The Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona has finished its five-year sky survey, capturing data on more than 47 million galaxies and quasars. This creates the most detailed map of the universe to date, exceeding initial expectations of 34 million objects. The findings could shed light on the apparent weakening of dark energy.
DESI began scanning the sky in 2021 from Kitt Peak National Observatory. The instrument observed extremely faint galaxies using as few as 100 or 200 photons due to their vast distances. David Schlegel at Lawrence Berkeley National Laboratory noted that prior cosmic maps included only about 5 million galaxies total, making DESI's data nearly ten times larger. “We’ve actually been on this curve now for my whole career where, every 10 years, we’re making 10-times-larger maps,” Schlegel said, projecting that every observable galaxy within 10 billion light years could be mapped by 2061 if the trend continues. The survey covers 14,000 square degrees of the sky, with plans to expand to 17,000. The full sky spans over 41,000 square degrees, though much is obscured by bright nearby objects like the Milky Way. Analysis of the data will take another year before release to researchers, while DESI continues operations for at least two and a half more years, with hopes for upgrades into the 2030s. An earlier 2024 dataset from DESI hinted that dark energy, which comprises about 70 percent of the universe, may be weakening over time rather than staying constant. This challenges the lambda-CDM model of cosmology and could prompt new theories. Ofer Lahav at University College London reflected on the shift: 40 years ago, samples had just thousands of galaxies, but now researchers face a data flood. “I think my students [today] may have the opposite problem; to have been flooded with data, and it’s very challenging to analyse it,” he said.