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New DESI results strengthen hints that dark energy may evolve

The fate of the universe hinges on the balance between matter and dark energy: the fundamental ingredient that drives its accelerating expansion. New results from the Dark Energy Spectroscopic Instrument () collaboration use the largest 3D map of our universe ever made to track dark energy's influence over the past 11 billion years. Researchers see hints that dark energy, widely thought to be a "cosmological constant," might be evolving over time in unexpected ways.

DESI is an international experiment with more than 900 researchers from more than 70 institutions around the world and is managed by the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab). The collaboration shared their findings today in that will be posted on the online repository arXiv and in a presentation at the in Anaheim, California.

"What we are seeing is deeply intriguing," said Alexie Leauthaud-Harnett, co-spokesperson for DESI and a professor at UC Santa Cruz. "It is exciting to think that we may be on the cusp of a major discovery about dark energy and the fundamental nature of our universe."

Taken alone, DESI's data are consistent with our standard model of the universe: Lambda CDM (where CDM is cold dark matter and Lambda represents the simplest case of dark energy, where it acts as a cosmological constant). However, when paired with other measurements, there are mounting indications that the impact of dark energy may be weakening over time and that other models may be a better fit.

Those other measurements include the light leftover from the dawn of the universe (the cosmic microwave background or CMB), exploding stars (supernovae), and how light from distant galaxies is warped by gravity (weak lensing).

"We're guided by Occam's razor, and the simplest explanation for what we see is shifting," said Will Percival, co-spokesperson for DESI and a professor at the University of Waterloo. "It's looking more and more like we may need to modify our standard model of cosmology to make these different datasets make sense together—and evolving dark energy seems promising."

So far, the preference for an evolving dark energy has not risen to "5 sigma," the gold standard in physics that represents the threshold for a discovery. However, different combinations of DESI data with the CMB, weak lensing, and supernovae datasets range from 2.8 to 4.2 sigma. (A 3-sigma event has a 0.3% chance of being a statistical fluke, but many 3-sigma events in physics have faded away with more data.) The analysis used a technique to hide the results from the scientists until the end, mitigating any unconscious bias about the data.

"We're in the business of letting the universe tell us how it works, and maybe the universe is telling us it's more complicated than we thought it was," said Andrei Cuceu, a postdoctoral researcher at Berkeley Lab and co-chair of DESI's Lyman-alpha working group, which uses the distribution of intergalactic hydrogen gas to map the distant universe. "It's interesting and gives us more confidence to see that many different lines of evidence are pointing in the same direction."

DESI is one of the most extensive surveys of the cosmos ever conducted. The state-of-the-art instrument captures light from 5,000 galaxies simultaneously. DESI is mounted on the U.S. National Science Foundation's Nicholas U. Mayall 4-meter Telescope at (a program of NSF NOIRLab) in Arizona. The experiment is now in its fourth of five years surveying the sky, with plans to measure roughly 50 million galaxies and quasars (extremely distant yet bright objects with black holes at their cores) by the time the project ends.

The new analysis uses data from the first three years of observations and includes nearly 15 million of the best measured galaxies and quasars. It's a major leap forward, improving the experiment's precision with a dataset that is more than double what was used in DESI's first analysis, which also hinted at an .

"It's not just that the data continue to show a preference for evolving dark energy, but that the evidence is stronger now than it was," said Seshadri Nadathur, professor at the University of Portsmouth and co-chair of DESI's Galaxy and Quasar Clustering working group. "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 that we haven't accounted for."

DESI tracks dark energy's influence by studying how matter is spread across the universe. Events in the very early universe left subtle patterns in how matter is distributed, a feature called baryon acoustic oscillations (BAO). That BAO pattern acts as a standard ruler, with its size at different times directly affected by how the universe was expanding. Measuring the ruler at different distances shows researchers the strength of dark energy throughout history. DESI's precision with this approach is the best in the world.

"For a couple of decades, we've had this standard model of cosmology that is really impressive," said Willem Elbers, a postdoctoral researcher at Durham University and co-chair of DESI's Cosmological Parameter Estimation working group, which works out the numbers that describe our universe. "As our data are getting more and more precise, we're finding potential cracks in the model and realizing we may need something new to explain all the results together."

The collaboration will soon begin work on additional analyses to extract even more information from the current dataset, and DESI will continue collecting data. Other experiments coming online over the next several years will also provide complementary datasets for future analyses.

"Our results are fertile ground for our theory colleagues as they look at new and existing models, and we're excited to see what they come up with," said Michael Levi, DESI director and a scientist at Berkeley Lab. "Whatever the nature of dark energy is, it will shape the future of our universe. It's pretty remarkable that we can look up at the sky with our telescopes and try to answer one of the biggest questions that humanity has ever asked."

Videos discussing the experiment's new analysis are available on the . Alongside unveiling its latest dark energy results at the APS meeting today, the DESI collaboration also announced that its Data Release 1 (DR1), which contains the first 13 months of main survey data, is now available for anyone to explore. With information on millions of celestial objects, the dataset will support a wide range of astrophysical research by others, in addition to DESI's cosmology goals.