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 material objects together and plays a fundamental role in shaping the universe, from the formation of galaxies to the orbits of planets.
However, as our understanding of the universe expanded, so did the mysteries surrounding it.
The Dark Matter Dilemma
One of the most perplexing 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, so it is invisible to telescopes and other detectors.
The existence of dark matter, first proposed by Dutch astronomer Jan Oort in 1932, is inferred only from the gravitational effects it has on visible matter, such as the rotation curves of galaxies and the motion of galaxies in 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 explained by visible matter alone.
Despite decades of research, the exact nature of dark matter remains one of the greatest mysteries of modern physics, with scientists exploring various theories such as weakly interacting massive particles (WIMPs) and axions to explain its properties and behavior.
The omnipresent 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 material objects together and plays a vital 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 controls the orbits of planets around the Sun, the motion of stars in 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 spacetime caused by the presence of matter 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 trying 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
From a new perspective, a recent study by Dr. Richard Lieu of The University of Alabama in Huntsville (UAH) hopes to solve the puzzle by adding a new twist to this age-old problem.
Published in Monthly Notices of the Royal Astronomical SocietyLieu’s paper shows for the first time how gravity can exist without matter.
This radical and thought-provoking research provides an alternative theory that could potentially alleviate the need for dark matter.
“My own inspiration comes from my quest to further solve 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 current state of affairs, specifically the notion of dark matter’s existence despite the lack of any direct evidence for a century.”
Topological defects may be the key
Lieu argues that the “excess” gravity necessary to hold a galaxy or cluster together could be caused by concentric sets of shell-like topological defects in structures commonly found throughout the universe.
These defects were most likely created during the early universe when there was a cosmological phase transition, a physical process where the overall state of matter changes collectively throughout the universe.
“It is currently unclear what exact form of phase transition in space could lead to topological defects of this kind,” says Lieu.
“Topological effects are very compact regions of space with 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 resembles dark matter
The shells proposed in Lieu’s paper consist of a thin inner layer of positive matter and a thin outer layer of negative matter.
While the total mass of both layers is exactly zero, a star lying on this shell is subject to a large gravitational force that pulls it towards the center of the shell.
Since the gravitational force essentially involves warping 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 the gravitational effects of astronomical objects.
“The gravitational bending of light by the set of concentric singular shells that make up a galaxy or cluster is caused by the beam of light being deflected slightly inward—that is, toward the center of the large-scale structure or set of shells—as it passes through a single shell,” notes Lieu.
He explains that when light passes through multiple shells, the cumulative effect results in a measurable deviation 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 in galaxies.
The role of massless envelopes in galaxy formation
The deflection of light and stellar orbital velocities are the only means by which to measure the strength of the gravitational field in a large-scale structure, whether it is a galaxy or a cluster of galaxies.
Lieu’s paper argues that the shells he hypothesizes are devoid of matter, suggesting that there may be no need to maintain a seemingly endless search for dark matter.
Questions for future research are likely to focus on how a galaxy or cluster forms by aligning these envelopes, as well as how the structures evolve.
“Of course, the availability of a second solution, while highly suggestive, is not in itself enough to discredit the dark matter hypothesis – it could be an interesting mathematical exercise at best,” Lieu concludes.
Lieu emphasizes that his research does not aim to address the issue of structure formation in the universe, and acknowledges that there are still open questions about the initial state of the 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 gravity without matter.
Dark Matter vs. Massless Gravity: Let the Games Begin
In short, the fascinating research of Dr. Richarda Lieu challenges the century-old notion of dark matter and offers a revolutionary insight into the nature of gravity.
By demonstrating how gravity can exist without matter through the concept of massless shells, Lieu’s work opens new avenues for understanding the universe and its fundamental forces.
While further investigation is needed to confirm or disprove the existence of these matterless shells, this study represents a significant leap forward in our understanding of the universe.
As the scientific community continues to explore the implications of Lieu’s findings, we may be on the threshold of a new era in astrophysics that reshapes our understanding of the mysterious force that binds galaxies and clusters together.
The entire study was published in the journal Monthly Notices of the Royal Astronomical Society.
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