A team of researchers from the universities of Geneva (UNIGE) and Toulouse III – Paul Sabatier has compared Einstein’s predictions with data from the Dark Energy Survey (DES). The findings reveal a slight discrepancy that varies depending on the period of cosmic history analyzed, raising questions about the validity of Einstein’s theories when applied to phenomena occurring on cosmic scales.

Einstein’s theory of general relativity describes the universe as a flexible and deformable space-time under the influence of matter. Imagine this space-time as an elastic surface that curves around massive objects, creating what are known as gravitational wells. When light passes through this warped surface, its path bends according to the shape of space-time, an effect that resembles that of an optical lens. This phenomenon is known as gravitational lensing. Since its first measurement in 1919 during a solar eclipse, observations of gravitational lenses have proven to be key tools for studying the distribution of matter in the universe and exploring its expansion.

Gravitational lens
Gravitational lensing of distant galaxies by the galaxy cluster Abell 2390, observed by the Euclid satellite. Credit: J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi / ESA/Euclid/Euclid Consortium/NASA

This recent study utilizes data from the Dark Energy Survey, a project that maps the shapes of hundreds of millions of galaxies in an effort to understand how matter is distributed and how the universe evolves. Rather than focusing solely on the distribution of matter, scientists from this team used the data to directly measure the distortion of space and time, allowing them to compare their observations with Einstein’s theoretical predictions.

By examining these observations, the Franco-Swiss team studied 100 million galaxies at four different points in cosmic history: 3.5, 5, 6, and 7 billion years ago. This variety of time periods allows researchers to observe how gravitational wells have changed over time, providing a view of more than half of the universe’s history.

The results revealed that 6 to 7 billion years ago, gravitational wells were as deep as Einstein predicted; however, in more recent times, about 3.5 to 5 billion years ago, these wells were slightly shallower than expected. This period coincides with the beginning of the universe’s accelerated expansion, suggesting that the two phenomena could be related and that the laws of gravity, as we currently understand them, might differ on a large scale.

Gravitational lens
This illustration shows how astronomers use gravitational lensing to study faint, distant galaxies by observing the distorted light emitted from them. Credit: NASA, ESA & L. Calçada / Wikimedia Commons

These observations test the robustness of Einstein’s theory in the cosmological realm. Nastassia Grimm, a postdoctoral researcher at the University of Geneva and co-author of the study, explains that the discrepancy found reaches a 3-sigma level of significance. In physics terms, this level of incompatibility draws significant interest from the scientific community and justifies the need for additional research. However, to definitively refute Einstein’s theory, it would be necessary to reach a 5-sigma threshold, which would require even more precise data.

In this context, scientists are hopeful that the Euclid space telescope, launched a year ago, will provide much more detailed measurements. This telescope is designed to observe approximately 1.5 billion galaxies over a six-year period, and its precision in observing gravitational lenses will allow researchers to look even further back in time and rigorously test Einstein’s equations. With Euclid, astronomers could achieve a deeper understanding of space-time distortions and potentially solve the mystery of cosmic acceleration.

The universe’s accelerated expansion remains a puzzle that challenges our understanding of physics and the cosmos. Observations by the team from the universities of Geneva and Toulouse represent a step toward a potential re-evaluation of the fundamental laws governing the universe. Although the theory of general relativity remains one of the cornerstones of modern physics, these findings suggest that, in the vast expanses of the cosmos, there may be aspects of gravity that we do not yet fully understand.

SOURCES

Université de Genève

Tutusaus, I., Bonvin, C. & Grimm, N. Measurement of the Weyl potential evolution from the first three years of dark energy survey data. Nat Commun 15, 9295 (2024). doi.org/10.1038/s41467-024-53363-6

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