The influence of dark energy could be changing over time, indicating that the conventional model of the universe might require a revision

Essential highlights:

• The DESI collaboration has released a fresh assessment of dark energy utilizing their initial three years of amassed data, encompassing nearly 15 million galaxies and quasars.
• Scientists have integrated the DESI findings with data from investigations of the cosmic microwave background, supernovae, and weak gravitational lensing.
• The conventional model of cosmology faces challenges in reconciling all observations when considered collectively—yet a framework where dark energy’s impact varies over time appears to align closely with the data.

The destiny of the universe is contingent on the equilibrium between matter and dark energy: the essential element that propels its accelerating expansion.

The Dark Energy Spectroscopic Instrument collaboration, comprising experts from the University of Michigan, is producing the most extensive 3D maps of the universe ever created to monitor dark energy’s impact over the preceding 11 billion years. With these latest findings, the team has bolstered the argument for what could represent a monumental shift in cosmology.

The findings suggest that dark energy, often regarded as a “cosmological constant,” may be evolving unpredictably over time.

“We are observing a transformation in human comprehension of the universe. I find that quite remarkable,” stated Uendert Andrade, a Leinweber research fellow at U-M and co-leader of the Year 3 Baryon Acoustic Oscillations team that delivered critical insights to analyze DESI’s findings.

DESI is a global initiative involving over 900 researchers from upwards of 70 institutions worldwide, overseen by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory.

The group disclosed its results in numerous papers scheduled to be available on the online repository arXiv and during a presentation at the American Physical Society’s Global Physics Summit in Anaheim, California.

“What we are observing is profoundly fascinating,” expressed Alexie Leauthaud-Harnett, co-spokesperson for DESI and a professor at the University of California, Santa Cruz. “It is exhilarating to consider that we might be on the verge of a significant breakthrough regarding dark energy and the foundational nature of our universe.”

DESI’s contingent from U-M also featured Dragan Huterer, a physics professor who co-led the team that interpreted and analyzed DESI’s initial year of cosmological data, which first indicated that dark energy was dynamic. Additionally, Gregory Tarlé, an emeritus physics professor who headed the group that designed the robotic systems managing DESI’s 5,000 robotic eyes, was involved in the endeavor.

“The trajectory of DESI has consistently been intertwined with the University of Michigan,” Andrade remarked. “It is thrilling to contribute to that legacy.”

The recent evaluation also involved U-M research scientist Michael Schubnell, who collaborated with Tarlé in constructing the robotic eyes, alongside graduate students Otavio Alves, Prakhar Bansal, Sikandar Hanif, and Jiaming Pan. Other contributors included Camille Avestruz, an associate professor of physics, undergraduate student Alexandra Wells, and postdoctoral researchers Humna Awan, Ming-Feng Ho, and Johannes Lange.

On the verge of a breakthrough

When considered independently, the data from DESI align with our standard universe model, known as Lambda CDM. CDM denotes cold dark matter, while lambda signifies the simplest scenario of dark energy, acting as a cosmological constant.

Nonetheless, when these findings are combined with other assessments, increasing evidence suggests that the influence of dark energy might be changing over time, indicating that alternative models might be more suitable. These additional measurements encompass the light remnants from the universe’s early moments (the cosmic microwave background, or CMB), explosive stars (supernovae), and the gravitational lensing effects on light from distant galaxies (weak lensing).

“We adhere to Occam’s razor, and the most straightforward clarification for our observations is evolving,” remarked Will Percival, co-spokesperson for DESI and a professor at the University of Waterloo. “It increasingly appears that we may need to adjust our conventional cosmological model to reconcile these disparate datasets—and changing dark energy looks promising.”

Thus far, the inclination toward an evolving dark energy has not attained “5 sigma,” the gold standard in physics that serves as the discovery benchmark. Nevertheless, varying combinations of DESI data with the CMB, weak lensing, and supernovae sets range from 2.8 to 4.2 sigma.

A 3-sigma event carries a 0.3% likelihood of being a statistical anomaly, but numerous 3-sigma occurrences in physics have dissipated with further data accumulation. The analysis utilized a method to obscure the results from researchers until the conclusion, helping to mitigate any unintentional bias regarding the data.

“Our goal is to allow the universe to reveal its workings, and perhaps the universe is indicating that it is more complex than we initially assumed,” expressed Andrei Cuceu, a postdoctoral researcher at Berkeley Lab and co-chair of DESI’s Lyman-alpha working group, which leverages the distribution of intergalactic hydrogen gas to chart the distant universe. “It’s intriguing and bolsters our confidence to observe that multiple lines of evidence are converging in the same direction.”

A narrative conveyed by 15 million galaxies and quasars

DESI stands as one of the most comprehensive surveys of the universe ever undertaken. This cutting-edge instrument can capture light from 5,000 galaxies at once and was developed 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, part of the NSF NOIRLab program in Arizona.

The project is currently in its fourth year out of five for sky surveys, and by its conclusion, it anticipates measuring approximately 50 million galaxies and quasars. Quasars are remarkably distant yet luminous objects hosting black holes at their centers.

The recent analysis employs data from the initial three years of observations and encompasses nearly 15 million of the most accurately measured galaxies and quasars. This represents a significant advancement, enhancing the experiment’s accuracy with a dataset that is more than twice that used in DESI’s first analysis, which also suggested an evolving dark energy.

“It’s not merely that the data persistently indicate a preference for evolving dark energy, but that the evidence is more robust than it was before,” noted Seshadri Nadathur, a professor at the University of Portsmouth and co-chair of DESI’s Galaxy and Quasar Clustering working group. “We’ve undertaken numerous additional tests compared to the initial year, and they are assuring us that the results are not influenced by any unidentified factors in the data that we haven’t considered.”

DESI analyzes dark energy’s impact by examining how matter is distributed throughout the universe. Events from the universe’s infancy left subtle traces in how matter is arranged, a phenomenon known as baryon acoustic oscillations, or BAO.

In the very early universe, following the Big Bang, a struggle ensued between light and matter, Andrade explained. The matter aimed to contract due to its gravity, while the light sought to rush outward, generating a pressure that countered gravity’s pull.

This clash produced a wave-like effect—comparable to a sound wave or ripples caused by a stone thrown into water—in the universe, which researchers now refer to as BAO.

“However, the universe’s expansion disrupted this conflict, essentially freezing this wave in time,” Andrade stated. “You have this ring that was coded into the universe early on, yet it persists and serves as a scale we can utilize for measurements.”

This BAO pattern functions as a standard ruler, with its apparent dimension in the sky directly influenced by the rate of the universe’s expansion. Measuring this ruler at various distances reveals the intensity of dark energy throughout history. DESI’s accuracy using this method is unparalleled globally.

“For several decades, we had this cosmological model that is truly impressive,” remarked Willem Elbers, a postdoctoral researcher at Durham University and co-chair of DESI’s Cosmological Parameter Estimation working group, which calculates the values that depict our universe. “As our data grow increasingly precise, we are uncovering potential flaws in the model and realizing we might require new insights to explain all the results cohesively.”

The collaboration is set to commence additional analyses to extract even more data from the current dataset, as DESI will keep gathering information. Other forthcoming experiments will also offer supplementary datasets for later analyses.

“Our findings provide fertile ground for our theoretical colleagues as they explore new and existing models, and we’re eager to witness their conclusions,” stated Michael Levi, DESI director and a scientist at Berkeley Lab. “Regardless of the nature of dark energy, it will determine the future of our universe. It’s quite extraordinary that we can gaze at the sky with our telescopes and attempt to resolve one of humanity’s most significant questions.”

Videos discussing the experiment’s new analysis can be found on the DESI YouTube channel.

While presenting its latest dark energy findings at the APS meeting today, the DESI collaboration also unveiled Data Release 1, or DR1, now accessible for anyone to explore. With data on millions of celestial objects, this dataset will facilitate a broad range of astrophysical research beyond DESI’s cosmological objectives.

DESI receives support from the DOE Office of Science and the National Energy Research Scientific Computing Center, a DOE Office of Science national user facility. Additional assistance for DESI comes from 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 DESI member institutions.

The DESI collaboration is privileged to have the opportunity to conduct scientific research on I’oligam Du’ag (Kitt Peak), a mountain of significant cultural importance to the Tohono O’odham Nation.

Written by Lauren Biron, Berkeley Lab