A new explanation for why we haven’t found dark matter suggests it doesn’t exist. Instead, the author thinks we have misunderstood gravity. He is not the first to suggest this, but the new proposal, of gravity without mass, created by topological defects in space-time, is particularly novel.
Dark matter was first proposed in 1932, based on the observation that galaxies move in ways consistent with them having more mass than the sum of their stars and gas. What started as a few surprising measurements has blossomed. Simple explanations, such as undercounting stellar populations, have utterly failed.
A wide range of options have been proposed of what dark matter could be made of, from primordial black holes to subatomic particles. So far we haven’t found any, except rogue planets and stellar-mass black holes each of which may account for only a fraction of one percent of what is missing.
This has led some physicists to wonder if we’ve got it all wrong. Maybe dark matter doesn’t exist and gravity works differently on a larger scale than we think. One version of this, known as Modified Newtonian Dynamics (MOND), has received a fair amount of attention, but has proven far more popular with online commentators than with physicists, who generally regard it as highly unlikely.
Professor Richard Lieu of the University of Alabama in Huntsville has gotten into this situation with his own twist on gravity, which posits that gravity can exist. If he is right, this will upend much of cosmology and make the search for dark matter an embarrassing distraction in the march of science, like phlogiston. However, there is still a long way to go before it is widely considered plausible, let alone probable.
Lieu argues that topological defects in space-time arose shortly after the Big Bang.
“Topological effects are very compact regions of space with a very high matter density, usually in the form of linear structures known as cosmic strings, although 2D structures such as spherical shells are also possible,” Lieu said in a statement.
‘The shells in my article consist of a thin inner layer of positive mass and a thin outer layer of negative mass; the total mass of both layers – the only thing you can measure in terms of mass – is exactly zero, but as a star, if it is on this scale, it experiences a strong gravitational pull that pulls it towards the center of the scale.”
The force here would have a strength equal to the reciprocal of the distance, instead of the reciprocal of the square of the distance, as is true under Newton’s law of gravity. Whether this is physically possible has not been proven, but Lieu claims to show that it works mathematically.
Lieu proposes that when light passes through an object such as a galaxy in a gravitational lens, the light is bent inward as it passes through these shells. This bending would be barely distinguishable from what would happen if more mass were pulling on it. The same applies to the motions of a star around a galactic center of mass.
Because, as Lieu points out, these two phenomena form the basis for our confidence that dark matter exists, an alternative process for both would make dark matter redundant.
On the other hand, the idea of grenades with positive and negative masses on each side is equally unproven, and it is considerably more difficult to explain why they would exist in the first place.
Lieu argues that the origin of these defects was related to a “cosmological phase transition” in the early universe, where matter throughout the universe underwent a change of state.
“My own inspiration came from my search for another solution to the gravitational field equations of general relativity,” Lieu said. This can “yield a finite force of gravity in the absence of any detectable mass.”
Lieu’s proposal requires several things for which we have no direct evidence, rather than just one, as in the case of dark matter. The researcher himself admits: ‘It is currently unclear which precise form of phase transition in the universe could give rise to these types of topological defects.’
He has suggested a few possible steps, such as that the shells were once flats or straight strings that wound up. At the moment, however, it all sounds almost as ad hoc as the epicycles that Ptolemaic astronomers used to explain the orbits of planets.
However, in defense of his hypothesis, Lieu might point out that the idea is so new that no one has looked for suitable evidence. On the other hand, billions of dollars and some of the best minds of our generation have been spent on the failed search for dark matter.
“This initiative, in turn, is driven by my frustration with the status quo, namely the idea of the existence of dark matter, despite the absence of any direct evidence for an entire century,” Lieu added.
“Of course, the availability of a second solution, even if highly suggestive, is not in itself sufficient to discredit the dark matter hypothesis – it could at best be an interesting mathematical exercise,” Lieu concluded. “But it’s the first.” proof that gravity can exist without mass.”
The proposal was published in Monthly Notices of the Royal Astronomical Society.