By calculating the distortion of time and space, a Franco-Swiss team is putting the famous physicist’s predictions to the test.
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.
A team from the universities of Geneva (UNIGE) and Toulouse III – Paul Sabatier compared the predictions of the famous physicist with measurements based on data from the Dark Energy Survey program. She discovered that a slight shift could appear, depending on the periods in the history of the cosmos at which these calculations were carried out. These results, to be read in Nature Communicationsquestion the validity of Einstein’s theories to explain the phenomena at work outside the solar systemon the scale of the Universe.
According to Albert Einstein’s theory, our Universe is deformed under the influence of the matter found there, a bit like a large flexible tablecloth. These deformations, caused by the gravity of celestial bodies, are called “gravitational sinks”. 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 the “gravitational lensing” effect.
© ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi
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.
Theory vs Data
But at the edge 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 theacceleration 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.
“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 led this work.
A slight shift
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 compatible with Einstein’s predictions. In contrast, 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 for Research in astrophysics et planetology (IRAP/OMP) of theToulouse III University – Paul Sabatier, first author of the study.
It was also in this same period close to 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 an incompatibility of 3 sigma with the measurements. In the language of physics, such an incompatibility threshold arouses our interest and calls for additional investigations. But this incompatibility is not enough great, at this stage, to refute Einstein’s theory.
To do this, a threshold of 5 sigma would have to be reached. 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, post-doctoral student at the Department of Physics. theory from the Faculty of Sciences of UNIGE, co-author of the study.
The team is preparing to analyze new data from the Euclid space telescope, launched a year ago. Euclid observed the Universe from space, his gravitational lensing measurements were 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 make it possible to better measure spatio-temporal distortions, to go back even further in time, and to further test Einstein’s equations.
