What is gravity without mass? Both Newton’s revolutionary laws describing its universal effect and Einstein’s proposal of a dimpled space-time have led us to think of gravity as falling solely within the domain of matter.
Now a wild new study suggests that gravity can exist without mass, easily eliminating the need for one of the most elusive substances in our universe: dark matter.
Dark matter is a hypothetical, invisible mass that is thought to make up 85 percent of the total size of the universe. Originally designed to account for galaxies staying together under high-speed rotation, it has yet to be directly observed, prompting physicists to propose all kinds of ideas to prevent this elusive material from being used as a way to plug holes in the current theories.
The latest offering along these lines comes from astrophysicist Richard Lieu of the University of Alabama in Huntsville, who has suggested that instead of dark matter connecting galaxies and other bodies, the universe may contain thin, shell-like layers of ‘topological defects’ giving rise to gravity without any underlying mass.
Lieu began trying to find another solution to Einstein’s field equations, which relate the curvature of space-time to the presence of matter in it.
As Einstein described in his 1915 theory of general relativity, space-time curves around bundles of matter and radiation streams in the universe, depending on their energy and momentum. That energy is of course related to the mass in Einstein’s famous equation: E=mc2.
An object’s mass is thus linked to its energy, which bends space-time – and this curvature of space-time is what Einstein described as gravity, a notch more advanced than Newton’s 17th century approach to gravity as a force between two objects with mass. . In other words: gravity seems inextricably linked to mass.
Not so, says Lieu.
In his work, Lieu began by solving a simplified version of the Einstein field equations that allows for finite gravity in the absence of any detectable mass. He says his efforts were “driven by my frustration with the status quo, which is the idea of the existence of dark matter, despite the absence of any direct evidence for an entire 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 mass, tucked into an outer layer of negative mass. The two masses cancel each other out, so the total mass of the two layers is exactly zero. But when a star lies on this shell, it experiences a strong gravitational pull that drags it toward the center of the shell.
“The thesis of my article is that at least the shells it posits are massless,” Lieu says. If these controversial suggestions carry any weight, “there is no need to continue this seemingly endless search for dark matter,” Lieu adds.
So the next question is how we can confirm or refute the shells that Lieu has proposed through observations.
“The increasing frequency of observations of ring- and shell-like formation of galaxies 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 mass.”
The research was published in Monthly notices of the Royal Astronomical Society.