The standard model of how the universe works involves the widely accepted theory that dark energy is a fundamental constant of nature that drives the accelerating expansion of the universe. But a new analysis announced today by the international Dark Energy Spectroscopic Instrument (DESI) collaboration raises the possibility that this theory may need to be updated.
The idea of having to revisit something so fundamental and broadly agreed upon has cosmologists, including some at UC Santa Cruz, seriously considering that they might be closing in on a deeper understanding of the nature of the universe. The DESI analysis, shared by the collaboration in a presentation at the American Physical Society’s Global Physics Summit in Anaheim, Calif., suggests that dark energy—widely thought to be a “cosmological constant”—might be evolving over time in unexpected ways.
“What we are seeing is deeply intriguing,” said Alexie Leauthaud-Harnett, co-spokesperson for DESI and an assistant professor of astronomy and astrophysics 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.”
The results used the largest 3D map of our universe ever made to track dark energy’s influence over the past 11 billion years. The map was produced by the DESI collaboration, which includes 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).
Other collaborating professors at UC Santa Cruz include Connie Rockosi and J. Xavier Prochaska, both also in the Department of Astronomy and Astrophysics.
Combining different observations
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).
Sven Heydenreich, a postdoctoral research scholar at UC Santa Cruz, is co-leading the effort to combine these measurements under the rationale that the ones made by DESI and those measuring weak gravitational lensing have independently been identified as very good probes of dark energy. It’s the kind of work that speaks to the collaboration's ability to push the boundaries of scientific consensus by applying their findings across independent observations to see if they affirm a more universal truth.
“In cosmology, the main challenge is that there are so many factors we must account for, each of which can significantly alter our conclusions if we are not careful,” Heydenreich said. “And so, the hope is that if we combine these separate measurements, we find more conclusive evidence for or against an evolution of dark energy over time.”
Working toward discovery
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. A 5-sigma event has a 1 in 3.5 million chance of being a statistical fluke. That said, 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.)
“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 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.
UC Santa Cruz’s Rockosi led the commissioning of the instrument at theMayall telescope, and her present role is as an instrument scientist, helping to keep it running in top shape.
“There is a dedicated team of people at the observatory and throughout the collaboration that work very hard to make sure DESI can take great data,” Rockosi said. “It's very rewarding to see this result.”
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 measured 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 also shared their findings today in multiple papers that will be posted on the online repository arXiv, and they will soon begin work on additional analyses to extract even more information from the current dataset. DESI will continue collecting data, and 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.”
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.
