For centuries, scientists have grappled with the fundamental forces that govern our universe, chief among them gravity, and more recently, dark matter.
Gravity is the invisible force that pulls objects with mass toward each other, playing 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.
The dark matter dilemma
One of the most perplexing of these mysteries is the concept of dark matter, a hypothetical form of matter believed to make up a significant portion of the total mass of the universe.
Unlike ordinary matter, which we can see and interact with directly, 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 the 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 very nature of gravity.
These observations suggest that there is much more matter present in the universe than can be accounted for by visible matter alone.
Despite decades of research, the exact nature of dark matter remains one of the greatest mysteries in modern physics, with scientists exploring various theories, such as weakly interacting massive particles (WIMPs) and spins, to explain its properties and her behavior.
The ever-present force of 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 toward each other and plays a crucial role in shaping the universe at all scales.
On Earth’s surface, gravity pulls objects toward the center of the planet, giving them weight and keeping them grounded.
On a larger scale, gravity governs the orbits of the planets around the sun, the motion of stars within galaxies, and the formation and evolution of galaxies and galaxy clusters.
According to Albert Einstein’s theory of general 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, with ongoing research seeking to reconcile it with the principles of quantum mechanics and explain phenomena such as dark matter and dark energy.
Seeing gravity and dark matter in a new light
Taking a new perspective, a recent study by Dr. Richard Lieu at the University of Alabama in Huntsville (UAH) hopes to solve the puzzle by adding a new twist to this old problem.
Published in Monthly Notices of the Royal Astronomical SocietyLieu’s paper demonstrates, for the first time, how gravity can exist without mass.
This radical and thought-provoking research offers an alternative theory that could potentially alleviate the need for dark matter.
“My inspiration came from my pursuit of another solution to the gravitational field equations of general relativity,” says Lieu, a distinguished professor of physics and astronomy at UAH.
“This initiative is in turn driven by my frustration with the status quo, namely the notion of the existence of dark matter despite the lack of any direct evidence for an entire century.”
Topological defects may hold the key
Lieu contends that the “excess” gravity needed to bind a galaxy or cluster together may be due to concentric clusters of 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 occurred, a physical process where the overall state of matter changes together throughout the universe.
“At the moment it is unclear what exact form of phase transition in the universe can cause topological defects of this type,” says Lieu.
“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 2D structures such as spherical shells are also possible.”
The effect of massless gravity resembles dark matter
The shells proposed in Lieu’s paper consist of a thin inner layer of positive mass and a thin outer layer of negative mass.
While the total mass of both layers is exactly zero, a star lying in this shell experiences a large gravitational force pulling it towards the center of the shell.
Since the gravitational force essentially involves the distortion of spacetime itself, it enables all objects to interact with each other, whether they have mass or not.
Massless photons, for example, have been confirmed to experience gravitational effects from astronomical objects.
“Gravitational bending of light from a singular concentric shell cluster comprising a galaxy or cluster is due to a light beam being deflected slightly inward—that is, toward the center of the large-scale structure, or shell cluster—as it passes through a shell,” notes Lieu.
He explains that as light traverses multiple shells, the cumulative effect results in a measurable deflection that closely resembles the gravitational influence typically attributed to the presence of significant amounts of dark matter, similar to the observed velocities of stellar orbits within galaxies. .
The role of massless shells in galaxy formation
The deflection of light and stellar orbital velocities are the only means by which 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 he deploys are massless, suggesting there may not be a need to perpetuate the seemingly endless search for dark matter.
Questions for future research are likely to focus on how a galaxy or cluster forms from the alignment of these shells, and how the structures evolve.
“Of course, the availability of a second solution, even if highly suggestive, is not in itself enough to discredit the dark matter hypothesis – it can be an interesting mathematical exercise at best,” Lieu concludes.
Lieu points out that his research is not intended to address the question of structure formation in the universe, and admits that there are still open questions about the initial state of shells and how to definitively confirm or disprove their existence through targeted observations.
Despite these limitations, Lieu claims that his work represents the first demonstration of the possibility of massless gravity.
Dark Matter vs Massless Gravity: Let the Games Begin
In summary, the fascinating research of Dr. Richard Lieu challenges the age-old notion 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.
While further investigations are needed to confirm or disprove the existence of these massless shells, this study represents an important step 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 that reshapes our understanding of the mysterious force that binds galaxies and clusters together.
The full study is published in the journal Monthly Notices of the Royal Astronomical Society.
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