The universe might be changing: New DESI data shows dark energy may evolve over time

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 (DESI) 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 over 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 multiple papers that will be posted on the online repository arXiv and in a presentation at the American Physical Society’s Global Physics Summit in Anaheim, California.

These DESI researchers include Ohio University Professor of Physics Hee-Jong Seo, as well as several of her students such as Jaide Swanson, a third-year graduate student, who contributed to the first DESI BAO paper last year, Alberto Rosado-Marin, a fifth-year graduate student researching cosmic inflation using data from the DESI survey, and Alexandra Well, a senior Honors Tutorial College student working on the DESI survey in collaboration with scientists across the country.

Together with the broader DESI team, the OHIO team of researchers have deeply engaged in this expedited analysis over the past few months, incorporating larger data sets and refining their conclusions. The results are nothing short of remarkable, suggesting that the study may signal a paradigm shift in our understanding of dark energy —which drives the accelerated expansion of the Universe, but remains unexplained by current physical theories.

The first result released last year, co-led by Seo, sparked intrigue when the team uncovered unexpected results. The analysis, conducted without prior knowledge of the outcomes, revealed that dark energy appears to change over time rather than staying constant, as previously thought. The new follow-up analysis, informed by additional data, has provided even stronger evidence of this evolving dark energy.

"The results we're seeing are truly fascinating," Seo said. "When we first detected a strong indication of evolving dark energy last year, we were excited, but there was also a healthy skepticism wondering if it might just be a statistical anomaly."

Over the past six months, the team has conducted a fast-tracked analysis, incorporating more data to address those doubts.

"It was essential to verify whether the signal persisted with a larger dataset—and through this latest research we found that it does. With this new analysis, it's becoming clear that we may be on the verge of a real shift in our understanding of the universe,” Seo added.

Another major member of Seo’s team is Nick Sanders, a second-year graduate student, who joined the DESI collaboration last year and has since made substantial contributions to the new BAO analysis. He works as part of the Galaxy and Quasar Clustering Group of DESI, creating the combined tracer catalog - one of several datasets that went into the analysis presented.

According to Seo, in mapping the Universe across vast cosmic timescales, DESI uses multiple types of galaxies as tracers, with their data slightly overlapping to ensure complete coverage.

“Nick was crucial in seamlessly combining these overlapping layers into a continuous map, helping to create a clearer picture of the Universe’s expansion history,” Seo said.

His insights and contributions demonstrate the growing role of students in this international scientific effort, with their ideas directly shaping the direction of the research.

"The speed at which we completed this analysis and the exciting results we’ve seen are a testament to the incredible teamwork involved," Seo added. "Nick and other students have played essential roles in refining and optimizing the process. Overall, I think it’s a great opportunity for them to participate in such a significant scientific discovery and to collaborate with the world’s leading experts in the field."

“This work is my first contribution to a collaboration of the likes of DESI. In just the last eight months, I've met so many people working in the field. It also has gotten my name out and circulating amongst others in the field. Just a few weeks ago I met someone for the first time, and they recognized my name from hearing about the work I've been doing. That was the first time that happened to me,” Sanders said.

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 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).

“Dark energy, which is believed to make up roughly 70% of the universe, has long baffled scientists,” Seo said. “While the universe’s accelerating expansion has been known about for decades, the underlying cause remains elusive. The observational evidences so far appeared to agree with something called a cosmological constant model which originally was introduced by Einstein. However, for years, researchers have struggled to understand the true nature of dark energy so it’s exciting that our team may have helped provide a crucial piece to the puzzle.”

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 sets 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.

DESI is one of the most extensive surveys of the cosmos ever conducted. The state-of-the-art instrument can capture light from 5,000 galaxies simultaneously and was constructed and is operated with funding from the DOE Office of Science. DESI is mounted on the U.S. National Science Foundation’s Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory (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 evolving dark energy.

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.

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.

"This research is an exciting and rewarding experience not only for me but for all of my students," Seo said. "Being part of a global collaboration like this opens up immense opportunities for career development, and it's a privilege to work on such an important issue in science."

Videos discussing the experiment’s new analysis are available on the DESI YouTube channel. Alongside unveiling its latest dark energy results at the APS meeting today, the DESI collaboration also announced that its Data Release 1 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.

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science national user facility. Additional support for DESI is provided by the U.S. National Science Foundation; the Science and Technology Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the French Alternative Energies and Atomic Energy Commission (CEA); the National Council of Humanities, Sciences, and Technologies of Mexico; the Ministry of Science and Innovation of Spain; and by the DESI member institutions.

The DESI collaboration is honored to be permitted to conduct scientific research on I’oligam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.