Yesterday—March 19, 2025—may eventually be remembered as "the day that changed everything we know about the universe." I'm only slightly exaggerating. This was when the Dark Energy Spectroscopic Instrument (DESI) collaboration released its latest analysis of the largest 3D map of the universe ever made, which uses nearly 15 million galaxies and other celestial objects to track cosmic evolution across the past 11 billion years. And the results appear to be nothing short of revolutionary.
Some Background
Dark energy is scientists' shorthand for the mysterious force that's powering an acceleration in the universe's expansion. Its discovery about a quarter-century ago came as a surprise: most cosmologists had expected that, if anything, the universe's expansion (which began with the explosive big bang some 13.8 billion years ago) would be slowing down or staying constant. Instead, in 1998 two different teams studying faraway supernovae and other astrophysical distance-markers found cosmic expansion was speeding up—a discovery so shocking that in 2011 it netted the physics Nobel Prize.
The simplest, most straightforward explanation for dark energy is that it's something called the cosmological constant—a concept tracing back to Einstein that posits a steady, space-suffusing repulsive force counteracting gravity at intergalactic scales. But, being a constant, this would mean dark energy's strength would be constant and unchanging, too.
What DESI Found
The latest DESI results strongly suggest is that dark energy is more complex, with its strength waning over time. The finding falls slightly short of the "five sigma" statistical threshold physicists have set as the gold standard for a genuine discovery, which translates to a one-in-3.5-million chance of being erroneous. But additional statistical certitude should come soon, via ongoing observations from DESI and a host of other cosmos-mapping projects.
What This Means
The implications for our best explanation of the whole cosmic enchilada—the standard model of cosmology—are hard to understate. This model assumes dark energy is a cosmological constant, and by doing so can account for the universe's large-scale structure (and much else) to an almost ludicrous degree. DESI's potent hint of dark energy being something different could be a crucial diagnostic of where cosmology's most hallowed model has gone wrong, and a signpost guiding the way to a better, deeper understanding of nothing less than the fundamental underpinnings of reality itself.
So stay tuned. Before too long, our evolving views of accelerating cosmic expansion may spark something much more immediate and mundane: a fresh trip to Sweden for some far-seeing scientists to receive yet another dark energy-themed Nobel prize in physics. —Lee Billings
