Dark energy breakthrough made with Kitt Peak telescope

Research team produces largest 3D map of the universe ever made

HENRY BREAN

Arizona Daily Star

The universe doesn't revolve around Kitt Peak, but the observatory southwest of Tucson is at the center of what could be the biggest breakthrough in the study of the cosmos in decades.

Using an advanced instrument mounted to the Mayall 4-meter Telescope at Kitt Peak, an international research team has produced the largest 3D map of the universe ever made and uncovered mounting proof of a confounding mystery: Once thought to be a 'cosmological constant,' the influence of dark energy actually appears to be weakening over time, in defiance of conventional wisdom about the universe.

Researchers unveiled their findings on March 19 as part of a massive new data release from the Dark Energy Spectroscopic Instrument — DESI for short — which has been scanning the sky above the 52-yearold Mayall Telescope since 2021.

'The universe never ceases to amaze and surprise us,' said DESI project scientist Arjun Dey in a written statement from the Tucson-based National Optical Infrared Astronomy Research Lab, or NOIRLab, which oversees Kitt Peak. 'By revealing the evolving textures of the fabric of our universe as never before, DESI and the Mayall Telescope are changing our very understanding of the future of our universe and nature itself.'

Dey is one of more than 900 researchers from around the world who are working together to diagram the large-scale structure of the universe and chart its expansion over the past 11 billion years or so.

The project, funded by the U.S. Department of Energy and managed by the Lawrence Berkeley National Laboratory in California, is now in year four of its 5-year mission to precisely measure roughly 50 million galaxies and other celestial objects.

The latest findings are based on the first three years of data collection at Kitt Peak, which includes measurements for some 13 million galaxies, 4 million individual stars and 1.6 million quasars, those distant cosmic spotlights powered by supermassive black holes.

Members of the DESI team shared what they have learned so far in multiple papers published online and presented earlier this month during the American Physical Society's Global Physics Summit in Anaheim, California.

Point of pride

Tucson-based NOIRLab astronomer Tod Lauer isn't directly involved in the DESI work, but he said it is a major achievement worthy of headlines. He called it 'the first really big advance' in our understanding of the universe in a quarter century, when dark energy was discovered in the first place.

'And this is a Tucson story. All the observations were done here. They were done at Kitt Peak,' Lauer said. 'This is something the town can be proud of.'

Kitt Peak also played a part in the initial dark-energy discovery in 1998. Some of the preliminary measurements that led to that stunning breakthrough were made using telescopes at the observatory, Lauer said.

The astronomers involved set out to use data from supernovas to more accurately measure the expansion rate of the universe. To their great surprise, they discovered that expansion was accelerating, not slowing as previously thought.

'It was an incredible shock,' Lauer said.

He compared it to tossing a baseball in the air, but instead of gravity bringing it back down to you, 'it speeds up and

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disappears into the wild blue yonder, never to be seen again.'

Dark energy is the name science has since adopted for the theoretical force that is somehow counteracting gravity and speeding up the universe's expansion.

'It's not understood physically,' Lauer said. 'People make up theories about it, but it's unknown. It's simply unknown.'

And now, on top of everything else, it looks like dark energy itself might be a moving target.

Big Bang music

DESI can capture the light from 5,000 galaxies at once, relaying it downstairs to 10 spectographs packed together in a room below the Mayall Telescope's main floor. Researchers are using the resulting measurements to chart the faint, wave-like reverberations from the explosion that sent the universe careening outward in all directions some 13.8 billion years ago.

'To be poetic about it, you can say we're listening to the music from the Big Bang,' Lauer said.

When coupled with other data, these echoes can serve as what he called 'a measuring rod' for determining the expansion rate of the universe across time.

'That has always been the bane of cosmology: finding a measuring rod,' he said.

Though DESI's findings have not reached the 'gold standard of scientific truth' just yet, they are getting close, Lauer said. 'A year ago, it looked intriguing. Now we have three years of data, and it looks even better.'

Researchers aren't just publishing papers about what they have found, either. The DESI team has made its entire dataset publicly available — all 270,000 gigabytes of it, spanning more than twice as many extragalactic objects as all previous 3D surveys of the universe combined.

The dataset covers seven times as much of the sky and contains roughly 10 times as much information as the team's previous release in April 2024, which included the first year of observations and preliminary readings taken during testing of the instrument.

By releasing everything, the team hopes to see a much larger pool of scientists comb through the data and develop their own ways to explain what dark energy is and how it behaves.

'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 senior scientist Michael Levi, who directs the DESI project 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.'

Lauer considers that a worthy pursuit all by itself, even though dark energy has stubbornly defied our attempts to understand it so far.

'If you have any ideas, we'd be glad to hear them,' he said with a laugh.

Contact reporter Henry Brean at hbrean@tucson.com.

On Twitter: @ RefriedBrean

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