General relativity has passed one of its most precise tests to date, thanks to observations of the past 11 billion years of cosmic evolution collected by the Dark Energy Spectrometer (DESI).
Albert Einstein’s 1915 theory of general relativity has remained humanity’s best explanation of gravity for the past 100 years. Cosmologists use the theory of general relativity to model how the universe has evolved from its earliest moments to its current state, showing how gravity brings together tiny clumps of matter to create vast It showed how galaxies and their clusters form. Although general relativity has passed all tests applied to it on relatively small scales, few tests have challenged it on very large scales.
Scientists have now performed such a large-scale test using DESI. They observed about 6 million galaxies and quasars. Quasars are bright centers of galaxies driven by a supply of supermassive black holes. Perhaps unsurprisingly, the experiment, which has tracked the evolution of the universe since its creation some three billion years ago, has once again shown that general relativity is the correct “recipe” for gravity.
“General relativity has been well tested at the scale of our solar system, but our assumptions also work on larger scales,” said study co-lead Pauline Zarouk, a cosmologist at France’s National Center for Scientific Research (CNRS). There was also a need to verify whether this was the case.” stated in a statement. “Studying the rate of galaxy formation allows us to directly test our theory, and so far it is consistent with what general relativity predicts on cosmological scales.”
DESI, located on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, is a state-of-the-art instrument consisting of 5,000 “robot eyes.” The experiment is currently in the fourth year of a five-year sky survey project that will eventually measure approximately 40 million galaxies and quasars.
Sky survey data is essential to understanding dark energy and dark matter, the mysterious substances that surpass the particles of “everyday matter” that make up stars, planets, the moon, and everything we see every day. possible, but will remain virtually invisible. Dark energy and dark matter, collectively known as the “dark universe,” suggest that everything we understand in the universe is only 5% of its content.
“Dark matter makes up about a quarter of the universe, and dark energy makes up another 70 percent, but we don’t actually know what they are,” said team members from Berkeley Lab and the University of California, Berkeley. said Mark Maus, a doctoral student at the school. in a statement. “The idea that we can take pictures of the universe and address these big fundamental questions is amazing.”
Weighing the ghosts of space
General relativity may be the best explanation of gravity that we know, but it cannot explain all the currently observed elements of the universe, especially the accelerating expansion of the universe and the gravitational effects of dark matter. It is currently believed that the acceleration of the expansion of the universe is due to a “placeholder” force called dark energy, but this force cannot be explained by cosmological models based on general relativity.
This failure to explain dark energy has led some scientists to advocate an alternative to general relativity based on adjustments to Isaac Newton’s theory of gravity, which has been supplanted by Einstein’s masterpiece theory. I did. These theories are commonly referred to as “modified theories of gravity” and explain observations of the universe without the need to introduce unknown substances such as dark energy.
DESI’s findings not only helped validate the lambda cold dark matter (LCDM) model, the main model of the universe based on general relativity, but also helped rule out some modified theories of gravity.
Additionally, the same results from DESI helped put an upper limit on the mass of so-called “ghost particles,” or neutrinos.
Neutrinos have a reputation as the ghosts of the particle zoo because they carry no electric charge and are virtually massless. As you read this sentence, trillions of these particles are flowing through your body at near the speed of light and remaining undetected.
Neutrinos are the only fundamental particles we have discovered, and their mass has not been precisely defined by scientists. Previous experiments had defined a lower mass limit for neutrinos, but the DESI results set an upper limit, giving researchers a more clearly defined mass range in which neutrinos should exist.
The new results come from an expanded analysis of the first year of DESI data released in April 2024. This data formed the largest 3D map of the universe ever created. These results were already noteworthy because they seemed to indicate that the strength of dark energy is changing over time.
April’s DESI results focused on a factor in galaxy clustering called baryon acoustic oscillations (BAO) oscillations in matter density that enable the growth of large-scale structures. This new examination of these results includes what the researchers call a “full shape analysis,” which reveals how galaxies and matter are distributed across the universe at different scales. Further investigation was conducted.
Further results from the second and third years of DESI operations are expected to be published in spring 2025.
“Both our BAO results and full-form analysis are impressive,” study co-leader Dragan Hutterer of the University of Michigan said in a statement. “This is the first time that DESI has observed the growth of cosmic structure. We are demonstrating tremendous new ability to investigate modified gravity and improve constraints on dark energy models. It’s just one part of it.”
DESI results are described in several papers published on the Research Repository site. arXiv Tuesday (November 19th).
