Dark Matter is Nature\u2019s poltergeist. We can see its effects, but we can\u2019t see it, and we don\u2019t know what it is. It\u2019s as if Nature is playing tricks on us, hiding most of its mass and confounding our efforts to determine what it is. <\/p>\n
<\/p>\n It\u2019s all part of the Universe\u2019s \u201cmissing mass\u201d problem. Actually, it\u2019s our problem. The Universe is what it is. It\u2019s our understanding of the Universe, mass, and gravity that\u2019s the problem. And a solution is proving to be elusive. <\/p>\n 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. <\/p>\n Could dark matter be primordial black holes? Could it be axions? How about WIMPS? Are dark photons its force carrier? There\u2019s lots of theoretical thought but no conclusion. <\/p>\n 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\u2019s early stages. <\/p>\n The new research is in a paper titled \u201cThe binding of cosmological structures by massless topological defects.\u201d The author is Richard Lieu, a distinguished professor of physics and astronomy at the University of Alabama at Huntsville. <\/p>\n \u201cThere is then no need to perpetuate this seemingly endless search for dark matter.\u201d<\/p>\n Dr. Richard Lieu, Professor, University of Alabama, Huntsville<\/cite><\/p><\/blockquote>\n<\/figure>\n As the paper\u2019s title makes clear, dark matter has a binding effect on structures like galaxies. Astronomers know that galaxies don\u2019t 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\u2019t provide enough mass to hold themselves together. They should simply dissipate into their constituent stars and clouds of gas. <\/p>\n But galaxies don\u2019t dissipate, and scientists have concluded that something is missing. Professor Lieu has another idea.<\/p>\n \u201cMy own inspiration came from my pursuit for another solution to the gravitational field equations of general relativity \u2014 the simplified version of which, applicable to the conditions of galaxies and clusters of galaxies, is known as the Poisson equation \u2014 which gives a finite gravitation force in the absence of any detectable mass,\u201d said Lieu. \u201cThis initiative is in turn driven by my frustration with the status quo, namely the notion of dark matter\u2019s existence despite the lack of any direct evidence for a whole century.\u201d<\/p>\n An entire century is a long time in the age of modern science. It\u2019s 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\u2019s large-scale structure, an impressive feat, but haven\u2019t figured out what it is. <\/p>\n \u201cThe 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,\u201d Dr. Lieu writes in his paper.<\/p>\n Lieu\u2019s 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. <\/p>\n 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\u2019t have mass.<\/p>\n \u201cIt is unclear presently what precise form of phase transition in the universe could give rise to topological defects of this sort,\u201d Lieu says. \u201cTopological 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 \u2014 which is all one could measure, mass-wise \u2014 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.\u201d<\/p>\n So, despite our inability to measure the mass, it\u2019s 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. <\/p>\n \u201cGravitational 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 \u2014 that is, towards the center of the large-scale structure, or the set of shells \u2014 as it passes through one shell,\u201d Lieu notes. \u201cThe 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.\u201d<\/p>\n 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. <\/p>\n \u201cBoth 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,\u201d Dr. Lieu says. \u201cThe 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.\u201d<\/p>\n In 2022, researchers discovered a giant arc in the sky. It spans 1 Gigaparsec and is nearly symmetrical. It\u2019s one of several large-scale structures that seems to go against the Standard Model and the Cosmological Principle it\u2019s based on. <\/p>\n \u201cThe observation of giant arcs and rings could lend further support to the proposed alternative to the DM model,\u201d Lieu writes in his paper. He also points out that the shells he proposes needn\u2019t be a complete sphere. <\/p>\n 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. \u201cThis paper does not attempt to tackle the problem of structure formation,\u201d Lieu says. In fact, Lieu acknowledges that there\u2019s currently no way to even observe how they might form. <\/p>\n \u201cA 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,\u201d Lieu says.<\/p>\n An experienced scientist, Lieu knows the limits of what he\u2019s proposing. <\/p>\n \u201cOf course, the availability of a second solution, even if it is highly suggestive, is not by itself sufficient to discredit the dark matter hypothesis \u2014 it could be an interesting mathematical exercise at best,\u201d Lieu concludes. \u201cBut it is the first proof that gravity can exist without mass.\u201d<\/p>\n\n
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