Einstein’s equations waver at the edge of the Universe – rts.ch

The fundamental laws of physics seek to describe the world in which we live: they succeed in their mission quite well, particularly with Einstein’s general relativity. But this theory fails to explain the acceleration in the expansion of the Universe which began roughly 5.5 billion years ago: this was measured for the first time in 1998. The great physicist, for his part, predicted a slowdown. Today, scientists are increasing their observations to unravel what remains one of the greatest scientific mysteries of today.

A team from the universities of Geneva and Toulouse III – Paul Sabatier compared Albert Einstein’s predictions with measurements based on data from the Dark Energy Survey (DES), an international program that maps hundreds of millions of galaxies and catalogs their shapes.

Cosmologists have exploited these results in an innovative way: “DES data were until now used to measure the distribution of matter in the Universe. We used them to directly measure the distortion of time and space. “space and thus compare our results with Einstein’s predictions”, explains to RTSinfo Camille Bonvin, associate professor in the Department of Theoretical Physics of the Faculty of Sciences of UNIGE, who led this work.

Because Albert Einstein had theorized it: our Universe is deformed with the matter found there, a bit like a large fabric in the middle of which there is a stone. The deformations caused by the gravity of celestial bodies – stones – are called gravitational wells: these deflect the trajectory of light, like a glass lens. Now, in this case, it is good gravitation which bends light and not a lens: this effect is called gravitational lensing (read box).

>> Galaxies distorted by gravitational lensing:

Disagree with observations

But this becomes complicated on the gigantic scale of our Universe, when the deformation of space and time is scrutinized with a large number of very distant galaxies: in this case, the trajectory of light is not deviated as expected .

According to thestudy published Monday in Nature Communications, the distortions of gravitational wells are smaller than predicted by Einstein’s equations: “We get a disagreement of 99.7%, which is already a pretty big discrepancy. In our scientific jargon, we let’s say that there is an incompatibility of 3 sigma with the measurements”, underlines Camille Bonvin, adding: “But that is not enough to call Einstein into question. We would like a disagreement of 99.99994%, or one. 5 sigma threshold.”

“It is essential to have more measurements, more precise, to confirm or refute these first results, and to know if this theory remains valid in our Universe, at very great distances”, remarks in a press release Nastassia Grimm, post-doctoral student at the Department of Theoretical Physics of the Faculty of Sciences of UNIGE, co-author of the study.

And these measurements are the telescope Euclidsent into space on July 1, 2023, which will bring them by March or April: “We are impatiently waiting for them,” rejoices Camille Bonvin. “It is more precise, because it measures from space. It will look at a larger period of the Universe and observe a phenomenal number of galaxies: around a billion and a half over six years of observation. Euclid will allow us to reduce uncertainties.”

>> Read the Large Format: Euclid will challenge Einstein’s general relativity

Fight between general relativity and dark energy

For now, the accelerated expansion of the Universe is explained by the possibility that there is a new form of energy in the Universe, very enigmatic, called dark energy: “It is everywhere and is very strange; it represents 70% of our Universe”, explains Camille Bonvin. “This dark energy would be like a kind of engine that accelerates the Universe, but we have never seen or detected it.”

Scientists do not yet know if the key to the mystery lies in this energy or if it is a question of flaws in the theory of general relativity which would not work at very large distances.

I would prefer that Einstein be shown to be wrong!

As for Camille Bonvin, her heart and mind as a cosmologist still lean to one side: “It is the observations that will tell us. Personally, I would prefer that we show that Einstein was wrong! It would be sad to abandon general relativity… but this theory would be subsumed into another, larger one. Just as is the case for Newton, who was not really put aside: his theory of gravity works for the Earth, but from. whether we move away and go to Mercury or the scale of the Solar System, we must move on to Einstein.”

For the physicist, keeping general relativity by adding dark energy is an entirely possible path: “But if we find that the acceleration of the Universe is due to gravity itself and to the fact that the theory of Einstein no longer works on the scale of the Universe, it would really be a leap forward for science!”, she concludes.

Stéphanie Jaquet