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Saturday, March 09, 2024

Controversial new theory of gravity rules out need for dark matter | Physics | The Guardian

Controversial new theory of gravity rules out need for dark matter

"Exclusive: Paper by UCL professor says ‘wobbly’ space-time could instead explain expansion of universe and galactic rotation

A photo combining an image of a supercluster with magenta-tinted clumps of an inferred dark matter map derived from observations by Nasa's Hubble space telescope.
A photo combining an image of a supercluster with magenta-tinted clumps of an inferred dark matter map derived from observations by Nasa's Hubble space telescope. Photograph: Alamy

Dark matter is supposed to account for 85% of the mass in the universe, according to conventional scientific wisdom. But proponents of a radical new theory of gravity, in which space-time is “wobbly”, say their approach could render the elusive substance obsolete.

The proposition, outlined in a new paper, raises the controversial possibility that dark matter, which has never been directly observed, is a mirage that a substantial portion of the physics community has been chasing for several decades. The theory is viewed as quite left-field and is yet to be thoroughly tested, but the latest claims are creating a stir in the world of physics.

Announcing the paper on X, Prof Jonathan Oppenheim, of University College London, said: “Folks, something seems to be happening. We show that our theory of gravity … can explain the expansion of the universe and galactic rotation without dark matter or dark energy.”

There are multiple lines of evidence for dark matter, but its nature has remained mysterious and searches by the Large Hadron Collider have come up empty-handed. Last year, the European Space Agency launched a mission, Euclid, aiming to produce a cosmic map of dark matter.

The latest paper, published on the Arxiv website and yet to be peer-reviewed, raises the question of whether it even exists, drawing parallels between dark matter and flawed concepts of the past, such as “the ether”, an invisible substance that was thought to permeate all of space.

“In the absence of any direct evidence for dark energy or dark matter it is natural to wonder whether they may be unnecessary scientific constructs like celestial spheres, ether, or the planet Vulcan, all of which were superseded by simpler explanations,” it states. “Gravity has a long history of being a trickster.”

In this case, the simpler explanation being proposed is Oppenheim’s “postquantum theory of classical gravity”. The UCL professor has spent the past five years developing the approach, which aims to unite the two pillars of modern physics: quantum theory and Einstein’s general relativity, which are fundamentally incompatible.

Oppenheim’s theory envisages the fabric of space-time as smooth and continuous (classical), but inherently wobbly. The rate at which time flows would randomly fluctuate, like a burbling stream, space would be haphazardly warped and time would diverge in different patches of the universe. The theory also envisions an intrinsic breakdown in predictability.

The paper, by Oppenheim and Andrea Russo, a PhD candidate at UCL, claims this take on the universe could explain landmark observations of rotating galaxies that led to the “discovery” of dark matter. Stars at the edges of galaxies, where gravity is expected to be weakest based on visible matter, ought to be rotating more slowly than stars at the centre. But in reality, the orbital motion of stars does not drop off. From this, astronomers inferred the presence of a halo of unseen (dark) matter exerting a gravitational pull.

In Oppenheim’s approach the additional energy required to keep the stars locked in orbit is provided by the random fluctuations in spacetime, which in effect add in a background hum of gravitation. This would be negligible in a high gravity interaction, such as the Earth orbiting the Sun. But in low gravity situations, such as the fringes of a galaxy, the phenomenon would dominate – and cumulatively could account for the majority of the energy in the universe.

“We show that it can explain the expansion of the universe and galactic rotation curves without the need for dark matter or dark energy,” Oppenheim said on X. “We do urge caution, however, since there is other indirect evidence for dark matter, so further calculations and comparison with data are needed. But if it holds, it would appear that 95% of the energy in the universe is due to the erratic nature of spacetime, signalling either a fundamental breakdown in predictability of physics, or we are immersed in an environment which does not obey the laws of classical or quantum theory.”

Not everyone is convinced, including the well-known theorists Prof Carlo Rovelli and Prof Geoff Penington, who have signed a 5,000:1 odds bet with Oppenheim against his theory being proven correct.

“I think it’s good that physicists explore a wide variety of approaches to very difficult problems like combining quantum mechanics with gravity,” said Penington.

“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.”

Others are more enthusiastic. “I think the authors are on to something really interesting here, exploring some beautiful and novel ideas,” said Prof Andrew Pontzen, a cosmologist at University College London. “However, the challenge for replacing dark matter is that there are so many different lines of evidence that suggest its presence. So far they have only addressed one of these lines. Only time will tell whether the new ideas can truly explain the huge variety of phenomena that point towards dark matter.”

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