According to the Standard Model of Cosmology, the expansion of our universe is driven by the simplest possible version of dark energy: an unchanging 'cosmological constant' called lambda.
That longstanding model may be about to change.
"It's the start of a new era," says Will Percival, professor and director of the Waterloo Centre for Astrophysics at the University of Waterloo, and associate faculty at Perimeter Institute for Theoretical Physics. "When I started as a PhD student, we didn't even know lambda existed. Then I spent most of my career finding evidence that lambda works. Now we're starting to see evidence that lambda is failing to align with observations. It's very exciting!"
Percival is co-spokesperson for the Dark Energy Spectroscopic Instrument (DESI) collaboration, which has built the largest 3D map of our universe to date by observing millions of galaxies and quasars. DESI is an international experiment with more than 900 researchers from over 70 institutions around the world and is managed by the U.S. Department of Energy's Lawrence Berkeley National Laboratory.
DESI is a state-of-the-art instrument and can capture light from up to 5,000 objects simultaneously.
DESI measures dark energy by tracking the size of bubble-like formations called Baryonic Acoustic Oscillations, caused by acoustic waves in the hot plasma of the early universe. These primordial bubbles can now be seen imprinted on the large-scale distribution of galaxies and matter in the universe.
Last spring, DESI's early results gave the first preliminary hint that dark energy may be evolving. The evidence is now stronger after analyzing more than double the amount of data, though it has not reached the '5 sigma' threshold physicists require to confirm a discovery.
In fact, the discrepancy doesn't arise from the DESI results alone, which, in isolation, seem consistent with the Standard Model. But when combined with other experiments measuring the Cosmic Microwave Background (CMB) radiation and supernova data, evolving dark energy becomes a better fit for what scientists are seeing. Different combinations of the datasets predict evolving dark energy with a certainty between 2.8 to 4.2 sigma. A 3-sigma event has a 0.3 percent chance of being a statistical fluke.
"I'm excited to see how it plays out. There's more work to do, but dark energy whose effect was initially stronger than the standard model and then transitioned to be weaker about 4 billion years ago, is a viable candidate for resolving the tensions we're seeing in the datasets," says Percival.
Dustin Lang, a research scientist at Perimeter Institute, is part of the imaging team for DESI, and his work on the project started more than a decade ago, laying the groundwork for the science results.
"What we're seeing is a back-and-forth between theory and observation," says Lang. "These observations have given theorists a little kick to come up with new models, or to find problems with the way we've interpreted the data. We hope it doesn't turn out to be some statistical fluke or something systematic, but DESI is very careful about that sort of thing, and has an extensive blinding strategy."
Now that the observational evidence for evolving dark energy is growing, scientists are beginning to evaluate what it might mean for the standard cosmological model.
"It's tough to say how this will impact our understanding of cosmology. There's something that doesn't match our expectation in these expansion rate measurements. Whether it's new physics or systematics in one or more datasets remains to be seen. The DESI cosmological parameters group has done a lot of work testing the robustness of our results, and I'm quite excited to see what the community thinks," says Alex Krolewski, a postdoctoral researcher at the University of Waterloo.
Krolewski is leading the work to combine the DESI galaxy data with CMB lensing - the gravitational distortion of CMB radiation caused by the distribution of matter in the universe. This effort will measure the growth of structure in the universe and provide further tests of dynamical dark energy, with results expected in 2026.
Hanyu Zhang, a University of Waterloo postdoctoral researcher and part of the DESI team testing the robustness of the results using synthetic data, agrees. "I'm truly excited about these outcomes. These measurements deliver compelling evidence for a dynamical dark energy."
DESI is in the middle of its primary observation run, with more to come in the years ahead. The collaboration has now made its first year of data available to anyone to explore, giving experts around the world the chance to probe the dataset for their own research programs.
While it remains to be seen what DESI's newest result might mean for the future of cosmology, Percival is excited by the prospects.
"We're guided by Occam's razor, and the simplest explanation for what we see is shifting. It's looking more and more like we may need to modify our Standard Model of Cosmology to make these different datasets make sense together - and evolving dark energy seems promising."
The DESI collaboration is honoured to be permitted to conduct scientific research on I'oligam Du'ag (Kitt Peak), a mountain with particular significance to the Tohono O'odham Nation.
