Dark energy: mysterious cosmic force appears to be weakening, say scientists

The Guardian

19-03-2025

Dark energy, the mysterious force powering the expansion of the universe, appears to be weakening, according to a survey that could 'overthrow' scientists' current understanding of the fate of the cosmos.
If confirmed, the results from the dark energy spectroscopic instrument (Desi) team at the Kitt Peak National Observatory in Arizona would have profound implications for theories about the evolution of the universe, opening up the possibility that its current expansion could eventually go into reverse in a 'big crunch'.
A suggestion that dark energy reached a peak billions of years ago would also herald the first substantial change in decades to the widely accepted theoretical model of the universe.
Prof Alexie Leauthaud-Harnett, a co-spokesperson for Desi and a cosmologist at the University of California, Santa Cruz, said: 'What we are seeing is deeply intriguing. 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.'
Dark energy was discovered in the late 1990s when astronomers used distant supernova explosions to investigate how the rate of cosmic expansion has changed over time. The expectation was that gravity should counteract the expansion that has been underway since the big bang, but instead, the supernovae indicated that the rate of expansion was accelerating, propelled by some unknown force that scientists called dark energy.
Dark energy has been assumed to be a constant, which would imply the universe will meet its end in a desolate scenario called the 'big freeze', when everything is eventually so far apart that even light cannot bridge the gap between galaxies. The latest findings, announced on Thursday at the American Physical Society's Global Physics Summit in Anaheim, California, challenge that prevailing view.
Desi uses its 5,000 fibreoptic 'eyes' to map the cosmos with unprecedented precision. Its latest data release captures 15m galaxies, spanning 11bn years of history, which astronomers have used to create the most detailed three-dimensional map of the universe to date.
The results suggest that dark energy reached a peak in strength when the universe was about 70% of its current age and it is now about 10% weaker. This would mean the rate of expansion is still accelerating, but that dark energy is gently lifting its foot off the pedal.
Prof Carlos Frenk, a cosmologist at the University of Durham and member of the Desi collaboration, said: 'What we're finding is that, yes, there is something pushing galaxies away from each other, but it is not constant. It is declining.'
The results do not meet the so-called five-sigma threshold of statistical certainty that is the gold standard in physics for claiming a discovery. But many in the collaboration have shifted in recent months from a position of scepticism to confidently backing the finding.
'I'm not on the fence,' said Frenk. 'I've looked at the data carefully. To me, this is a robust result. We're witnessing the overthrow of the old paradigm and the emergence of a new paradigm.'
Prof John Peacock, a cosmologist at the University of Edinburgh and a Desi collaborator who voiced scepticism about evolving dark energy at a Royal Society meeting last year, has been similarly persuaded. 'Extreme claims require extreme evidence,' he said. 'There's almost nothing in science that I would bet my house on. But I would put £1,000 on this result.'
Others continue to reserve judgment. Prof George Efstathiou of the University of Cambridge, who was not involved in the findings, said: 'My take-home from this analysis is that the … measurements do not yet provide decisive evidence for evolving dark energy. They may do as Desi accumulates more data.'
If dark energy keeps decreasing to the point where it becomes negative, the universe is predicted to end in a reverse big bang scenario known as the big crunch.
Scientists do not know why dark energy, which is generally estimated to account for about 70% of the universe – with the rest made up of dark and ordinary matter – might be waning or whether this would indicate the laws of physics are changing or that a crucial component is missing from them.
Prof Ofer Lahav, an astronomer at University College London and Desi collaborator, said: 'It's fair to say we have no idea what dark matter or dark energy is. The constant dark energy [theory] is already sufficiently challenging. I feel like: 'As if things were not complicated enough.'
'But you can also look at it more positively. For 20 years we've been stuck with dark energy. Now physicists have new questions.'