Gravitational lensing of distant galaxies by the galaxy cluster Abell 2390, observed by the Euclid satellite. © ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi
New research is challenging our fundamental understanding of the universe’s expansion
GENEVA, Switzerland — For over two decades, astronomers have been puzzled by the mysterious “dark energy” driving the ever-quickening expansion of our universe. Now, a team of scientists may have uncovered a crack in the bedrock of our cosmic understanding – one that could rewrite the rules of the universe as we know them.
By closely analyzing the warping of light from distant galaxies, the researchers found a subtle but perplexing discrepancy between what Albert Einstein’s theory of general relativity predicts about the expansion of the universe and what they’re actually observing. This incompatibility, while not yet definitive, has stirred up major questions about the validity of Einstein’s groundbreaking equations when applied to the largest scales of the cosmos.
‘‘Our results show that Einstein’s predictions have an incompatibility of 3 sigma with measurements. In the language of physics, 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. For that to happen, we would need to reach a threshold of 5 sigma. It is therefore essential to have more precise measurements to confirm or refute these initial results, and to find out whether this theory remains valid in our Universe, at very large distances,’’ emphasizes Nastassia Grimm, a postdoctoral researcher in the Department of Theoretical Physics at the University of Geneva, in a media release.
To conduct their analysis, published in the journal Nature Communications, the team examined data from the Dark Energy Survey, a massive mapping project that has tracked the shapes and positions of hundreds of millions of distant galaxies. By observing how the gravity of these faraway objects bends and distorts the paths of light traveling towards Earth – a phenomenon known as gravitational lensing – the researchers were able to construct a detailed 3D map of the hidden matter and energy that fills the universe.
Crucially, this map allowed them to measure the “gravitational wells” – the distortions in space-time caused by the pull of gravity – at four different points in cosmic history, stretching back over half the age of the universe. And that’s where they found the puzzling discrepancy.
‘‘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,’’ reveals Isaac Tutusaus, an assistant astronomer at the Institute of Research in Astrophysics and Planetology (IRAP/OMP) at Université Toulouse III – Paul Sabatier and the study’s lead author.
Intriguingly, this period of shallower gravitational wells coincides with the time when the expansion of the universe started to accelerate – a mystery that has baffled cosmologists since its discovery in the late 1990s. The team believes the two phenomena could be linked and that gravity may operate differently on the grandest cosmic scales than Einstein’s equations suggest.
‘‘It is therefore essential to have more precise measurements to confirm or refute these initial results, and to find out whether this theory remains valid in our Universe, at very large distances,’’ Grimm emphasizes.
Fortunately, the upcoming Euclid space telescope mission is expected to provide the team with an even more detailed 3D map of the universe, which they hope will shed further light on this cosmic conundrum.
For now, the search continues for a fuller understanding of the mysterious forces driving our universe’s ceaseless expansion. But one thing is certain: the foundations of our cosmic knowledge are shaking, and a paradigm shift in our understanding of gravity and space-time may be on the horizon.
Paper Summary
Methodology
In this study, researchers measured the Weyl potential, which tells us about changes in the universe’s structure, using data from the Dark Energy Survey (DES). This survey captured data over several years to help scientists understand the forces shaping the universe. Specifically, they used galaxy-galaxy lensing and clustering data to track changes in the Weyl potential over time, dividing observations into four periods based on the distance of galaxies (or redshift bins).
These bins allowed the researchers to see how the universe’s structure evolves at different times. Importantly, this method didn’t assume any specific theories about gravity, making the results versatile and widely applicable to current models of gravity.
Key Results
The study found that in the first two time periods (the closest ones), the Weyl potential was lower than expected based on standard models. This lower measurement hints at a potential mismatch between what we see in the distant universe (measured by cosmic microwave background radiation) and more recent observations.
Simply put, the universe’s structure might be evolving differently than what current models suggest, especially in these closer periods. However, for the last two time periods, measurements were closer to what the standard model predicts, so any difference might only be happening in the more recent cosmic history.
Study Limitations
One main limitation of this study is that measuring the Weyl potential relies on large-scale structures, which can be impacted by minor distortions or “noises” in the data from each time period. Additionally, certain assumptions made in the methods (like choosing specific angular scales and using data from particular redshifts) can affect the precision of the results. Another challenge is that some aspects of gravitational theories are hard to separate just from this dataset alone, requiring additional studies for stronger conclusions.
Discussion & Takeaways
These findings could have interesting implications for our understanding of gravity and the universe. The results suggest a possible tension between older, far-off measurements and more recent observations. This could mean that the universe’s structure is evolving in unexpected ways. It might also encourage scientists to consider new models of gravity that go beyond what we currently know. If future studies with larger datasets confirm these results, this could lead to adjustments in how we understand the universe’s evolution and gravity itself.
Funding & Disclosures
This research was primarily conducted at the Institut de Recherche en Astrophysique et Planétologie (IRAP) in France and the University of Geneva in Switzerland, as part of the Dark Energy Survey Collaboration. Additional funding support came from institutions like CNRS, UPS, and CNES in France. No significant conflicts of interest or other disclosures were reported by the researchers.
