Something seems to be missing from the universe, and the favored model of physics calls it “dark matter” – but despite a century of searching, it remains a no-show. A new paper proposes an alternative hypothesis, showing how gravity could exist without mass and produce many of the same effects we ascribe to dark matter.
Einstein’s theory of general relativity is still our best model for describing gravity. As you might remember from high school physics class, gravity is the force that arises from masses resting on the fabric of spacetime. The more mass an object has, the deeper the “dip” in spacetime and the stronger the gravitational pull.
But starting in the 1930s, some strange astronomical observations began to raise questions. Galaxy clusters seemed to be moving much too fast to stay stable based on visible matter, suggesting that far more matter was present than we could see. That led to the hypothesis that huge amounts of invisible stuff – which was dubbed dark matter – pervaded the universe. The idea has held surprisingly strong in observations in the decades since, backed up by the motions of stars within galaxies and the bending and magnifying of light through gravitational lenses.
A good hypothesis is always testable, and so physicists concocted plenty of experiments designed to detect a range of plausible dark matter particles. But so far, all have come up empty, leading some scientists to propose alternatives like modified gravity or even a “dark fluid” permeating the cosmos.
A new paper, by Dr. Richard Lieu at the University of Alabama in Huntsville (UAH), suggests a new idea entirely. Topological defects in the cosmos, which could have been formed during a phase transition in the early universe, could exert a gravitational influence on nearby objects and passing light, but have zero mass themselves.
“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,” said Lieu. “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.”
This could explain how stars can move faster than they “should” be able to according to their visible mass, and how galaxies and clusters hold themselves together. And if these shells form groups of concentric rings, they could also explain observations of gravitational lenses, which magnify distant light sources.
“Gravitational bending of light by a set of concentric singular shells comprising a galaxy or cluster is due to a ray of light being deflected slightly inwards – that is, towards the center of the large-scale structure, or the set of shells – as it passes through one shell,” said Lieu. “The sum total effect of passage through many shells is a finite and measurable total deflection which mimics the presence of a large amount of dark matter in much the same way as the velocity of stellar orbits.”
It might sound a bit too convenient to invent a new phenomenon out of nowhere, but it’s not without some merit. First, negative mass sounds like a sci-fi concept, but it has been modeled before, and some of its expected properties – such as an object flowing backwards when you push on it – have even been demonstrated in fluids and particles. Second, gigantic ring structures seen in space, which can’t currently be explained through dark matter, could be evidence of these topological defects.
It’s an intriguing idea, albeit one that still has a few holes to plug up. For one, the paper doesn’t attempt to explain how the defects form in the first place. There’s also the problem of how these shell structures could be confirmed or ruled out through observations. And finally, Lieu admits that it might not be enough to remove the need for dark matter entirely, but could just reduce its role.
Still, it’s the first model that suggests gravity could exist without mass, and future work could investigate how the structures might form and how they could guide galaxies and clusters to form.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.
Source: UAH
