Unless there is more matter in the cosmos than what is visible, dark matter—a hypothetical type of matter suggested by gravitational effects—cannot be described by general relativity. The theory, first proposed by Dutch astronomer Jan Oort in 1932 to explain the so-called “missing mass” required for objects like galaxies to cluster together, is still as unexplained today as it was almost a century ago.
For the first time, researchers at The University of Alabama in Huntsville (UAH) have demonstrated how gravity can exist without mass, offering an alternate explanation that may mitigate the necessity for dark matter.
Scientists were inspired by the search for an alternative solution to general relativity’s gravitational field equations. This alternative solution, known as the Poisson equation, provides a finite gravitational force in the absence of any detectable mass and applies to the conditions of galaxies and clusters of galaxies.
Lieu is a distinguished professor of physics and astronomy at UAH, a part of the University of Alabama System. “This initiative is 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.”
According to scientists, concentric sets of shell-like topological defects in structures commonly found throughout the universe may be the source of the “excess” gravity required to hold a galaxy or cluster together. These defects were most likely created during the early universe when a phase transition occurred. A physical event known as a cosmological phase transition occurs when the general state of matter changes simultaneously throughout the universe.
Lieu says, “It is unclear presently what precise form of phase transition in the universe could give rise to topological defects of this sort. 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. However, 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.”
The gravitational force allows all objects, massless or otherwise, to interact with one another because it essentially entails the bending of space-time itself. For instance, it has been established that celestial objects exert gravitational pull on massless photons.
“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 that mimics the presence of a large amount of dark matter in much the same way as the velocity of stellar orbits.
“Both the deflection of light and stellar orbital velocities is the only way one gauges the strength of the gravitational field in a large-scale structure, be it a galaxy or a cluster of galaxies. My paper contends that at least the shells it posits are massless. There is no need to perpetuate this seemingly endless search for dark matter.”
“This paper does not attempt to tackle the problem of structure formation. 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.”
“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. But it is the first proof that gravity can exist without mass.”
Journal Reference:
Richard Lieu. The binding of cosmological structures by massless topological defects. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stae1258
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