By A&S News

Using the Dark Energy Spectroscopic Instrument (DESI) to observe 15 million galaxies and quasars, a team of astronomers has created the largest 3D map of our universe to date and tracked dark energy’s influence on the evolution of the cosmos over the past 11 billion years.

Combining their data with other experiments, the researchers uncovered signs that dark energy — the “force” powering the universe’s accelerating expansion — may be weakening over time instead of remaining constant.

This suggests that our understanding of how the universe works may need an update. 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.

“While we first saw hints of this in our previous results, the additional data has now strengthened this indication significantly,” says Ting Li, an assistant professor in the David A. Dunlap Department of Astronomy & Astrophysics and Dunlap Institute for Astronomy & Astrophysics, and chair of the DESI Milky Way Survey Working Group. “This finding suggests we may be on the brink of discovering entirely new physics beyond our current understanding.”

“We need more data to confirm this with certainty, but if this is true, it means we do not understand the stuff that makes up 67 per cent of our universe!” says Tanveer Karim, a postdoctoral fellow at the Dunlap Institute and member of the DESI collaboration.

“The 3D map that DESI has produced is the most detailed 3D image of the Universe produced to-date,” he says. “Before DESI, the largest such sample was the BOSS/eBOSS survey which measured distances to galaxies up to redshift of 1.1, that is when the universe was 5.5 billion years old. DESI has pushed this limit to redshift of 1.6, or when the universe was 4 billion years old.”

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

“What we are seeing is deeply intriguing,” says 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 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, aka weak lensing.

“We’re guided by Occam’s razor, and the simplest explanation for what we see is shifting,” says 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 the range from 2.8 to 4.2 sigma.

“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,” says 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 U.S. Department of Energy’s 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 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.

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 University of Toronto has played a very active and substantial role in the DESI collaboration,” says Li. “At U of T, our team comprises three faculty members: Professor Ray Carlberg, Professor Josh Speagle and myself — as well as four postdoctoral fellows including three Arts & Science Fellows and one AI Schmidt Fellow, three graduate students and numerous undergraduate students, all actively contributing to the DESI project.”

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,” says 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,” says 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.”

DESI is supported by the Department of Energy’s 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.

With files from the Lawrence Berkeley National Laboratory.