There are hints that dark energy may be getting weaker

The largest 3D map of the universe ever created is providing hints about the evolution of the cosmos, and they suggest that we may be wrong about the behaviour of dark energy, which makes up most of the universe. It seems that this mysterious force may be weakening over time.

“If it holds up, this is a very big deal,” says Adam Riess at Johns Hopkins University in Maryland, who found the first evidence for dark energy 25 years ago. That is because the standard model of cosmology, called lambda-CDM, suggests that the strength of dark energy should be static over time.

Dark energy is thought to cause the accelerating expansion of the cosmos – if it is not static, that could also have huge implications for our ideas about the beginning of the universe, its size and its ultimate fate. Reiss, who was not involved in the new work, says the implications could mean “we will have to do some serious soul-searching regarding [our understanding of] gravity and fields”.

The strange findings come from the Dark Energy Spectroscopic Instrument (DESI) in Arizona – and even DESI collaborators are not quite sure what to make of the fact that their data suggests dark energy may have recently gotten weaker. “It’s all we’ve been talking in the collaboration about for months… whether this is interesting or not,” says DESI spokesperson Kyle Dawson at the University of Utah.

DESI researchers examined the strength of dark energy by measuring the large-scale structure and distribution of galaxies in the cosmos, which illuminates how the universe has expanded over time. The researchers then combined this information with three sets of data on supernovae, which act as so-called “standard candles” to determine the distances to cosmic objects thanks to their predictable brightnesses.

Surprisingly, each of the three samples of supernovae yielded a different answer to the change in the universe’s rate of expansion over time. All three suggested that the effects of dark energy may have decreased in recent aeons, but the strength of these suggestions varied, so researchers are not quite sure how to interpret the data.

“Two of the supernova samples disagree with each other, and they’re very, very similar samples,” says Dawson. “I don’t know which one’s right, it’s possible that the truth lies in between, but it really looks like the differences lie in the way [the supernova researchers] evaluated the data.”

Discrepancies in models are denoted by a factor called sigma, which measures the likelihood that a similar clash could have happened by chance if the models did disagree with one another. “About 3-sigma is the level we usually sit up and pay attention and call an ‘indication’ of something,” says Riess. Anything lower than that would not generally be particularly exciting to researchers – it would be too likely to be a simple coincidence.

The discrepancies between lambda-CDM and the combination of supernova and DESI measurements ranged from 2.5-sigma to 3.9-sigma. “Both statements are true: it is sufficient tension, it’s interesting; and it’s not sufficient tension to say that anything is definitely there,” says Dawson.

Dark energy makes up nearly 70 per cent of the universe, so any error in our understanding of its nature could have widespread impacts on physics. Proving whether that error is really there, though, will take more precise measurements in the coming years.

“If [this is] true, it would be the first real clue we have gotten about the nature of dark energy in 25 years,” says Riess.