Gravity has shaped our cosmos. Its attractive influence turned tiny differences in the amount of matter present in the early universe into the sprawling strands of galaxies we see today.

A new study using data from the Dark Energy Spectroscopic Instrument, or DESI, has traced how this cosmic structure grew over the past 11 billion years, providing the most precise test to date of gravity at very large scales.

DESI is an international collaboration of more than 900 researchers from more than 70 institutions around the world, including the University of Michigan, and is managed by the Department of Energy’s Lawrence Berkeley National Laboratory.

The new DESI result validates our leading model of the universe and limits possible theories of modified gravity, which have been proposed to explain the observed accelerated expansion of the universe.

“This is the first time that DESI has looked at the growth of cosmic structure,” said Dragan Huterer, professor of physics at the University of Michigan and co-lead of DESI’s group analyzing and interpreting the cosmological data.

“DESI data have a tremendous ability to probe modified gravity and improve constraints on models of dark energy. And it’s only the tip of the iceberg.”

In their new study, DESI researchers found that gravity behaves as predicted by Einstein’s theory of general relativity.

“General relativity has been very well tested at the scale of solar systems, but we also needed to test that our assumption works at much larger scales,” said Pauline Zarrouk, a cosmologist at the French National Center for Scientific Research working at the Laboratory of Nuclear and High-Energy Physics, who co-led the new analysis. “Studying the rate at which galaxies formed lets us directly test our theories and, so far, we’re lining up with what general relativity predicts at cosmological scales.”

The study also provided new upper limits on the mass of neutrinos, the only fundamental particles whose masses have not yet been precisely measured.

Previous neutrino experiments found that the sum of the masses of the three types of neutrinos should be at least 0.059 eV/c2. For comparison, an electron has a mass of about 511,000 eV/c2. DESI’s results indicate that the sum should be less than 0.071 eV/c2, leaving a narrow window for neutrino masses.

The DESI collaboration has shared results in several studies posted to the online repository arXiv.

The complex analysis used nearly 6 million galaxies and quasars whose light was emitted between 1 and 11 billion years ago. With just one year of data, DESI has made the most precise overall measurement of the growth of structure, matching previous efforts in precision that took decades to make.

How Michigan helped create DESI

DESI is a state-of-the-art instrument that can capture light from 5,000 galaxies simultaneously. It does this using 5,000 tiny, robotic “eyes” that independently capture light from their target of interest.

Gregory Tarlé, U-M professor emeritus of physics, led the team that made the robotics that precisely control the position of each eye, which collect light with optical fibers.

“We control the position to better than one-tenth the diameter of a human hair,” Tarlé said. “These robotics move the position of the fiber very accurately to look at different galaxies and they do this every 15-20 minutes.”

The results provide an extended analysis of DESI’s first year of data. The researchers anticipate sharing another update on their measurements of dark energy and the expansion history of our universe in spring 2025. The collaboration is currently analyzing the first three years of collected data.

The experiment is now in its fourth of five years surveying the sky and plans to collect roughly 40 million galaxies and quasars by the time the project ends.

“Without the instrumentation built by Greg’s group, we’re not having this discussion,” Huterer said. “Without those 5,000 eyes in the sky, we wouldn’t be doing any of this now.”

Although DESI’s survey officially began in 2021, the process of approving and funding the instrument began more than a decade ago.

“After almost a decade of work, to have everything working with the instrument and to be getting real science out of it, it just feels good,” Tarlé said.

You may read the rest of the story on the University of Michigan News website.

More Information:
Dragan Huterer
Gregory Tarlé
Michael Schubnell
Uendert Andrade
Otavio Alves
Sikandar Hanif
Jiaming Pan