A potential new theory of gravity that ruffles the fabric of the universe, allowing space and time to vary erratically, could solve some of the largest mysteries in physics and do away with the need for dark matter, say its proponents. However, others say much more evidence is needed before this “post-quantum gravity” can be taken seriously.
Most cosmologists believe that a substance called dark matter makes up around 85 per cent of the matter in the universe, despite having never been seen directly after decades of searching. Instead, its presence has been inferred from unusual astronomical observations, such as galaxies spinning faster than they should according to Albert Einstein’s theory of gravity or tiny fluctuations in the cosmic microwave background (CMB) radiation that can’t be explained by only the gravity of the matter we see in the universe.
Now, and at University College London and his colleagues say that their new conception of gravity can explain the faster-than-expected rotation of galaxies without dark matter.
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While many efforts to explain the universe focus on developing a theory of quantum gravity – unifying quantum mechanics and gravity, which currently break down when trying to understand extreme objects such as black holes – Oppenheim takes a different approach. In his post-quantum gravity, everyday matter is still subject to the rules of quantum mechanics, but space-time remains non-quantum, or classical, while being allowed to vary randomly and ever so slightly as the universe evolves.
Oppenheim and a number of colleagues have been working on this idea for some time, but now they have made two key breakthroughs. The first is to show that it works at all length scales, from the quantum to the cosmic, a test that any potential theory of gravity must pass. Oppenheim and Russo have also calculated how gravity would act for very weak gravitational forces and over large scales, such as in the outer rotational arms of galaxies, by using mathematical machinery borrowed from quantum mechanics. They found that gravitational forces acting far from galactic centres would match what is inferred from observations of rotational speed at the outer edges of galaxies.
“This is the most promising new idea I’ve heard in some time,” says at Case Western Reserve University in Ohio. However, he says, many physicists might only consider its merits if they start seeing problems with the current working model of the universe, which takes dark matter as real.
The idea could also work in tandem with another dark matter alternative called modified Newtonian dynamics (MOND). This tweaks the mathematics of Newtonian mechanics to reproduce phenomena like fast galaxy rotation, but doesn’t suggest a physical explanation for this tweak. “This theory could provide the basis for MOND, and that’s a really desirable thing,” says McGaugh.
However, there are other observations that post-quantum gravity will need to reproduce before it is taken seriously by physicists, says at King’s College London, such as subtle fluctuations in the CMB radiation, which apparently provides a snapshot of how matter and dark matter were distributed in the universe’s first moments. There are some hints this could be possible, but “it’s very, very early days,” says at UCL, who has worked on the idea with Oppenheim but wasn’t involved in the recent research. “The calculations that have been done so far are encouraging, but they’re very basic and there’s a lot that can go wrong.”
Along with the rotational speeds of galaxies and the CMB signals, Oppenheim and his colleagues will also need to show why galaxies within galaxy clusters move faster than they should according to how much mass we can observe. “Until they can do those three things, it’s not quite time to get the champagne,” says Fairbairn.
At this point, critics of post-quantum gravity remain unconvinced. In 2022, Oppenheim made a bet with physicist on whether space-time is quantum or not, at odds of 5000:1. Rovelli says this new work hasn’t changed his outlook.
at the University of California, Berkeley, who also took part in and officiated the bet, says: “I think it’s good that physicists explore a wide variety of approaches to very difficult problems like combining quantum mechanics with gravity. Personally, I don’t think this particular approach is likely to be the correct one. I’ve obviously put my money where my mouth is on that front, and there is nothing new in the recent papers that would make me change that assessment.”
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