A new explanation for why we haven’t found dark matter suggests it doesn’t exist. Instead, the author thinks that we have misunderstood gravity. He is not the first to suggest this, but the new proposal, of massless gravity created by topological defects in spacetime, is particularly novel.
Dark matter was first proposed in 1932, based on the observation that galaxies are moving in such a way that they have more mass than the sum of their stars and gas. What started as a few surprising measurements has blossomed. Simple explanations, such as stellar populations being undercounted, have definitely failed.
A wide range of options have been proposed for what dark matter might consist of, from primordial black holes to subatomic particles. So far, we haven’t found any of them, except for rogue planets and stellar-mass black holes that each can only account for a fraction of one percent of what is missing.
This has led some physicists to wonder if we’ve got it all wrong. Perhaps there is no dark matter, and instead, gravity works differently on larger scales than we think. A version of this, known as Modified Newtonian Dynamics (MOND) has received little attention, but has proven far more popular with internet commentators than with physicists, who generally regard it as highly unlikely.
Professor Richard Lieu of the University of Alabama in Huntsville has stepped into this situation with his spin on gravity, which says it can exist without mass. If he is right, it will turn much of cosmology on its head and make the search for dark matter an embarrassing detour in the march of science, like phlogiston. However, it has a long way to go before it is widely regarded as credible, let alone possible.
Lieu proposes that topological defects were created in spacetime shortly after the Big Bang.
“Topological effects are very compact regions of space with a very high density of matter, 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 crabs in my paper consist of a thin inner layer of positive mass and a thin outer layer of negative mass; the total mass of both layers—which is all that can be measured, in terms of mass—is exactly zero, but when a star lies in this shell, it experiences a great gravitational force pulling it toward the center of the shell.”
The force here would have a force equal to the inverse of the distance, rather than the inverse of the distance squared, as applied under Newton’s law of gravity. Whether this is physically possible is unproven, but Lieu claims to show that it works mathematically.
Lieu proposes that as light passes through an object like a galaxy in a gravitational lens, it is bent slightly inward by passing through these shells. This bending would be barely distinguishable from what would happen if there were more mass pulling on it. The same applies to the motions of a star around a galactic center of mass.
Since, as Lieu notes, these two phenomena are the basis for our belief that dark matter exists, an alternative process for both would make dark matter unnecessary.
On the other hand, the idea of ​​shells with positive and negative mass on each side is equally unproven, and it is much more difficult to explain why they would exist at all.
Lieu proposes that the creation of these defects was accompanied by a “cosmological phase transition” in the early universe, where matter throughout the universe underwent a change of state.
“My inspiration came from my pursuit of another solution to the gravitational field equations of general relativity,” Lieu said. This can give “a finite force of gravity in the absence of any discernible 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: “At the moment it is unclear what exact form of phase transition in the universe can cause topological defects of this type.”
He has suggested several possible steps, for example that the shells were once planes or straight strings that coiled. At present, however, it all sounds about as ad hoc as the epics that Ptolemaic astronomers used to explain the orbits of the planets.
However, in defense of his hypothesis, Lieu can point out that the idea is new enough that no one has looked for adequate evidence. On the other hand, billions of dollars and some of the best minds of our generation have been devoted to the unsuccessful search for dark matter.
“This initiative is driven by my frustration with the status quo, namely the notion of the existence of dark matter despite the lack 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 enough to discredit the dark matter hypothesis – it can be an interesting mathematical exercise at best,” Lieu concluded. “But it’s the first. proof that gravity can exist without mass.”
The proposal is published in the Monthly Notices of the Royal Astronomical Society.
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