Last spring, a team of nearly 1,000 cosmologists announced that dark energy — the enigmatic agent propelling the universe to swell in size at an ever-increasing rate — might be slackening. The bombshell result, based on the team’s observations of the motions of millions of galaxies combined with other data, was tentative and preliminary. Today, the scientists report that they have analyzed more than twice as much data as before and that it points more strongly to the same conclusion: Dark energy is losing steam.
“We are much more certain than last year that this is definitely a thing,” said Seshadri Nadathur, a member of the Dark Energy Spectroscopic Instrument (DESI) collaboration, the group behind the new result.
Their finding, presented today at the Global Physics Summit in Anaheim, California, aligns with that of a second group of cosmologists, the 400-strong Dark Energy Survey (DES). Having also observed a huge swath of the cosmos, DES reported evidence of varying dark energy in a paper earlier this month and in a talk today at the Anaheim meeting.
“It’s interesting that things are pushing in this direction and that multiple experiments are seeing some tension” with the idea that dark energy is constant, said Michael Troxel, a member of the DES team based at Duke University.
If the evidence of evolving dark energy holds up as more data accrues — and this is not guaranteed — it would upend cosmologists’ understanding of our ultimate destiny. Dark energy that has a constant density and pressure would doom our cosmos to expand for all time until unbridgeable gulfs separate every particle from all the others, snuffing out all activity. But dark energy that evolves makes alternative futures possible. “It challenges the fate of the universe,” said Mustapha Ishak-Boushaki, a cosmologist at the University of Texas at Dallas and DESI team member. “It’s game-changing.”
Evolving or weakening dark energy would also rewrite our picture of present-day reality. The most straightforward idea is that dark energy is the energy of the vacuum of space itself, which should be an unchanging feature of quantum physics. Evolving dark energy would herald the presence of something extra, some previously undetected ingredient in the fundamental recipe of the cosmos. The missing part could be as simple as a new type of particle, or it could reveal a subtle failure of Einstein’s theory of gravity. It might even lead researchers down a path that ends at a new fundamental theory of physics.
“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.
Astrophysicists first detected the influence of dark energy in the late 1990s. Two teams observed dozens of faraway supernovas and found that the most distant ones had traveled even farther from our Milky Way galaxy than had been expected. Something appeared to be speeding up the expansion of the universe.
Theoretical physicists knew exactly what that something should be: the energy of space itself. In his theory of gravity, Einstein noted a mathematical slot for a “cosmological constant” — energy that has a constant density and pressure everywhere, causing repulsion. As for the source of this energy, physicists knew that quantum fields, the entities responsible for particles like electrons and photons, contribute an energetic sizzle to otherwise-empty space. This energy would be too mild to matter over a few meters, but on a cosmic scale it should add up, sweeping galaxies away from one another faster and faster as more space (and therefore more of this vacuum energy) accumulates. The discovery that the universe’s expansion is indeed accelerating vindicated physicists’ understanding of both quantum fields and gravitation, even as it raised new questions.