What is gravity without mass? Both Newton’s revolutionary laws describing its universal effect and Einstein’s proposal of deepened spacetime, we thought of gravity as exclusively in the realm of matter.
Now, a wild new study suggests that gravity can exist without matter, conveniently eliminating the need for one of the most volatile substances in our universe: dark matter.
Dark matter is a hypothetical, invisible mass believed to make up 85 percent of the total volume of the universe. It was originally proposed to account for galaxies holding together at high rotation rates, but has not yet been directly observed, leading physicists to propose all sorts of ideas to avoid using this elusive material as a way to plug holes in current theories.
The latest offering along these lines comes from astrophysicist Richard Lieu of the University of Alabama in Huntsville, who proposed that instead of dark matter, galaxies and other bodies bind the universe together. may contain thin, shell-like layers of “topological defects” which give rise to gravity without any underlying matter.
Lieu began trying to find other solutions to Einstein’s field equations that relate the curvature of spacetime to the presence of matter in it.
As Einstein described in his 1915 theory of general relativity, space-time revolves around bundles of matter and streams of radiation in space, depending on their energy and momentum. This energy is of course related to mass in Einstein’s famous equation: E=mc2.
So an object’s mass is associated with its energy, which bends spacetime—and this curvature of spacetime is what Einstein described as gravity, a degree more sophisticated than Newton’s 17th-century approximation of gravity as a force between two massive objects. . In other words, gravity appears to be inextricably linked to matter.
Not so, Lieu surmises.
In his work, Lieu set out to solve a simplified version of Einstein’s field equations that allows for a finite gravitational force in the absence of any detectable matter. He says his efforts were “fueled by my frustration with the status quo, namely the notion of dark matter’s existence despite the lack of any direct evidence for a century.”
Lieu’s solution consists of shell-shaped topological defects that can occur in very compact regions of space with very high matter density.
These sets of concentric shells contain a thin layer of positive matter tucked inside an outer layer of negative matter. The two masses cancel each other out, so the total mass of the two layers is exactly zero. But when the star lies on this shell, it experiences a large gravitational force that pulls it towards the center of the shell.
“The claim of my paper is that at least the shells it assumes are immaterial,” says Lieu. If these controversial suggestions have any weight, “there’s no need to keep up this seemingly endless search for dark matter,” Lieu adds.
So the next question is how to possibly confirm or disprove the shells that Lieu suggested through observation.
“The increasing frequency of observations of ring and shell galaxy formations in the Universe provides evidence for the type of source proposed here,” Lieu writes in his paper. Although he admits that his proposed solution is “highly suggestive” and cannot by itself discredit the dark matter hypothesis.
“It could be an interesting mathematical exercise at best,” Lieu concludes. “But it’s the first [mathematical] proof that gravity can exist without matter.”
The study was published in Monthly Notices of the Royal Astronomical Society.