A new explanation for why we have not found dark matter proposes that it doesn’t exist. Instead, the author thinks we have been misunderstanding gravity. He’s not the first to suggest that, 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 are moving 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 stellar populations having been undercounted, have definitely failed.
A wide array of options have been proposed for what dark matter might be composed of, from primordial black holes to subatomic particles. So far, we haven’t found any of them, other than rogue planets and stellar mass black holes that can each 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. One version of this, known as Modified Newtonian Dynamics (MOND) has got a fair bit of attention, but has proven a lot more popular with online commentators than with physicists, who generally regard it as very improbable.
Professor Richard Lieu of the University of Alabama at Huntsville has waded into this situation with his own spin on gravity, one which holds that it can exist without mass. If he’s right, it will turn much of cosmology on its head and make the quest for dark matter an embarrassing diversion in science’s march, like phlogiston. However, there’s a long, long way to go before it is widely treated as plausible, let alone likely.
Lieu proposes that topological defects were created in space-time soon 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 shells 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 one could measure, mass-wise—is exactly zero, but when a star lies on this shell it experiences a large gravitational force pulling it towards the center of the shell.”
The force here would have a strength equal to the inverse of distance, rather than the inverse of distance squared as applies 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 an object like a galaxy in a gravitational lens, it is bent slightly inward by its passage through these shells. This bending would be barely distinguishable from what would occur if there was more mass pulling on it. The same goes for the movements of a star around a galactic center of mass.
Since, as Lieu notes, these two phenomena are the basis for our confidence that dark matter exists, an alternative process for both would render dark matter unnecessary.
On the other hand, the idea of shells with positive and negative mass on each side is equally unproven, and it’s considerably harder to explain why they would exist at all.
Lieu proposes that the creation of these defects was associated with a “cosmological phase transition” in the early universe, where across the entire universe, matter underwent a change of state.
“My own inspiration came from my pursuit for another solution to the gravitational field equations of general relativity,” Lieu said. This can give “a finite gravitation force 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 unclear presently what precise form of phase transition in the universe could give rise to topological defects of this sort.”
He has suggested a few possible steps, for example that the shells were once planes or straight strings that became wound up. However, it currently all sounds almost as ad hoc as the epicycles Ptolemaic astronomers used to explain the orbits of planets.
In defense of his hypothesis, however, Lieu could point out that the idea is sufficiently new that no one has been searching for suitable evidence. On the other hand, billions of dollars and some of the finest minds of our generation have been devoted to the unsuccessful search for dark matter.
“This initiative is in turn driven by my frustration with the status quo, namely the notion of dark matter’s existence despite the lack of any direct evidence for a whole century,” Lieu added.
“Of course, the availability of a second solution, even if it is highly suggestive, is not by itself sufficient to discredit the dark matter hypothesis—it could be an interesting mathematical exercise at best,” Lieu concluded. “But it is the first proof that gravity can exist without mass.”
The proposal is published in Monthly Notices of the Royal Astronomical Society.
