The impact of dark energy may be evolving over time, suggesting that the standard model of how the universe works may need an update

Key points:

• The DESI collaboration published a new analysis of dark energy using their first three years of collected data, which spans nearly 15 million galaxies and quasars.
• Researchers combined the DESI data with information from studies of the cosmic microwave background, supernovae and weak gravitational lensing.
• The standard model of cosmology struggles to explain all the observations when taken together—but a model where dark energy’s influence changes over time seems to fit the data well.

The fate of the universe hinges on the balance between matter and dark energy: the fundamental ingredient that drives its accelerating expansion.

The Dark Energy Spectroscopic Instrument collaboration, which includes researchers from the University of Michigan, is generating the largest-ever 3D maps of the universe to track dark energy’s influence over the past 11 billion years. With its latest results, the team has strengthened the case for what would be a seismic shift in cosmology.

The results hint that dark energy, widely thought to be a “cosmological constant,” might be evolving over time in unexpected ways.

“We are witnessing a change in how humans understand the universe. I think that’s pretty incredible,” said Uendert Andrade, a Leinweber research fellow at U-M and co-leader of the Year 3 Baryon Acoustic Oscillations team that provided key insights to interpret DESI’s results.

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.

The collaboration shared its findings 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.

“What we are seeing is deeply intriguing,” said Alexie Leauthaud-Harnett, co-spokesperson for DESI and a professor at the University of California, 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.”

DESI’s U-M contingent also included Dragan Huterer, a professor of physics who was co-lead of the group analyzing and interpreting DESI’s first year of cosmological data that first suggested dark energy was dynamic. Gregory Tarlé, a professor emeritus of physics who led the team that built the robotics systems that control DESI’s 5,000 robotic eyes, was also on the project.

“DESI’s path has always run through the University of Michigan,” Andrade said. “It’s very exciting to help keep that up.”

The new analysis also included U-M research scientist Michael Schubnell, who also worked with Tarlé to construct the robotic eyes, and graduate students Otavio Alves, Prakhar Bansal, Sikandar Hanif and Jiaming Pan. Contributing to other efforts on the project were Camille Avestruz, an associate professor of physics, undergraduate student Alexandra Wells and postdoctoral fellows Humna Awan, Ming-Feng Ho and Johannes Lange.

Approaching the threshold of discovery

Taken alone, DESI’s data are consistent with our standard model of the universe, called Lambda CDM. 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 evolving 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).

“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 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.

“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.”

A story told by 15 million galaxies and quasars

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 and, by the time the project ends, it plans to measure roughly 50 million galaxies and quasars. Quasars are extremely distant yet bright objects with black holes at their cores.

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.

“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, a 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, or BAO.

In the very early universe, after the Big Bang, a conflict raged between the universe’s light and matter, Andrade said. The matter wanted to collapse in on itself because of its gravity, but the light wanted to rush outward, creating a pressure pushing in the opposite direction of gravity.

This fight created a wavelike phenomenon—similar to a sound wave or ripples from a rock tossed into a pond—in the universe that researchers now call BAO.

“But the expansion of the universe broke up the fight and this wave was basically frozen in spacetime,” Andrade said. “You have this ring that was imprinted on the universe at an early age, but it’s still there and it provides a scale we can use for measurements.”

That BAO pattern acts as a standard ruler, with its apparent size on the sky 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 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, or DR1, 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; Science and Technology Facilities Council of the United Kingdom; Gordon and Betty Moore Foundation; Heising-Simons Foundation; French Alternative Energies and Atomic Energy Commission; National Council of Humanities, Sciences, and Technologies of Mexico; Ministry of Science and Innovation of Spain; and by 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.

Written by Lauren Biron, Berkeley Lab