The Universe might not be as Einstein predicted

GenevaThe Universe is expanding, and it's doing so ever faster, although astronomers don't quite know why. To find out, a team of cosmologists from the University of Toulouse III - Paul Sabatier and the University of Geneva have put the current model of the Universe predicted by Einstein's own theories to the test. The results, published in the magazine Nature Communications, indicate that the theory of general relativity, which the German physicist developed more than a hundred years ago, may be insufficient to accurately describe the evolution of the universe. "We have enough evidence to believe that Einstein's theory is not definitive," says Isaac Tutusaus, a Catalan cosmologist at the University of Toulouse III - Paul Sabatier and co-author of the study.

"These observations are extremely important because they challenge the foundations of cosmology and help us understand what our universe is made of," says Camille Bonvin, co-author of the study, from her office at the University of Geneva, where she holds an associate professor position in the Department of Theoretical Physics.

A catalog of 100 million galaxies

One of the most widely held hypotheses explaining this accelerated expansion involves the existence of a new type of energy that causes the Universe to stretch, known as dark energy. Although its abundance can be estimated, the origin and nature of this energy are still unknown.

The Dark Energy Survey project was launched to measure the amount of dark energy contained in our Universe. This project uses a camera installed on the Victor M. Blanco telescope at the Cerro Tololo Inter-American Observatory, near the Chilean city of La Serena. One of the project's main objectives is to create a three-dimensional map of the galaxy population in a portion of the Universe. "Gravity causes galaxies to clump together and form structures. Studying these structures gives us information about dark matter and energy, which make up 95% of the Universe," says Tutusáus.

The team of researchers used part of the data collected by the Dark Energy Survey. Specifically, they analyzed how the Universe on a large scale modifies the path of light, an effect known as gravitational lensing, as it acts like a cosmic magnifying glass. The bending of light is an accurate indicator of the amount of matter—composed of galaxies, galaxy clusters, and other massive structures—that lies between the emitting source and us. "Due to the distortion, we observe that the galaxies are not in their true positions and appear distorted relative to their actual location," Bonvin comments.

The study collects data from 100 million galaxies. These come from different moments throughout the history of the Universe, and allow us to evaluate how the warping of space-time has evolved at different times. "What we have seen is that in the measurements of the most recent Universe, the warping effect is weaker than predicted by Einstein's theory," says Tutusáus. This discrepancy coincides precisely with the time when the expansion of the Universe began to accelerate.

A sprawling story

It was thanks to the deflection of light caused by the influence of the Sun that, in 1919, Arthur Eddington demonstrated the validity of the theory of general relativity that Einstein had published four years earlier. The equations of this theory describe how space and time are deformed under the influence of very massive bodies such as planets, stars, and galaxies. Based on these equations, cosmologist and priest Georges Lemaître predicted that the Universe must be expanding and that it began at a specific moment. Lemaître sowed the seeds of what we know today as the Big Bang. Lemaître's hypothesis, on the other hand, was not at all liked by Einstein himself, who considered the cleric's conclusion to be physically meaningless. So much so that Einstein modified his equations to describe a static Universe. However, in 1929, observations made by the American astronomer Edwin Hubble determined that the Universe is indeed expanding. Einstein had to admit that he had made the biggest mistake of his career.

The current model of the universe, known as ΛCDM (lambda-CDM), describes a wide range of phenomena. However, cosmologists have found some limitations that lead them to question whether, as the recent study suggests, Einstein could be "wrong" again.

Einstein will not have the last word

Despite being significant enough, the observed discrepancy is not yet definitive, as it could simply be due to statistical fluctuations. For this reason, scientists are cautious before concluding that the model of the Universe needs a major overhaul. "A theory is never true forever; it is only true for as long as it can explain the observations," says Bonvin, adding: "To refute Einstein, we need to reduce the uncertainties in our measurements even further."

Therefore, the next phase of observations includes using data from 1.5 billion galaxies recorded by the Euclid Space Telescope, located about 1.5 million kilometers from Earth and whose mission is to map space with very high precision, including the amount of dark matter and energy. The new data will provide fundamental information about the content of our Universe and the validity of the models that describe it.

Tutusáus greatly appreciates the fact that the data collected through these collaborations are open and that all researchers can benefit. "Sharing knowledge is essential to advancing our understanding of the Universe," he concludes.