“We’re in the business of letting the universe tell us how it works, and maybe the universe is telling us it’s more complicated than we thought it was,” Berkeley Lab postdoctoral researcher Andrei Cuceu and co-chair of DESI’s Lyman-alpha working group explained. That’s the bite of the hype on the latest discovery by the Dark Energy Spectroscopic Instrument (DESI), one that took decades to carefully chart the universe to try to uncover the secrets of dark energy. The Lambda Cold Dark Matter (ΛCDM) model explained the expansion of the universe for decades, but according to new evidence, dark energy, the enigmatic force driving the accelerating expansion of the universe, is not as stable as once believed.
The breakthrough DESI study with three years of observations of nearly 15 million galaxies and quasars discovered there to be a indication of dark energy evolving over time. This is the opposite of our current model of cosmology in which dark energy is a “cosmological constant” a static force present in the nature of space-time. “It is exciting to think that we may be on the cusp of a major discovery about dark energy and the fundamental nature of our universe,” said Alexie Leauthaud-Harnett, co-spokesperson for DESI and professor at UC Santa Cruz.
To understand why discoveries like these are so significant, it is necessary to understand the significance of dark energy as a factor of cosmological evolution. Dark energy was first found by looking at the supernovae that happened far away in the late 1990s, and it constitutes almost 70% of the universe’s energy. Dark energy is a force that causes the acceleration of the expansion of the universe, a finding that conflicted with previous forecasts of how gravity would slow down this expansion. Dark energy has been speculated by physicists to be connected with quantum energy in the open universe or to a scalar field known as quintessence. But the outcome of DESI hints at something more dynamic one wherein dark energy power could have declined over the course of time.
The possibility of controlling dark energy is a deeply significant one. If so, the fate of the universe is not quite so merely unlimited expansion. Instead, choices such as dampening expansion, or even reversal, are on the table. “It challenges the fate of the universe,” said University of Texas at Dallas cosmologist and DESI collaboration member Mustapha Ishak-Boushaki. “It’s game-changing.”
DESI’s precision in tracking dark energy’s influence stems from its ability to study the distribution of matter across vast cosmic distances. Using a feature called baryon acoustic oscillations (BAO), researchers measure subtle patterns in how matter is spread, providing a “standard ruler” to gauge the universe’s expansion history. This technique, combined with DESI’s ability to simultaneously observe 5,000 galaxies, has yielded the most detailed 3D map of the universe ever created.
With DESI observations combined with other cosmic microwave background, supernovae, and weak lensing data, the evidence for dynamical dark energy piles up. Contrasts with the ΛCDM model arise through statistical tests of as much as 4.2 sigma a value close to the 5 sigma level at which physicists feel others should confirm before claiming discovery. “We’re guided by Occam’s razor [the principle that the simplest explanation is the best], and the simplest explanation for what we see is shifting,” states Will Percival, University of Waterloo professor and DESI co-spokesperson.
Despite the excitement, researchers caution that the findings are not yet definitive. “We should have a clearer picture within five years,” predicted French physicist Etienne Burtin, referencing the influx of new data expected from upcoming experiments like NASA’s Nancy Grace Roman Space Telescope and Europe’s Euclid mission. These projects, alongside DESI’s continued observations, promise to refine our understanding of dark energy and its role in the universe’s evolution.
The changing dark energy also conflicts with the underlying physical laws. Is it a sign of a pleasant glitch in Einstein’s theory of relativity? Or a sign of an unknown new particle or field? “It sounds like it will be a paradigm shift, something that will change our understanding and the way we are putting all the pieces together,” Ishak-Boushaki said.
For now, the DESI collaboration remains focused on analyzing its vast dataset, which has already doubled in size since its first analysis.“Our results are fertile ground for our theory colleagues as they look at new and existing models, and we’re excited to see what they come up with,” said Michael Levi, DESI director and a scientist at Berkeley Lab.
As researchers lift the veil of mystery about dark energy, this is one thing that’s certain the universe is a whole lot more complicated than we might have dreamed. With every new discovery, we move one step closer to one of the most basic questions of humanity what is the fate of the universe? The search continues into our current era, driven by the humble but promising hints that fill the universe.
DESI observations have introduced a new generation of cosmology that may change the universe’s past, present, and future.
