We think dark energy makes up most of the universe, but we have no idea what it actually is. In 2025, the Dark Energy Spectroscopic Instrument (DESI) in Arizona may offer clues, particularly in relation to how this strange force has changed as the universe matured.
“Either there is some new form of dark energy that we don’t know about yet or this could be a paradigm shift, maybe [the data will show] that there is something we don’t understand about space and time,” says at the University of Texas at Dallas.
In the spring, the DESI researchers will share their analysis of three years of the instrument’s data. They will describe how around 31 million galaxies form clusters across the universe and how this cosmic structure has changed in the past 11 billion years, as far as DESI can see. An early look released in April 2024 suggests dark energy, which is thought to be making our universe expand ever faster, may have been stronger in the past.
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The idea that dark energy could have changed over time came as a big surprise, says at Lawrence Berkeley National Laboratory in California. “This really has shaken the community,” she says. But these early results couldn’t fully rule out the more traditional cosmological model, where dark energy has a constant value, so more data is eagerly awaited, she says.
“At this stage, it’s more of a hint than it is a discovery,” says at Brown University in Rhode Island. “A lot of researchers said, ‘Well, I really would like to see year three data now.'”
The same is true of several other cosmological questions that DESI may be able to address, says Allali. For one, it may show new shifts in the Hubble constant, which measures the rate at which the universe is expanding. The exact value of this number is controversial because different methods of determining it haven’t always agreed. Allali and his colleagues have previously suggested that part of the problem may be the presence of yet another mysterious entity, which they named dark radiation.
Previous DESI analyses haven’t ruled out this exotic idea, but new data may weigh in more heavily on the issue, especially if the Hubble constant is indeed different to what physicists previously thought, says Allali. “This is super exciting for people like me who want to solve huge cosmological problems.”
Another long-standing issue DESI could address is pinpointing the masses of ghostly particles called neutrinos. These barely interact with other particles, but we can still determine their properties by studying the cosmic structure that is being uncovered by DESI. Initial data narrowed the difference between the upper and lower bounds on neutrino mass – it got close to a single number – but Allali says clarity will only come with more data. If DESI can pinpoint the neutrino mass further, it may be the result that “has the broadest impact on all of physics” from the instrument, he says.
