The bombshell results that demand a new theory of the universe

New Scientist · Mar 2, 2026 · Collected from RSS

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2023 Sunyixun If you imagine the story of the universe as a film endlessly in post-production, cosmologists would be its obsessive editors, constantly tweaking the narrative. The version they are working with is an astonishing cinematic achievement: it starts with a bang, space-time erupting out of nothing, before unfurling majestically with the formation of stars and then galaxies, sculpted by the gravitational pull of both visible matter and mysterious dark matter, all the while serenely expanding thanks to a shadowy force known as dark energy. But it can’t be the final cut. The more we peer into space, the more it seems incomplete: the story contains niggling inconsistencies and key protagonists remain maddeningly elusive. For decades, cosmologists have been struggling to refine the script. Now, they finally have fresh inspiration from the cosmos. A powerful telescope has mapped millions of distant galaxies to trace the expansion of the universe with unprecedented precision. What it appears to be revealing is that dark energy behaves so weirdly, it can’t be what we thought it was. This article is part of our special issue on the crisis in cosmology Explore the full package here If confirmed, it is an exhilarating twist. Theorists are contemplating a complete rewrite of dark energy. How it all pans out is far from clear. But many are warming to the idea that we are about to produce a richer, more detailed cosmic story – one that looks very different from the current version. “We’re at an interesting moment,” says Adam Riess, an astrophysicist at Johns Hopkins University in Maryland, who won a share of the 2011 Nobel prize in physics for his part in the discovery of dark energy. If someone were filming a documentary charting the making of our cosmological movie, he adds, “I would say: ‘Don’t go to the bathroom now.’” The standard model of cosmology Our current best picture of the origins and evolution of the universe was pieced together over the course of a century. It began in 1915 with Albert Einstein’s theory of general relativity, which describes gravity as the result of massive objects warping space-time. At the time, the universe was thought to be static, so Einstein added a calming term to his equations called the “cosmological constant”. But in 1929, astronomer Edwin Hubble observed distant galaxies speeding away from one another, indicating that the universe is expanding and prompting Einstein to ditch his constant. Then came the big bang theory. While it is gospel these days, it wasn’t until the 1960s that the rival steady-state theory gave way, as astronomers discovered a sea of primordial radiation left over from the big bang – the cosmic microwave background (CMB) – with properties that matched predictions. As our ability to peer deep into space improved, the big bang theory was no longer enough. In the 1980s, astronomers found that the gravity of visible matter was insufficient to hold galaxies together or explain the formation of galaxy clusters. The fix was to invoke invisible dark matter. A decade later, observations of distant exploding stars led by Riess and his colleagues revealed, contrary to all expectations, that the expansion of the universe is speeding up. The cosmological constant was reinstated, albeit rebadged as dark energy. And this, essentially, is the current standard model of cosmology, known as lambda-CDM. The Greek letter lambda denotes the cosmological constant and CDM stands for cold dark matter, assumed to be made of heavy, slow-moving particles. Added to general relativity and with a few key assumptions – most importantly that the universe, on average, looks the same in all directions – it offers a compelling framework for how large-scale structure formed from quantum fluctuations in the early universe through a brief burst of exponential inflation in the first moments. The Dark Energy Spectroscopy Instrument’s new map of the universeDESI Collaboration/DOE/KPNO/N​OIRLab/NSF/AURA/R. Proctor Lambda-CDM ranks among science’s greatest triumphs. It combines elegance with breathtaking reach, using just six parameters to describe the entire history of the cosmos, making a host of precise predictions that have been verified by increasingly exacting observations. “It has been extraordinarily successful,” says Mike Turner, a theoretical cosmologist at the University of Chicago in Illinois. “Compare it with what we had when I became a cosmologist around 1980 and, oh my God, it’s more than we could ever have imagined. It’s absolutely stunning.” And yet, as Turner says, it is “now much less than we’re willing to settle for”. That is partly just the restless nature of science: even the most successful theories are only ever approximations of a deeper understanding and, as we stress-test them with new observations, we uncover loose ends and cracks. In the case of lambda-CDM, the loose ends are obvious. Dark matter and dark energy were only ever placeholders: they were invoked in response to observations, but without physical explanations. Despite decades of effort, physicists have yet to directly detect dark matter particles. And while dark energy is thought of as vacuum energy, the result of quantum fluctuations in empty space, it has always been troubling from a theoretical perspective. Quantum theory predicts that its strength must be some 10120 times greater than what is required to drive the expansion of the universe we see. “Right now, dark energy and dark matter… they’re tack-ons,” says Turner. They both serve functions. There is strong empirical evidence that they exist. “But they are just phenomenological descriptions, so they’re pointing to something more fundamental.” The Hubble tension Cracks have begun to appear, too, the most notorious of which has a long history, but became recognised as the Hubble tension in 2015. It is so named because two different ways of measuring the rate at which the universe is expanding, known as the Hubble constant, disagree. When cosmologists extrapolate forwards from the CMB using the current model, they get a value of about 67 kilometres per second per megaparsec. But when astronomers measure the local universe directly, using supernovae and variable stars, the value is around 73. “It’s an end-to-end test of the universe,” says Riess, who argues that the fact that the two ends don’t meet is a strong hint there is something seriously wrong with lambda-CDM. Still, most cosmologists have been unwilling to give up on it. All the proposals made so far for how to resolve the Hubble tension undermine the existing model’s near-perfect fit to the CMB and the large-scale structure we see today. It is also possible that the measurements underlying the tension contain subtle systematic errors. The way we measure late-universe expansion in particular relies on an intricate chain of inference, each link dependent on painstaking calibration and assumptions about stars and galaxies. The suspicion is that, with more data, the tension will disappear. “There’s just too much going on there for you to say something truly definitive,” says Pedro Ferreira, a cosmologist and astrophysicist at the University of Oxford. Riess doesn’t buy that. His measurements of late-universe expansion have been checked again and again, he points out, and nobody has found an error – even if some astronomers argue that independent distance measurements from the James Webb Space Telescope could resolve the tension. “It’s been a decade since we discovered the Hubble tension and it hasn’t gone away,” he says. “It’s only grown more pronounced.” The real reason the community has been reluctant to move beyond lambda-CDM, Riess argues, is that scientists are loath to let go of any theory, especially such a successful one, until they have a better one. “People are uncomfortable just wandering in the wilderness.” What we need, by that logic, are observations that more clearly point the way to something better. The good news on that front is that a new generation of telescopes designed to probe dark energy has begun to deliver in dramatic fashion, such as the Dark Energy Spectroscopic Instrument (DESI). The DESI results Mounted on a telescope in Arizona, DESI combines a huge mirror with 5000 robotically controlled optical fibres that automatically lock onto distant galaxies, one after the other, in quick succession – far faster than previous dark energy surveys. Since 2021, it has been surveying millions of galaxies to gauge their redshift, or how much the light they emit has stretched due to cosmic expansion, an indicator of their distance from us. And because galaxies are at different redshifts, we can compare a characteristic spacing in their distribution – a slight preference for galaxies to be separated by a particular distance – to reconstruct how the universe’s expansion rate has changed over time. Gabriela Secara, Perimeter Institute, NASA To calibrate those distances, DESI has also been measuring a subtle imprint left over from the early universe, known as baryonic acoustic oscillations (BAOs). Like ripples on a pond frozen in ice, these BAOs preserve a pattern in the separation of galaxies that provides cosmologists with a “standard ruler” for measuring cosmic expansion. The idea was to produce the most accurate, precise, three-dimensional reconstruction of cosmic expansion ever made. And the latest version, released in March 2025 and based on three years’ worth of data, or 15 million galaxies, contained a bombshell that has sent shockwaves through cosmology. When DESI researchers combined this with the latest data from supernovae – which tightly constrain the expansion of the nearby universe – and the CMB, then checked how well it all fits with lambda-CDM, they found that the current model doesn’t match up, at least not as well as one that allows the strength of dark energy to change over time. The headline finding was stark: dark energy appears to be weakening, and isn’t a cosmolo