Why is the expansion of our universe accelerating? Twenty-five years after its discovery, this phenomenon remains one of the greatest scientific mysteries. Solving this will require testing the fundamental laws of physics, including Albert Einstein’s theory of general relativity. A team from the Universities of Geneva (UNIGE) and Toulouse III – Paul Sabatier compared Einstein’s predictions with data from the Dark Energy Survey. Scientists discovered a small discrepancy that varies with different periods in cosmic history. These results, published in Nature communicationchallenge the validity of Einstein’s theories for explaining phenomena beyond our solar system on a universal scale.
According to Albert Einstein’s theory, the universe is deformed by matter, like a large, flexible sheet. These distortions, caused by the gravity of celestial bodies, are called ”gravitational sinks”. When light passes through this irregular framework, its trajectory is bent through these pits, similar to the effect of a glass lens. However, in this case it is gravity and not the glass that bends the light. This phenomenon is known as ‘gravitational lensing’.
Observing them provides insight into the components, history and expansion of the universe. The first measurement, made during a solar eclipse in 1919, confirmed Einstein’s theory, which predicted a bending of light twice that predicted by Isaac Newton. This difference arises from Einstein’s introduction of an important new element: the distortion of time, in addition to the distortion 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 want 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 mapping the shapes of hundreds of millions of galaxies – a team from the Universities of Geneva (UNIGE) and Toulouse III – Paul Sabatier provides new insights.
Until now, data from the Dark Energy Survey has been used to measure the distribution of matter in the universe. In our research, we used this data to directly measure the distortion of time and space, allowing us to compare our findings with Einstein’s predictions,” said Camille Bonvin, associate professor at the Department of Theoretical Physics at the UNIGE Faculty of Science, who led the investigation.
A small discrepancy
Thanks to the data from the Dark Energy Survey, scientists can look deep into space, and therefore far 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 have evolved over time, covering more than half of the history of the cosmos.
‘We discovered that in the distant past – 6 to 7 billion years ago – the depth of the wells matched Einstein’s predictions well. But closer to today, 3.5 to 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 the 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 sources – could be the same: on a large scale, gravity could operate under different laws of physics than those predicted by Einstein.
Challenge Einstein?
”Our results show that Einstein’s predictions have a 3 sigma incompatibility 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 that to happen, we would have to reach a 5 sigma threshold. It is therefore essential to have more precise measurements to confirm or refute these first results, and to find out whether this theory remains valid in our universe, at very large distances,” emphasizes Nastassia Grimm, postdoctoral researcher at 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, which was launched a year ago. As Euclid observes the universe from space, his gravitational lens measurements will be significantly more accurate. Furthermore, it is expected to observe approximately 1.5 billion galaxies within the six years of the mission. This will allow for more accurate measurements of space-time distortions, allowing us to look further back in time and ultimately test Einstein’s equations.
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