Dark matter is a theoretical type of matter that scientists believe exists because of gravitational effects that can’t be explained by current theories of gravity unless there is more unseen matter in the universe. This idea has been around for nearly a century, first suggested by Dutch astronomer Jan Oort in 1932 to explain the “missing mass” needed for galaxies to form and hold together.
Dr. Richard Lieu from the University of Alabama in Huntsville has proposed a new theory that explains gravity without needing dark matter. He argues that gravity can exist without mass, which could challenge the current understanding of dark matter.
Dr. Lieu’s research focuses on finding alternative solutions to the equations of general relativity. He suggests that the extra gravity required to hold galaxies and clusters together might come from shell-like structures known as topological defects, which were likely formed during a significant change in the state of matter in the early universe.
In summary, instead of needing dark matter to explain the extra gravity, Dr. Lieu’s theory proposes that this gravity might come from these cosmic structures created during the universe’s formation.
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.”
Gravitational force affects the fabric of space-time, allowing objects to interact regardless of their mass. For instance, massless photons (light particles) are known to be influenced by gravity from astronomical objects.
Dr. Lieu explains that light can bend around concentric shells in a galaxy or cluster, causing it to be slightly deflected inward as it passes through each shell. The cumulative effect of passing through many shells leads to a measurable deflection that can mimic the gravitational effects typically attributed to dark matter. This is similar to how we measure the strength of gravity based on the orbits of stars.
Dr. Lieu argues that these shells can be massless, which means we might not need to continue searching for dark matter to explain the gravitational effects we observe.
Future research will likely explore how galaxies or clusters form from these shells and how these structures evolve over time.
Lieu concludes, “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