For centuries, scientists have grappled with the fundamental forces that govern our universe, the most important of which are gravity and, more recently, dark matter.
Gravity is the invisible force that pulls objects of mass together and plays a crucial role in shaping the cosmos, from the formation of galaxies to the orbits of planets.
However, as our understanding of the universe has expanded, so have the mysteries surrounding it.
Dark matter dilemma
One of the most baffling mysteries is the concept of dark matter, a hypothetical form of matter believed to make up a significant portion of the universe’s total mass.
Unlike ordinary matter, which we can directly see and interact with, dark matter does not emit, absorb or reflect light, making it invisible to telescopes and other detection instruments.
The existence of dark matter, first suggested by Dutch astronomer Jan Oort in 1932, is inferred solely from the gravitational effects it exerts on visible matter, such as the rotation curves of galaxies and the motion of galaxies within clusters. This leads scientists to question the nature of gravity itself.
These observations suggest that there is much more matter in the universe than can be explained by visible matter alone.
Despite decades of research, the exact nature of dark matter remains one of the biggest mysteries in modern physics, with scientists exploring various theories, such as weakly interacting massive particles (WIMPs) and axions, to explain its properties and behavior.
Ever-present gravity
Gravity is one of the four fundamental forces of nature, along with electromagnetism, the strong nuclear force and the weak nuclear force. It is the force that pulls objects with mass together, and plays a crucial role in shaping the universe at all scales.
On the Earth’s surface, gravity pulls objects toward the center of the planet, giving them weight and keeping them on the ground.
On a larger scale, gravity controls the orbits of planets around the sun, the movement of stars in galaxies, and the formation and evolution of galaxies and galaxy clusters.
According to Albert Einstein’s general theory of relativity, gravity arises from the curvature of space-time, caused by the presence of mass and energy. The more massive an object is, the greater its gravitational influence on other objects.
Despite its ubiquity and importance, gravity remains one of the least understood forces in physics. Continuous research is being done to reconcile these with the principles of quantum mechanics and to explain phenomena such as dark matter and dark energy.
Seeing gravity and dark matter in a new light
From a new perspective, a recent study by Dr. Richard Lieu of the University of Alabama in Huntsville (UAH) to solve the puzzle by adding a new twist to this age-old problem.
Published in the Monthly notices of the Royal Astronomical SocietyLieu’s paper shows for the first time how gravity can exist without mass.
This radical and thought-provoking research provides an alternative theory that could potentially alleviate the need for dark matter.
“My own inspiration came from my search for another solution to the gravitational field equations of general relativity,” says Lieu, a distinguished professor of physics and astronomy at UAH.
“This initiative, in turn, is driven by my frustration with the status quo, namely the idea of the existence of dark matter, despite the absence of any direct evidence for an entire century.”
Topological defects may hold the key
Lieu argues that the ‘excess’ gravity needed to bind a galaxy or cluster together could be due to concentric sets of shell-like topological defects in structures commonly found in the cosmos.
These defects most likely arose during the early universe, when a cosmological phase transition occurred, a physical process in which the overall state of matter in the entire universe changes together.
“It is currently unclear what precise form of phase transition in the universe could give rise to these types of topological defects,” says Lieu.
“Topological effects are very compact regions of space with a very high matter density, usually in the form of linear structures known as cosmic strings, although 2D structures such as spherical shells are also possible.”
The massless gravitational effect is similar to dark matter
The grenades proposed in Lieu’s article consist of a thin inner layer with positive mass and a thin outer layer with negative mass.
Although the total mass of both layers is exactly zero, a star lying on this shell experiences a strong gravitational force that pulls it towards the center of the shell.
Because gravity essentially involves the warping of space-time itself, it allows all objects to interact with each other, whether they have mass or not.
For example, massless photons have been confirmed to experience gravitational effects from astronomical objects.
“The gravitational bending of light by a series of concentric single shells forming a galaxy or cluster results from a ray of light being bent slightly inward – that is, toward the center of the large-scale structure, or series of shells – because this one goes through one shell,” Lieu notes.
He explains that when light moves through multiple shells, the cumulative effect results in a measurable deflection that is very similar to the gravitational influence typically attributed to the presence of a significant amount of dark matter, similar to the observed velocities of stellar orbits in galaxies.
Role of massless shells in galaxy formation
The deflection of light and the orbital speeds of stars are the only way one can measure the strength of the gravitational field in a large-scale structure, be it a galaxy or a cluster of galaxies.
Lieu’s paper claims that the shells it posits are massless, suggesting that there may be no need to continue the seemingly endless search for dark matter.
Questions for future research will likely focus on how a galaxy or cluster is formed by the alignment of these shells, as well as how the evolution of the structures occurs.
“Of course, the availability of a second solution, even if highly suggestive, is not in itself sufficient to discredit the dark matter hypothesis; it could be an interesting mathematical exercise at best,” Lieu concludes.
Lieu emphasizes that his research is not aimed at addressing the issue of structure formation in the universe, and acknowledges that there are still open questions about the original state of the shells and how their existence can be definitively confirmed or refuted through of targeted observations.
Despite these limitations, Lieu claims that his work is the first demonstration of the possibility of gravity without mass.
Dark matter versus massless gravity: let the game begin
In summary, Dr. Richard Lieu challenges the age-old idea of dark matter and offers a revolutionary perspective on the nature of gravity.
By demonstrating how gravity can exist without mass through the concept of massless shells, Lieu’s work opens new avenues for understanding the universe and its fundamental forces.
Although further research is needed to confirm or refute the existence of these massless shells, this study represents a significant leap forward in our understanding of the cosmos.
As the scientific community continues to explore the implications of Lieu’s findings, we may be on the cusp of a new era in astrophysics, one that reshapes our understanding of the mysterious force that connects galaxies and clusters.
The full study was published in the journal Monthly notices of the Royal Astronomical Society.
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