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Star trails over the Mayall Telescope in Arizona, which houses the Dark Energy Spectroscopic Instrument<\/p>\n
Luke Tyas\/Berkeley Lab<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
Dark energy is one of the most mysterious features of our universe \u2013 we don\u2019t know what it is, but it controls how the universe is expanding, as well as its ultimate fate. Now, a study of millions of celestial objects has revealed that we may have been thinking about it all wrong, with potentially dramatic consequences for the cosmos.<\/p>\n
\u201cThis is the biggest hint we have about the nature of dark energy in the approximately 25 years since we discovered it,\u201d says Adam Riess at Johns Hopkins University in Maryland.<\/p>\n
The result comes from three years\u2019 worth of data gathered by the Dark Energy Spectroscopic Instrument (DESI) in Arizona. By combining this data with other measurements, such as maps of the cosmic microwave background radiation and supernovae, the DESI team has concluded that dark energy may have changed over time \u2013 directly contradicting the standard model of cosmology, called lambda-CDM.<\/p>\n
\u201cThis is the cutting edge of human knowledge,\u201d says DESI team member Will Percival at the University of Waterloo in Canada. \u201cWe\u2019re seeing something amazing with the whole universe.\u201d<\/p>\n
<\/p>\n DESI is mounted on a telescope and works by measuring the \u201cred shift\u201d of light emitted by distant galaxies, or how the wavelengths of that light are stretched as it travels through the universe. From this, researchers can determine how much the universe has expanded during the light\u2019s journey and calculate how this expansion is changing. So far, the team has analysed light from nearly 15 million galaxies and other bright objects in the sky.<\/p>\n For decades, physicists have agreed that the universe is expanding at a fixed rate of acceleration, a cosmological constant known as lambda that has been interpreted as the push of dark energy. But in April 2024, DESI\u2019s measurements showed the first hints that the universe may actually be accelerating less quickly over time \u2013 making the cosmological constant not so constant.<\/p>\n Riess, who isn\u2019t on the DESI team, says that at the time he wasn\u2019t sure whether the finding would persist with more data. In fact, it has only gotten stronger. \u201cIt\u2019s very exciting to me that it appears that [the team] did not find any problem in the analysis after another year, and after they\u2019ve added more data. If anything, the result is more significant,\u201d he says.<\/p>\n That said, the finding still doesn\u2019t meet the \u201c5 sigma\u201d statistical level that physicists conventionally use to mark a discovery as genuine, rather than a statistical fluke. The current analysis reaches at most 4.2 sigma, but team member Mustapha Ishak-Boushaki at the University of Texas at Dallas says the team believes that, as DESI keeps taking data, the result should reach 5 sigma within two years. \u201cThis result about dark energy is something that we did not expect to happen in our lifetime,\u201d he says.<\/p>\n One reassurance, says Ishak-Boushaki, is that the finding doesn\u2019t rely only on data from DESI, but also on several other surveys of the universe. Riess compares the situation to a multi-legged stool, where breaking one leg \u2013 or removing one dataset \u2013 doesn\u2019t make the conclusion fully crumble.<\/p>\n Assuming the legs hold, the universe could look very different from our current picture of it. If dark energy keeps becoming weaker, the universe may reach a state where it is expanding at a constant rate instead of faster and faster, says Ishak-Boushaki. Some dramatic scenarios also become more plausible, such as the \u201cbig crunch\u201d, where the cosmos starts contracting instead of expanding and eventually collapses in on itself, he says.<\/p>\n The exact future of the universe remains an open question, and DESI isn\u2019t the only tool researchers are using to answer it. Riess points to several other surveys of the universe, such as NASA\u2019s Nancy Grace Roman Space Telescope and the Vera Rubin Observatory in Chile, which are designed to help shed light on the true nature of dark energy.<\/p>\n While mathematical models for a universe with changing dark energy still need to catch up with these observations, Percival says he expects future theoretical work will help design even more experiments that will directly test our assumptions about this mysterious force.<\/p>\n \u201cAs far as theoretical models, Pandora\u2019s box just opened. We were stuck with a cosmological constant,\u201d says Ishak-Boushaki. \u201cWe are not stuck anymore.\u201d<\/p>\n Topics:<\/p>\n<\/section><\/div>\n