Researchers believe gravity could exist even without mass, reducing the need for hypothetical dark matter.

Dark matter is a hypothetical form of matter created by the effects of gravity that general relativity cannot explain unless there is more matter in the universe than we can see. The phenomenon remains almost as mysterious as it was nearly a century ago, when Dutch astronomer Jan Oort first proposed the theory in 1932 to explain the so-called “missing mass” needed for things like galaxies to assemble.

Now, Dr. Richard Liu of the University of Alabama in Huntsville (UAH) has published an article in the Monthly Notices of the Royal Astronomical Society that shows for the first time how gravity can exist even without mass, offering an alternative theory that could obscure the need for gravity to be alleviated. “My own inspiration came from looking for an alternative solution to the gravitational field equations of general relativity, a simplified version of which can be applied to the conditions of galaxies and clusters of galaxies, known as the Poisson equation, which gives finite gravity. No detectable mass exists,” says Liu, a distinguished professor of physics and astronomy at UAH, part of the University of Alabama system. “This effort was driven by my dissatisfaction with the status quo: the idea that dark matter exists, despite a century of lack of direct evidence.” The researchers argue that the “excess” gravity needed to hold galaxies and clusters of galaxies together may be common throughout the universe and may be due to concentric shell-like topological defects in the structure, possibly originating in the early universe, where a phase transition has taken place. A cosmological phase transition is a physical process in which the overall state of matter throughout the universe changes. “It is unclear at present what exact form of phase transition in the universe could give rise to this kind of topological defect,” Liu said. “Topological effects are very compact regions of space that contain very dense matter, usually in the form of linear structures called cosmic strings, but also 2D structures such as spherical shells. “The shell in my article consists 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 we can measure in terms of mass) is exactly zero, but when a star sits on this shell, it experiences a large gravitational force that pulls it towards the center of the shell.” Gravity is fundamentally related to the curvature of space-time itself, which makes it possible for all objects to interact with each other, regardless of whether they have mass or not. For example, it has been confirmed that massless photons are subject to gravitational forces from celestial objects. “The gravitational curvature of light passing through a series of concentric individual shells that form galaxies and clusters of galaxies is due to the fact that the light ray is deflected slightly inwards, towards the center of the cluster, as it passes through a shell, which is similar to the way a large structure or a series of shells – ” Liu points out. “The overall effect of passing through many shells is a finite and measurable total deflection, reflecting the presence of a large amount of dark matter at a speed roughly equal to the speed of the stellar orbits.

“Both the deflection of light and the orbital velocity of stars are the only measures of the strength of the gravitational field in large-scale structures, whether galaxies or clusters of galaxies. The argument of my paper is that “at least the shells he hypothesizes are massless, and there is no need to continue this seemingly endless search for dark matter.” Future research questions may focus on how galaxies or clusters of galaxies are formed by the arrangement of these shells, and how the evolution of the structure occurs. “This paper does not attempt to address the problem of structure formation. What is open to debate is whether the shells were originally flat or whether they were straight strings that were wound up by angular momentum.” It also raises the question of how the proposed shell can be confirmed or refuted through targeted observations. Of course, the availability of the second solution, even if very suggestive, is not enough to disprove the dark matter hypothesis. It might be, at best, an interesting mathematical experiment,” Liu concludes. “But this is the first proof that gravity can exist even without mass.”

source: https://dx.doi.org/10.1093/mnras/stae1258