By AFP | 2025-03-26

Dark energy, the mysterious force thought to be driving the ever-faster expansion of the universe, appears to be changing over time, according to new observations released March 19.

If dark energy is in fact weakening, it would likely mean that scientists will have to rewrite their understanding of how the universe works.

The new findings come from the Dark Energy Spectroscopic Instrument (DESI), which sits on a telescope at the Kitt Peak National Observatory in the US state of Arizona.

"What we are seeing is deeply intriguing," said Alexie Leauthaud-Harnett, a spokesperson for the DESI collaboration, which brings together 70 institutions around the world.

"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," she said in a statement.

DESI's thin optical fibers can simultaneously observe 5,000 galaxies or quasars -- blazing monsters with a black hole at their heart -- for 20 minutes.

This innovation allows scientists to calculate the age and distance of these objects and to create a map of the universe so they can detect patterns and trace its history.

'Tensions' emerging

Scientists have known for a century that the universe is expanding, because they have observed massive clusters of galaxies moving away from each other.

In the late 1990s, scientists shocked the field by discovering that the universe's expansion has been speeding up over time.

They gave the name dark energy to the phenomenon driving this acceleration, the effects of which seem to be partially offset by ordinary matter -- and an also unknown thing called dark matter.

The universe is thought to be made of 70% dark energy, 25% dark matter -- and just 5% normal matter.

Science's best understanding of how the universe works, which is called the standard cosmological model, refers to dark energy as being constant -- meaning it does not change.

Albert Einstein introduced the idea in his theory of relativity.

The standard model is "satisfactory" but some "tensions" are emerging between observations, Arnaud de Mattia, a French physicist involved in analyzing the DESI data, told AFP.

Several different ways of measuring the expansion of the universe exist, including looking at the lingering radiation from after the Big Bang and at exploding stars called supernovae and investigating how gravity distorts the light of galaxies.

When the DESI team combined its new data with other measurements, it found "signs that the impact of dark energy may be weakening over time," according to a statement.

"When we combine all the cosmological data, it favors that the universe's expansion was accelerating at a slightly higher rate around seven billion years ago," de Mattia said.

But for the moment there is "absolutely not certainty" about this, he added.

'Inflection point'

French physicist Etienne Burtin was confident that "we should have a clearer picture within five years."

That confidence has formed because loads of new data are expected from DESI, Europe's Euclid space telescope, NASA's upcoming Nancy Grace Roman space telescope and the Vera Rubin Observatory in Chile.

"This new generation of surveys -- in the next few years -- will nail this," Joshua Frieman, a theoretical astrophysicist at the University of Chicago, told AFP.

But for now, "we're at this interesting inflection point," added Frieman, a dark energy scholar and former DESI member.

Confirming the "evolving dark energy" theory will be a "revolution on the level of the discovery of accelerated expansion," which itself was the subject of a physics Nobel, said Burtin.

"The standard cosmological model would have to be different," he added.

The DESI research, which involved three years' worth of observations of 15 million galaxies and quasars, was presented at a conference of the American Physical Society in California.