Einstein’s equations collide with the mysteries of the Universe

Why is the expansion of our Universe accelerating? Twenty-five years after its discovery, this phenomenon remains one of today’s greatest scientific mysteries. To unravel it, we need to put the fundamental laws of physics to the test, including Albert Einstein’s general relativity. A team from the Universities of Geneva (UNIGE) and Toulouse III – Paul Sabatier has compared the predictions of the famous physicist with measurements based on data from the Dark Energy Survey program. They discovered a slight discrepancy, depending on the period in the history of the cosmos at which the calculations were made. These results, to be read in Nature Communications, challenge the validity of Einstein’s theories to explain phenomena at work outside the solar system, on the scale of the Universe.

According to Albert Einstein’s theory, our Universe deforms under the influence of the matter it contains, rather like a large, flexible sheet. These deformations, caused by the gravity of celestial bodies, are known as gravitational wells. When light passes through this frame of irregularities, its trajectory is deflected by these wells, as if by a glass lens. But here, it’s gravitation, not glass, that bends the light. This is known as the “ gravitational lensing ” effect.

Observation of this effect provides information on the constituents, history and expansion of the Universe. Its first measurement, in 1919 during a solar eclipse, confirmed Einstein’s theory, which predicted a deviation of light twice as great as that predicted by Isaac Newton. This difference is explained by the addition of a new “ingredient” by Einstein: the deformation of time, in addition to the deformation of space, to obtain the exact curvature of light.

Theory vs. data

But at the very edge of the Universe, do these equations work? This is the question posed by many scientists trying to quantify the density of matter in the cosmos and understand the acceleration of its expansion. A team from the Universities of Geneva (UNIGE) and Toulouse III – Paul Sabatier, using data from the Dark Energy Survey – an international program that surveys the shape of hundreds of millions of galaxies – has come up with new answers.

“Previously, Dark Energy Survey data had been used to measure the distribution of matter in the Universe. In our study, we used them to directly measure the distortion of time and space, and thus compare our results with Einstein’s predictions”, explains Camille Bonvin, associate professor in the Department of Theoretical Physics at the UNIGE Faculty of Science, who led this work.

A slight shift

Dark Energy Survey data allows us to look far into space, and therefore far into the past. The Franco-Swiss team was able to carry out analyses on 100 million galaxies, at four different points in the history of the Universe: 3.5, 5, 6 and 7 billion years ago. These measurements revealed how gravitational wells grew over time, covering more than half the history of the cosmos.

“We have discovered that far back in time, 6 and 7 billion years ago, the depth of the wells is completely compatible with Einstein’s predictions. On the other hand, in the period closer to today, 3.5 and 5 billion years ago, they are a little shallower than predicted by Einstein”, reveals Isaac Tutusaus, assistant astronomer at the Institut de recherche en astrophysique et planétologie (IRAP/OMP) at Toulouse III – Paul Sabatier University, first author of the study.

The expansion of the Universe also began to accelerate in the same period “close” to the present day. It is therefore possible that the answer to these two strange phenomena – the acceleration of the Universe and the slower growth of gravitational wells – is the same: gravitation could respond, on a large scale, to physical laws different from those of Einstein.

Enough to invalidate Einstein?

“Our results show that Einstein’s predictions have a 3 sigma incompatibility with measurements. In the language of physics, such an incompatibility threshold arouses our interest and calls for further investigations. But this incompatibility is not great enough, at this stage, to disprove Einstein’s theory. For that, a threshold of 5 sigma would have to be reached. It is therefore essential to have more precise measurements to confirm or refute these initial results, and to know whether this theory remains valid in our universe, at very great distances”, stresses Nastassia Grimm, postdoctoral fellow in the Department of Theoretical Physics at the UNIGE Faculty of Science, 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 are much more precise. In addition, Euclid will observe a phenomenal number of galaxies: around one and a half billion are expected after six years of observation. This will enable us to better measure space-time distortions, go even further back in time, and put Einstein’s equations to the ultimate test.

Further Resources

• Scientific paper : « Measurement of the Weyl potential evolution from the first three years of dark energy survey data« , Isaac Tutusaus, Camille Bonvin, Nastassia Grimm, Nature Communications, novembre 2024
• Press Release of the Université de Toulouse III – Paul Sabatier

IRAP Contact

• Isaac Tutusaus, isaac.tutusaus@irap.omp.eu