At Kitt Peak National Observatory near Tucson, Arizona, a telescope views far into the cosmos recording the locations of millions of galaxies. The four-meter telescope is currently mapping the universe to solve one of the biggest riddles in astrophysics: why is the universe expanding faster as time passes?  

Now a multi-institutional collaboration of more than 900 researchers from over 70 institutions around the world provided shared results posted to the online repository arXiv. It turns out that once again, Einstein’s theory of general relativity holds up. This time on a billion-light-year scale.

Based on observations of six million galaxies using the telescope’s Dark Energy Spectroscopic Instrument (DESI), mounted on the U.S. National Science Foundation’s Nicholas U. Mayall 4-meter Telescope at Kitt Peak, the findings also point to a deeper story about dark energy—the driver behind this accelerated expansion.

For over a century, General Relativity has explained gravity’s pull in our solar system and beyond. DESI’s latest findings extend that validation to the largest observable scales. By mapping the distribution of galaxies, researchers have shown that Einstein’s model of gravity still holds true even when applied over billions of years of cosmic history.

Leading the analysis was physicist Mustapha Ishak-Boushaki of the University of Texas in Dallas.

“For this round of DESI results, I focused my efforts at UT Dallas on conducting a large part of the analysis on gravity, which puts constraints on how matter in the universe moves and how large-scale structures, such as clusters of galaxies, evolve,” said Ishak-Boushaki, an astrophysicist whose research career has focused on questions in cosmology.

“The results from DESI, combined with datasets from other experiments, are consistent with general relativity theory operating at cosmic scales, although they do not completely exclude other theories of modified gravity.”

The complex analysis used nearly 6 million galaxies and quasars and lets researchers see up to 11 billion years into the past. With just one year of data, DESI has made the most precise overall measurement of the growth of structure, surpassing previous efforts that took decades to make. 

Still, the effort is not complete. Researchers continue to investigate theories of modified gravity to help to explain why the expansion of the cosmos is speeding up. DESI’s results currently point to Einstein, but other theories are not totally ruled out.

By simultaneously recording light from 5,000 galaxies, DESI is able to generate a three-dimensional cosmos map. This lets scientists track the development of cosmic formations across time. Examining how matter clusters at various levels, the most recent analysis takes a broad view, examining how matter clusters at different scales.

The results exceed simply supporting relativity. They back the theory that dark energy might not be continuous. Rather, it might actually develop, altering its influence on the cosmos over time.

“Dark matter makes up about a quarter of the universe, and dark energy makes up another 70 percent, and we don’t really know what either one is,” said Mark Maus, a PhD student at Berkeley Lab and UC Berkeley who worked on theory and validation modeling pipelines for the new analysis. “The idea that we can take pictures of the universe and tackle these big, fundamental questions is mind-blowing.”

DESI’s data isn’t limited to testing theories of gravity. It also emphasizes neutrinos, particles so tiny and elusive their features remain mostly unknown. DESI’s results reduced the conceivable range of neutrino masses, which is essential regarding their impact on the universe.

These particles, nearly weightless, interact weakly with other matter. Yet their combined mass plays a role in shaping the cosmos, affecting how galaxies form and cluster over time.

DESI is still early in its mission. The project has entered its fourth year of data collection, with plans to observe 40 million galaxies and quasars before it’s finished. The work already surpasses what previous surveys achieved over decades.