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 the greatest scientific mysteries of today. Breaking through requires testing the fundamental laws of physics, including Albert Einstein’s general relativity.

According to Einstein’s theory, our Universe is deformed under the influence of the matter found there, a bit like a large flexible sheet. These deformations, caused by the gravity of celestial bodies, are called “gravitational wells”. When light passes through this frame made up of irregularities, its trajectory is deviated by these wells, as if under the effect of a glass lens.

But here, it is gravity and not the glass that bends the light. This is what we call the “gravitational lensing” effect (video explanations). Observing this effect provides information on the components, 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 of Isaac Newton.

This difference is explained by the addition of a new “ingredient” by Einstein: the distortion of time, in addition to the distortion of space, to obtain the exact curvature of light.

But at the edges of the Universe, do these equations work? This is the question asked by many scientists who seek to quantify the density of matter in the cosmos and understand the acceleration of its expansion. Thanks to an unprecedented use of data from the Dark Energy Survey (an international program to record the shape of hundreds of millions of galaxies), a team from the universities of Geneva (UNIGE) and Toulouse III–Paul Sabatier provides new answers in “NatureCommunications.”

“Until now, data from the Dark Energy Survey were 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 of the Faculty of Sciences. of UNIGE, who directed this work.

Data from the Dark Energy Survey makes it possible to look very far into space, and therefore very far into the past. The Franco-Swiss team was thus able to carry out analyzes on 100 million galaxies, at four different times in the history of the Universe: 3.5, 5, 6 and 7 billion years ago. These measurements made it possible to know how gravitational wells grew over time, over a period that covers more than half the history of the cosmos.

“We discovered that, very far in the past, 6 and 7 billion years ago, the depth of the wells is completely consistent 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 Institute of research in astrophysics and planetology in Toulouse, first author of the study. It was also in this same period “near” today that the expansion of the Universe began to accelerate.

It is therefore possible that the response 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 inconsistency with the measurements. In the language of physics, such an incompatibility threshold arouses our interest and calls for further investigation.

But this incompatibility is not great enough, at this stage, to refute Einstein’s theory. For this, it would be necessary to reach a threshold of 5 sigma (If we obtain a result with a statistical significance of five sigmas, this means that it is almost certain that the difference observed is due to a new phenomenon and not to a statistical fluctuation, Editor’s note). It is therefore essential to have more, more precise measurements to confirm or refute these first results, and to know if this theory remains valid in our universe, at very great distances,” underlines Nastassia Grimm, postdoctoral researcher in the Department of Theoretical Physics of UNIGE, co-author of the study.

The team is preparing to analyze new data from the Euclid space telescope, launched a year ago. By observing the Universe from space, its gravitational lensing measurements are much more precise. In addition, Euclid will observe a phenomenal number of galaxies: around a billion and a half after six years of observation. This will make it possible to better measure spatio-temporal distortions, to go back even further in time, and to further test Einstein’s equations.