Why the universe’s expansion is accelerating remains a profound mystery in physics. Since its discovery over 25 years ago, scientists have struggled to explain what forces could drive such a phenomenon, which fundamentally challenges our understanding of cosmic dynamics. Central to this investigation is a deep analysis of the foundational laws of physics, particularly Albert Einstein’s theory of general relativity.
Einstein’s general relativity describes gravity not merely as a force but as a deformation of space and time. According to this theory, matter causes space to curve around it, creating “gravitational wells,” similar to placing a heavy object on a flexible sheet. The presence of mass warps the sheet, and objects moving nearby are influenced by the curvature, following the bent paths created by these gravitational wells. When light passes through these regions, its path is similarly distorted, a phenomenon known as “gravitational lensing.” This effect acts like a cosmic lens, bending light around massive objects and allowing scientists to observe and measure distant phenomena in the universe.
Gravitational lensing has proven invaluable for understanding the composition, history, and expansion rate of the universe. First observed in 1919 during a solar eclipse, this effect validated Einstein’s predictions: light from stars near the sun’s edge was bent, producing a deflection twice as large as what Isaac Newton’s theories would predict. This groundbreaking observation was possible because Einstein’s theory introduced a new component: time itself is also warped by gravity. The combined distortions of space and time create a more precise curvature that influences the paths of both matter and light.
Today, scientists are exploring whether these equations still hold true at the vast edges of the cosmos. If deviations from Einstein’s predictions are detected, they could help explain the accelerated expansion of the universe, possibly indicating a need for new physics. The Dark Energy Survey (DES), a project that maps hundreds of millions of galaxies across time and space, has recently provided data to help tackle these questions. Researchers from the University of Geneva (UNIGE) and the University of Toulouse III—Paul Sabatier have analyzed this data to gain insights into the behavior of gravitational wells over billions of years.
Typically, DES data has been used to examine how matter is distributed across the universe. In this recent study, however, the researchers directly measured the distortions in space and time caused by gravitational wells, comparing these observations with the predictions made by Einstein’s equations. Camille Bonvin, an associate professor in UNIGE’s Department of Theoretical Physics and leader of the study published in Nature Communications, notes that this approach provides an unprecedented way to observe gravitational evolution over time.
The DES data allows scientists to look back at various stages of cosmic history. By examining 100 million galaxies at four distinct points—3.5, 5, 6, and 7 billion years ago—the researchers could trace how gravitational wells have changed over the past several billion years. Their results revealed that Einstein’s predictions align well with the observed depths of these wells at earlier times, around 6 to 7 billion years ago. However, in more recent cosmic history, around 3.5 to 5 billion years ago, they observed that these wells are slightly shallower than Einstein’s equations would predict.
This period also coincides with the onset of the universe’s accelerating expansion, suggesting a possible link between the shallower gravitational wells and the increasing rate of expansion. The team’s findings raise the intriguing possibility that gravity may behave differently on the largest scales than we currently understand. Assistant astronomer Isaac Tutusaus from the Institute of Research in Astrophysics and Planetology (IRAP/OMP) at Toulouse III—Paul Sabatier, and the study’s lead author, explains that these discrepancies might suggest gravity could follow different physical laws on cosmological scales.
In physics, discrepancies are often measured in terms of “sigma” levels, which indicate the likelihood that a result is due to chance rather than a real effect. Currently, the inconsistency between Einstein’s theory and the team’s measurements is at a level of 3 sigma. In physics, this is a noteworthy discrepancy, suggesting that something interesting may be happening, but it falls short of the 5 sigma threshold typically required to declare a finding that challenges established theories.
Nastassia Grimm, a postdoctoral researcher at UNIGE and co-author of the study, stresses that further investigations with even more precise measurements are essential to confirm or refute these preliminary results. Only by examining Einstein’s equations on ever-larger scales can scientists determine whether they hold up across the universe or if they break down at certain distances, indicating the need for new models of cosmic behavior.
Fortunately, advancements in technology are making this possible. The research team is preparing to analyze new data from the Euclid space telescope, launched recently to observe the universe from an orbital vantage point. Euclid’s measurements of gravitational lensing will be far more precise than ground-based observations, and it is expected to examine around 1.5 billion galaxies over its six-year mission. This extensive survey will allow researchers to probe further back in time and space, gathering crucial data to test whether Einstein’s theory of relativity holds across the universe’s vast distances.
As Euclid’s data comes in, it will enable scientists to observe and quantify space-time distortions on a larger and more detailed scale than ever before. These insights could be pivotal in solving the mystery of the universe’s accelerating expansion and determining whether we need new physics to explain the cosmos at its largest scales. Until then, the data from projects like the Dark Energy Survey will continue to deepen our understanding, revealing hints about whether general relativity is truly universal or merely a close approximation for a portion of cosmic history.
Source: University of Geneva