New discovery challenges Einstein’s theory of general relativity

The accelerating expansion of the Universe has puzzled scientists since its discovery in 1998. This phenomenon, one of the most profound mysteries in physics, challenges our understanding of the cosmos.

Some theories suggest that this acceleration is driven by a cosmological constant, while others point to dynamic dark energy or even modifications of Einstein’s general relativity. To test these possibilities, researchers rely on large-scale structure surveys that map the distribution of matter and galaxies in the Universe.

Traditional models like ΛCDM have provided a strong foundation for understanding the Universe, but their limitations are becoming apparent. As a result, scientists have developed two key approaches to testing theories beyond ΛCDM.

One focuses on broad categories of theories, such as Horndeski theories, which unify scalar-tensor models with second-order equations of motion. This method directly ties observable data to theoretical components but suffers from significant degeneracies among the functions involved, limiting its effectiveness with current data.

The second approach takes a more direct, phenomenological route. It modifies Einstein’s equations by introducing two functions, μ and η, which describe changes in gravity and metric distortions.

μ addresses alterations in Poisson’s equation, while η captures differences in time and spatial distortions. These parameters are invaluable for analyzing gravitational lensing data from projects like the Dark Energy Survey (DES) and galaxy clustering measurements from BOSS and eBOSS.

However, this approach has its challenges. The constraints on μ and η are interdependent across different redshifts, requiring either a fixed evolution model or sophisticated reconstruction techniques. Moreover, the framework assumes the validity of Euler’s equation for dark matter, which introduces potential degeneracies that complicate data interpretation.

Einstein’s general relativity revolutionized our understanding of gravity, depicting the Universe as a flexible fabric deformed by massive objects. This theory, confirmed by the 1919 solar eclipse, predicted the deflection of light by gravity, a phenomenon known as gravitational lensing. By studying how light bends around celestial bodies, scientists gain critical insights into the Universe's structure and expansion.

But are Einstein’s equations valid on the largest cosmic scales? Researchers from the University of Geneva (UNIGE) and Toulouse III – Paul Sabatier have sought to answer this question. Using DES data, they analyzed the distortion of time and space, comparing their findings to Einstein’s predictions.

Camille Bonvin, an associate professor at UNIGE, highlights the novelty of this approach: "Until now, Dark Energy Survey data have been used to measure the distribution of matter in the Universe. In our study, we used this data to directly measure the distortion of time and space, enabling us to compare our findings with Einstein’s predictions."

The DES provided data on 100 million galaxies across four time periods: 3.5, 5, 6, and 7 billion years ago. These measurements revealed how gravitational wells—regions where gravity deforms space-time—evolved.

According to Isaac Tutusaus, lead author of the study, "We discovered that in the distant past — 6 and 7 billion years ago — the depth of the wells aligns well with Einstein’s predictions. However, closer to today, 3.5 and 5 billion years ago, they are slightly shallower than predicted by Einstein."

This discrepancy coincides with the period when the Universe’s expansion began accelerating. The data hint at a possible link between the two phenomena, suggesting that gravity might operate differently on cosmic scales. Tutusaus notes, "The slower growth of gravitational wells and the accelerating expansion might share the same underlying cause."

The study found a 3-sigma level of incompatibility between Einstein’s predictions and the observed data. While this is significant, it falls short of the 5-sigma threshold needed to definitively challenge the theory. Nastassia Grimm, a co-author and postdoctoral researcher at UNIGE, explains, "Such an incompatibility threshold arouses our interest and calls for further investigations. But this incompatibility is not large enough, at this stage, to invalidate Einstein’s theory."

Further research is necessary to confirm these findings. The team is turning to the Euclid space telescope, launched last year, which promises more precise gravitational lensing data. Over its six-year mission, Euclid will observe 1.5 billion galaxies, offering unprecedented insights into the Universe's history and allowing scientists to test Einstein’s equations more rigorously.

As cosmologists delve deeper into the mysteries of the Universe, the boundaries of established theories are being pushed. The work of the UNIGE and Toulouse teams represents a critical step in this journey.

With upcoming data from Euclid, the scientific community is poised to refine its understanding of gravity and cosmic expansion. These efforts may ultimately reshape our perception of the Universe, bringing us closer to solving one of science’s greatest enigmas.

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