An astrophysicist has shown that gravity may be able to exist without mass, a theory that suggests dark matter may not be necessary to explain phenomena observed in our universe.
Dark matter is a hypothetical form of matter, thought to account for 80 percent of the matter in the universe. However, dark matter does not emit, absorb or reflect light, so we can't see it. Instead, its existence is inferred from the gravitational effects it has on visible matter, for example, because it distorts light from distant stars.
Meanwhile, gravity is the force that pulls all things with mass or energy toward one another. Astronomers have long suspected invisible dark matter to be the source of additional gravity that holds galaxy clusters together.
Now, Richard Lieu, a professor of physics and astronomy at the University of Alabama in Huntsville, has a new theory that could lessen the need for dark matter to exist.
Lieu suggests that the additional gravity needed to hold a galaxy or cluster together could instead come from shell-like, topological defects in structures commonly found throughout the cosmos.
These defects were most likely created during the early universe when a cosmological phase transition took place. This is a physical process where the overall state of matter changes together across the entire universe.
In a statement, Lieu said: "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."
Albert Einstein's theory posits that gravity warps space-time, causing interactions between objects regardless of whether they have mass. For example, even photons without mass are influenced by gravity, as light is warped as it passes by dense celestial objects.
For Lieu, observations of the deflection of light and the velocities of stellar orbits in galaxies may be the result of these mass-less shell-like defects. He suggests this theory helps to explain phenomena better than the theoretical existence of dark matter.
Lieu added: "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. 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."
However, he does not explain how these mass-less shells may be formed.
"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."
The full findings of the study were published in the Monthly Notices of the Royal Astronomical Society.
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