Thanks to observations of the evolution of the universe over the past 11 billion years collected by the Dark Energy Spectrograph (DESI), general relativity has passed one of its most exacting tests.
Albert Einstein’s 1915 theory of general relativity remains humanity’s best explanation of gravity for the past 100 years. Cosmologists have used general relativity to model how the universe evolved—from its earliest moments to its current state—and to show how gravity brings small chunks of matter together to form vast galaxies and their clusters. But while general relativity has passed every test applied on a relatively small scale, few tests have challenged it on a very large scale.
Now scientists have used DESI to conduct these large-scale tests. They observed nearly 6 million galaxies and quasars, the bright hearts of galaxies that feed supermassive black holes. Perhaps unsurprisingly, this test, which traces the evolution of the universe from about 3 billion years ago, has once again shown that general relativity is the right “method” for gravity.
“General relativity has been very well tested at the scale of the solar system, but we needed to test whether our assumptions would work at much larger scales,” said study co-leader Pauline Zarrouk, a cosmologist at the French National Center for Scientific Research (CNRS). He said. said in a statement. “Studying the rate at which galaxies form allows us to test our theory directly, and so far it is consistent with what general relativity predicts at the cosmic scale.”
DESI, mounted on the Nicholas U. Mayall 4-meter telescope at Kitt Peak National Observatory, is a state-of-the-art instrument comprised of 5,000 “robot eyes.” The experiment is now in the fourth year of a five-year sky survey project that will eventually observe some 40 million galaxies and quasars.
Sky survey data could be essential to understanding dark energy and dark matter. Dark matter is a mysterious substance that is heavier than the particles of “ordinary matter” that make up stars, planets, moons, and everything else we see around us every day. Remains effectively invisible. Dark energy and dark matter, collectively referred to as the “dark universe,” suggest that everything we understand about the universe represents only 5% of the story.
“Dark matter makes up about a quarter of the universe, and dark energy makes up another 70%. We don’t really know what either is,” said team member Mark Maus, a doctoral student at Berkeley Lab and UC Berkeley. said: From the statement. “The idea that we can take pictures of the universe and address these big, fundamental questions is truly amazing.”
Weighing a Space Ghost
General relativity may be the best explanation of gravity that we know, but it cannot explain all the elements of the universe that we currently observe, especially the accelerating expansion of space and the gravitational effects of dark matter. The acceleration of the universe’s expansion is due to a “placeholder” force called dark energy that cannot be explained in current cosmological models based on general relativity.
This failure to explain dark energy has led some scientists to propose an alternative to general relativity based on adjustments to Isaac Newton’s theory of gravity, which replaced Einstein’s masterpiece theory. These theories are commonly referred to as “modified gravitational theories” and explain observations of the universe without the need to introduce unknowns such as dark energy.
In addition to helping to validate the Lambda cold dark matter (LCDM) model, a leading model of the universe based on general relativity, DESI’s findings also helped rule out some theories of strain gravity.
The same results from DESI also helped set an upper limit on the mass of so-called “ghost particles,” or neutrinos.
Neutrinos lack electric charge and are virtually massless, giving them the reputation of being the ghosts of the particle menagerie. As you read this sentence, trillions of these particles are flowing through your body at nearly the speed of light and remaining undetected.
Neutrinos are the only elementary particles we’ve discovered without scientists being able to accurately define their mass. Previous experiments defined a low mass for neutrinos, but the DESI results set an upper limit, giving researchers a clearer mass range in which neutrinos must reside.
The new results come from an expanded analysis of the first year of DESI data published in April 2024. This data formed the largest 3D map of the universe created to date. These results were already notable because they seemed to show that the intensity of dark energy was changing over time.
The April DESI results focused on elements of galaxy clustering called baryonic acoustic oscillations (BAO) oscillations of matter density that enable the growth of large-scale structures. This new examination of these results included a “global morphological analysis,” in which researchers further examined how galaxies and matter are distributed at various scales throughout the universe.
Additional results from the second and third years of DESI operation are scheduled to be released in spring 2025.
“Both our BAO results and the full morphological analysis are excellent,” study co-leader Dragan Huterer of the University of Michigan said in a statement. “This is the first time DESI has observed the growth of cosmic structures. We are demonstrating tremendous new capabilities to probe modified gravity and improve the constraints of dark energy models. This is just the tip of the iceberg. .”
DESI results are described in several papers published on the research repository site. arXiv Tuesday (November 19).
