5,000 optical fibers are aimed at the sky inside the dome of the Kitt Peak National Observatory on a mountaintop in southern Arizona. Each fiber is a robotically operated mini-telescope that scans galaxies at speeds that would have seemed unattainable to astronomers even twenty years ago. They created the Dark Energy Spectroscopic Instrument, or DESI, with the primary goal of measuring the force pushing the universe apart and determining whether or not it has changed over cosmic time. The response it consistently gives is not what physicists were hoping for. And it’s becoming more difficult to rule out that response with every new dataset.
Results from the second year of observations were released in March 2025 by the DESI collaboration, which consists of over 900 researchers from more than 70 institutions worldwide, including teams at Durham, UCL, and Portsmouth. Something strange had been hinted at during the first year: dark energy, the enigmatic force propelling the universe’s accelerating expansion, might not be constant. Dark energy is treated as a fixed, unchanging property of space itself in the standard cosmological model, which is largely based on Albert Einstein’s theory of general relativity.
| Category | Details |
|---|---|
| Instrument | Dark Energy Spectroscopic Instrument (DESI) |
| Location | Kitt Peak National Observatory, near Tucson, Arizona |
| Technology | 5,000 robotically controlled optical fibres, each acting as a mini-telescope |
| Collaboration Size | 900+ researchers from 70+ institutions worldwide |
| Key Finding | Dark energy appears to be changing over time — not constant as Einstein’s model requires |
| First Hints | 2024 (Year 1 DESI data) |
| Confirmed Stronger | March 2025 (Year 2 DESI data) |
| Statistical Status | Not yet at formal “discovery” threshold — but growing stronger |
| Key Researchers | Prof. Ofer Lahav (UCL), Prof. Seshadri Nadathur (Portsmouth), Prof. Catherine Heymans (Edinburgh) |
| Related Mission | ESA’s Euclid Space Telescope (launched 2023) |
| Competing Theory | Modified Newtonian Dynamics (MOND), fifth-force “screening” models |
| NASA Involvement | Dr. Slava Turyshev (JPL) exploring solar system tests of dark energy theories |
| Reference Website | DESI – Dark Energy Spectroscopic Instrument |
The model breaks down fundamentally if that assumption is incorrect—that is, if dark energy becomes weaker or stronger over time—and physicists would need to find a new model to replace it. “The evidence is stronger now than it was,” stated University of Portsmouth professor Seshadri Nadathur. “We’ve also performed many additional tests compared to the first year, and they’re making us confident that the results aren’t driven by some unknown effect in the data.”
Even the discovery of dark energy in 1998 was shocking. Up until that point, the general consensus was that the universe would eventually slow down due to gravity’s pull after expanding outward since the Big Bang, much like a ball thrown upward eventually arcs back down. Rather, the expansion was accelerating, according to observations made by Australian and American scientists. They gave it the name “dark energy,” which was essentially a way to mark the shape of their ignorance and move on, because no one could satisfactorily explain what was causing it. For the next twenty-five years, dark energy—Einstein’s own term, which he had famously introduced but later rejected—was treated as the cosmological constant in the standard model, absorbing this unsettling fact. It was successful. Almost. It performed well enough to create a highly successful model of the structure and history of the universe.
The cosmological constant may not be constant, according to DESI. Because of the speed of light, which means that looking farther into space means looking farther back in time, DESI is able to measure the force that appears to behave differently when observed at different points in the universe’s history. Many scientists believed that the discrepancy in the data in 2024 was due to noise in the initial dataset, an artifact that would disappear with more observations. Rather, it has become more noticeable. “Dark energy appears to be even weirder than we thought,” said Prof.
Catherine Heymans of Edinburgh University, Scotland’s Astronomer Royal. Since the data was relatively new in 2024, nobody was entirely certain of it, and many believed that more work needed to be done. However, there is now more information and the scientific community is closely examining it. The formal statistical threshold that physicists use to declare a discovery has not yet been crossed by the result. However, it is obviously heading in that direction.
The controlled excitement that permeates the scientific language surrounding this discovery is difficult to ignore. These are cautious individuals who have been taught to be skeptical and are aware of the numerous promising anomalies that have vanished upon closer inspection. However, University College London professor Ofer Lahav, who is regarded by colleagues as one of the most cautious and esteemed researchers involved, told the BBC: “It is a dramatic moment.” We might be seeing a paradigm change in how we perceive the universe. A cosmologist who has spent decades witnessing extraordinary claims crumble under closer examination finds the term “paradigm shift” difficult to understand.
This opens up a truly bizarre theoretical landscape. What would cause dark energy to change over time is unknown. Models involving a hypothetical “fifth force” of nature—something other than gravity, electromagnetism, and the two nuclear forces—that exhibits distinct behaviors in areas of high and low density are among the contenders. There are various varieties of these “screening” models. According to the “chameleon” model, we don’t detect the fifth force locally because it is strong in low-density cosmic voids but suppressed to almost nothing in dense gravitational environments like our solar system. According to the “Vainshtein” model, the force is essentially immobilized by the gravitational pull of massive objects and only reemerges beyond a very large radius, possibly 400 light-years from the Sun, which is far beyond what is currently testable. Slava Turyshev, a NASA physicist, has been investigating the possibility of designing future solar system missions to test these theories locally, but he admits that better instruments and theoretical predictions are needed to know what to look for.
DESI will continue to operate in the interim. Its goal is to create a three-dimensional map of the distribution of matter over enormous periods of cosmic time by focusing on about 50 million galaxies and other bright objects. Launched in 2023 and currently releasing its first scientific images, the European Space Agency’s Euclid telescope will continue to explore with even more detail. They are both searching for a definitive response to a question that has loomed over cosmology since 1998: what is dark energy, and does it change? The map of the universe that physics has spent a century creating will need to be redrawn if the answer is yes, if this persistent blip turns out to be a confirmed discovery. Perhaps Einstein was right. However, he may be lacking.
