Dark Matter is Nature’s poltergeist. We can see its effects, but we can’t see it, and we don’t know what it is. It’s as if Nature is playing tricks on us, hiding most of its mass and confounding our efforts to determine what it is.
It’s all part of the Universe’s “missing mass” problem. Actually, it’s our problem. The Universe is what it is. It’s our understanding of the Universe, mass, and gravity that’s the problem. And a solution is proving to be elusive.
Whatever the missing mass is or whatever causes the effects we observe, we have a placeholder name for it: dark matter. And it makes up 85% of the matter in the Universe.
Could dark matter be primordial black holes? Could it be axions? How about WIMPS? Are dark photons its force carrier? There’s lots of theoretical thought but no conclusion.
New research in the Monthly Notices of the Royal Astronomical Society says that our hunt for dark matter may be off-track. Instead of looking for a type of particle, the solution might lie in a type of topological defect found throughout the Universe that has its roots in the Universe’s early stages.
The new research is in a paper titled “The binding of cosmological structures by massless topological defects.” The author is Richard Lieu, a distinguished professor of physics and astronomy at the University of Alabama at Huntsville.
“There is then no need to perpetuate this seemingly endless search for dark matter.”
As the paper’s title makes clear, dark matter has a binding effect on structures like galaxies. Astronomers know that galaxies don’t have enough measurable mass to hold themselves together. By measuring the mass of the stars and gas in galaxies, it became clear that the visible components of the galaxies don’t provide enough mass to hold themselves together. They should simply dissipate into their constituent stars and clouds of gas.
But galaxies don’t dissipate, and scientists have concluded that something is missing. Professor Lieu has another idea.
“My own inspiration came from my pursuit for another solution to the gravitational field equations of general relativity — the simplified version of which, applicable to the conditions of galaxies and clusters of galaxies, is known as the Poisson equation — which gives a finite gravitation force in the absence of any detectable mass,” said Lieu. “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.”
An entire century is a long time in the age of modern science. It’s not surprising that Nature has the power to confound us, but it is somewhat surprising that very little progress has been made on the problem. Scientists have made great progress in understanding how dark matter influences the Universe’s large-scale structure, an impressive feat, but haven’t figured out what it is.
“The nature of dark matter (DM), defined specifically in this letter as an unknown component of the cosmic substratum responsible for the extra gravitational field that binds galaxies and clusters of galaxies, has been an enigma for more than a century,” Dr. Lieu writes in his paper.
Lieu’s work leans on phase transitions in the Universe. These are episodes when the state of matter in the Universe changes. Not locally but across the entire cosmos. One example is when the Universe cooled enough to allow the strong force to bind quarks into protons and neutrons.
Dr. Lieu contends that topological defects could have formed during one of these phase transitions. These defects can take the shape of shell-like compact regions where matter density is much higher. When arranged in concentric rings, these defects behave like gravity but don’t have mass.
“It is unclear presently what precise form of phase transition in the universe could give rise to topological defects of this sort,” Lieu says. “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 2-D structures such as spherical shells are also possible. 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.”
So, despite our inability to measure the mass, it’s there, and other objects respond to it. Mass warps space-time and affects even massless photons. That fact underlies our ability to use gravitational lensing. We use the mass of galaxy clusters in gravitational lensing. A set of spherical shells, as Lieu talks about, could cause the same effect.
This illustration shows the gravitational lensing phenomenon. Astronomers use it to study very distant and very faint objects. Note that the scale has been greatly exaggerated in this diagram. In reality, the distant galaxy is much further away and much smaller. Image Credit: NASA, ESA & L. Calcada“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,” Lieu notes. “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.”
Since astronomers measure galaxy and galaxy cluster masses by measuring the light they deflect and the way they affect the orbit of stars, astronomers could be measuring topological defects rather than particles that comprise dark matter.
“Both the deflection of light and stellar orbital velocities is the only means by which one gauges the strength of the gravitational field in a large-scale structure, be it a galaxy or a cluster of galaxies,” Dr. Lieu says. “The contention of my paper is that at least the shells it posits are massless. There is then no need to perpetuate this seemingly endless search for dark matter.”
In 2022, researchers discovered a giant arc in the sky. It spans 1 Gigaparsec and is nearly symmetrical. It’s one of several large-scale structures that seems to go against the Standard Model and the Cosmological Principle it’s based on.
These are three separate data images of the Giant Arc discovered in 2022. The paper provides details. Image Credit: Lopez et al. 2022, 10.1093/mnras/stac2204“The observation of giant arcs and rings could lend further support to the proposed alternative to the DM model,” Lieu writes in his paper. He also points out that the shells he proposes needn’t be a complete sphere.
If these shells exist, their alignment would also govern the formation and shape of galaxies and clusters. Future research will determine exactly how these shells form. “This paper does not attempt to tackle the problem of structure formation,” Lieu says. In fact, Lieu acknowledges that there’s currently no way to even observe how they might form.
“A contentious point is whether the shells were initially planes or even straight strings, but angular momentum winds them up. There is also the question of how to confirm or refute the proposed shells by dedicated observations,” Lieu says.
An experienced scientist, Lieu knows the limits of what he’s proposing.
“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 concludes. “But it is the first proof that gravity can exist without mass.”
