For nearly a century, dark matter has been one of the most enigmatic concepts in astrophysics. This hypothetical form of matter is inferred from gravitational effects that cannot be explained by general relativity alone, suggesting the existence of unseen mass in the universe. Proposed by Dutch astronomer Jan Oort in 1932 to explain the missing mass required to hold galaxies together, dark matter remains a mystery.

However, Dr. Richard Lieu from the University of Alabama in Huntsville (UAH) has published a groundbreaking study that offers an alternative explanation. His research demonstrates, for the first time, that gravity can exist without mass, potentially reducing the necessity for dark matter.

My inspiration came from seeking another solution to the gravitational field equations of general relativity. The simplified version applicable to galaxies and galaxy clusters is known as Poisson’s equation, which provides a finite gravitational force in the absence of any detectable mass, says Lieu, a distinguished professor of physics and astronomy at UAH. This effort is driven by my frustration with the status quo, namely the notion of dark matter’s existence despite a century-long lack of direct evidence.

Lieu proposes that the excessive gravity needed to bind galaxies or clusters could instead arise from concentric sets of topological defects in structures commonly found throughout the cosmos. These defects likely formed during the early universe when a phase transition occurred. A cosmological phase transition is a process where the overall state of matter changes throughout the universe.

It is currently unclear what specific form of phase transition in the universe could give rise to such topological defects, says Lieu. Topological effects are very compact regions of space with a very high matter density, typically in the form of linear structures known as cosmic strings, though 2-D structures like 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 that could be measured in terms of mass, is exactly zero. But when a star encounters this shell, it experiences a strong gravitational force pulling it towards the shell’s center.

Since gravitational force fundamentally involves the warping of space-time itself, it allows all objects to interact, regardless of whether they have mass. For example, massless photons are confirmed to experience gravitational effects from astronomical objects.

The gravitational bending of light by a set of singular concentric shells comprising a galaxy or cluster occurs because a light beam is slightly deflected inward—toward the large-scale structure’s center or the shell set—as it passes through a shell, Lieu notes. The total summed effect of passing through many shells is a finite, measurable deflection that mimics the presence of a large amount of dark matter in the same way stellar orbital velocities do.

Both the deflection of light and stellar orbital velocities are the only means by which the strength of the gravitational field in a large-scale structure, be it a galaxy or a galaxy cluster, is measured. My paper asserts that at least the proposed shells are massless. Therefore, there is no need to perpetuate this seemingly endless search for dark matter.

Future research will likely focus on how a galaxy or cluster forms through the alignment of these shells and how the evolution of structures occurs.

This paper does not attempt to address 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’s also the question of how to confirm or refute the proposed shells through dedicated observations. Of course, the availability of a second solution, even if highly suggestive, is not enough to discredit the dark matter hypothesis—at most, it could be an interesting mathematical exercise, concludes Lieu. But it is the first proof that gravity can exist without mass.

Sources

The University of Alabama in Huntsville | Richard Lieu, The binding of cosmological structures by massless topological defects, Monthly Notices of the Royal Astronomical Society, Volume 531, Issue 1, June 2024, Pages 1630–1636, doi.org/10.1093/mnras/stae1258

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