General relativity has undergone one of its most precise tests yet, thanks to observations from the Dark Energy Spectroscopic Instrument (DESI), which has been tracking the evolution of the cosmos over the past 11 billion years.

Albert Einstein’s 1915 theory of general relativity has been the leading explanation for gravity for over a century. It has been used to model the universe’s evolution, explaining how gravity pulled matter together to form galaxies and clusters of galaxies. While general relativity has been thoroughly tested on small scales, it hasn’t been as extensively tested on large cosmological scales. Now, with the help of DESI, scientists have conducted such a test by observing nearly 6 million galaxies and quasars, providing insight into the universe’s evolution since it was about 3 billion years old. The results confirm that general relativity continues to accurately describe gravity on large scales.

“General relativity has been well-tested within solar systems, but we needed to see if it works on much larger scales,” said Pauline Zarrouk, co-leader of the study from the French National Center for Scientific Research. “By studying how galaxies formed, we’re directly testing our theories, and so far, we’re aligned with general relativity’s predictions at cosmological scales.”

DESI, installed at Kitt Peak National Observatory on the Nicholas U. Mayall 4-meter Telescope, is a cutting-edge instrument with 5,000 “robotic eyes.” The survey is now in its fourth year of a five-year project, which will eventually survey about 40 million galaxies and quasars.

The data from DESI may be key to understanding dark energy and dark matter, the mysterious substances that make up roughly 95% of the universe’s content, yet remain largely invisible. “Dark matter makes up about a quarter of the universe, and dark energy accounts for another 70%, but we don’t yet know what either of them are,” said Mark Maus, a Berkeley Lab PhD student. “The idea that we can capture images of the universe and tackle these huge questions is mind-blowing.”

While general relativity is the best current description of gravity, it doesn’t explain all observed phenomena, such as the accelerating expansion of the universe or the gravitational effects of dark matter. This has led to the development of alternative theories of gravity that modify Einstein’s theory, in an attempt to explain cosmic observations without invoking dark energy.

The DESI findings not only reinforce the Lambda Cold Dark Matter (LCDM) model based on general relativity, but they also help rule out some modified gravity theories. Additionally, the results have placed an upper limit on the mass of neutrinos, elusive particles that are nearly massless and interact weakly with other matter. DESI’s data provides a more precise mass range for neutrinos, refining our understanding of these ghostly particles.

The latest results come from an analysis of DESI’s first year of data, released in April 2024, which produced the largest 3D map of the universe to date. These findings were notable because they indicated that the strength of dark energy may be changing over time. The data also examined baryon acoustic oscillations (BAO), a factor in galaxy clustering that allows large-scale cosmic structures to form.