New 3D map of Universe could solve dark energy mystery

So the Universe should be accelerating much faster than it is if the dark energy is, essentially, the cosmological constant. Still, all the observations to date indicate that it’s constant. The best theoretical fit thus far is known as the Lambda CDM model, which incorporates both a weakly interacting cold dark matter and dark energy. One alternative theory proposes that the Universe may be filled with a fluctuating form of dark energy dubbed “quintessence.” There are also several other alternative models that assume the density of dark energy has varied over the history of the Universe. The first hints that dark energy might vary over time appeared in the first full year of data. While there was basic agreement with the Lambda CDM model, when those results were combined with data from other studies—involving the cosmic microwave background radiation and Type Ia supernovae—subtle differences cropped up that suggested the dark energy might be weakening. It amounted to between 3.5 and 3.9 sigmas in terms of confidence. But the subsequent 2025 results covered the first three years of data with confidence levels between 2.8 and 4.2 sigma—just shy of the five-sigma threshold that is the gold standard for discovery. Analysis on this latest round of data won’t be completed until 2027 or 2028 if all goes smoothly. But finishing the original planned survey, on time, with significantly more data than expected, is a major achievement in itself. Over its five years of operation, DESI has mapped over 47 million galaxies, with more to come. “Some people have been working on this for decades, so it’s just amazing to see it come to completion,” DESI co-spokesperson Alexie Leauthaud of the University of California, Santa Cruz, told Ars. “Anyone who does science knows that you rarely achieve more than you proposed you would. And you never achieve more on time. DESI is maybe the only team I’ve ever worked with—that has actually done more than it was going to do—on time.” “When the original DESI survey began, we had ‘dark’ tiles over many different parts of the sky, so we weren’t as limited by conditions,” Myers told Ars. “But for the final dark tile, we had to thread the needle more, which made it seem like ‘weather’ was more of a problem. A lot of the time we were waiting to complete the final dark tile from the original program; we’d been observing plenty of ‘bright’ and ‘backup’ tiles, as well as even some tiles from newer dark programs, which span more of the sky.” DESI’s operation has been extended until 2028 to get a deeper look at more distant and faint “luminous red” galaxies, as well as nearby dwarf galaxies and stellar streams. Plans are already underway for DESI-II, which will require a small instrument upgrade. As for further tests of the Lambda CDM model, future analysis will be able to incorporate observational data from the Vera Rubin Telescope’s sky survey as well as the Euclid Space Telescope. “We’re going to need a lot of datasets and mix them in different ways to try to figure out what the Universe is trying to tell us,” said Leauthaud. The big question mark is whether there will be future funding for DESI and DESI-II, given the precarious state of science funding in the US. Honscheid acknowledged the uncertainty but is cautiously optimistic, in part because of the project’s success to date, and because DESI-II’s upgrade is a relatively small-ticket item. “I’m optimistic for DESI-II, but I’m also gravely concerned more broadly by the funding landscape and the attack on science,” said Leauthaud. “Even though we may be lucky, I’m still extremely concerned for my colleagues in astronomy who have lost funding, students whose careers have been jeopardized, postdocs who have had to leave. More broadly, beyond astronomy and astrophysics, I’ve been extremely concerned about the impact on climate science and NOAA. We rely on weather services to help with our observations.”