What if dark matter didn’t exist?

What is dark matter?

Dark matter is a hypothetical form matter that cannot be observed directly with current instruments because it does not emit, absorb, or reflect light or other forms of electromagnetic radiation. It is, however, detectable by its gravitational effects on visible matter, such as stars and galaxies. The concept of dark matter was first introduced in 1933 by Swiss astronomer Fritz Zwicky, who observed that galaxies in a cluster were moving much faster than their visible mass would have predicted. This observation suggested the presence of additional invisible mass. Since then, further evidence has confirmed the presence of this form of invisible matter. Observations show that the rotation speed of stars in galaxies remains high even far from the galactic center, which cannot be explained by the presence of visible matter alone. Light from distant objects is also bent by galaxy clusters more pronouncedly than their visible mass would cause, indicating additional invisible mass. Finally, cosmological simulations show that dark matter is necessary to explain the formation of large structures in the Universe, such as galaxy clusters. Many theories have naturally been proposed to explain its nature. Among them, the most popular include WIMPs (Weakly Interacting Massive Particles), massive particles interacting very weakly with normal matter, except by gravity, or axions, hypothetical very light particles. Despite everything, none of these hypotheses have been confirmed and dark matter remains elusive. However, it is necessary to continue searching. Determining its nature could indeed revolutionize our understanding of physics and the Universe. Credits: Wirestock/istock

What if dark matter doesn’t actually exist?

Dr. Richard Lieu of the University of Alabama in Huntsville (UAH) has indeed proposed an innovative alternative theory. Specifically, the researcher claims that the “excess” gravity needed to bind a galaxy or cluster (the famous missing mass called dark matter) could instead be due to sets of topological defects likely created in the early universe when a phase transition occurred. In detail, a topological defect is an irregularity in the structure of space-time that can form during phase transitions, a physical process in which the general state of matter changes throughout the universe. In the early universe, such transitions would have then favored the creation of these defects. A notable example of a transition is when the universe cooled enough to allow the strong force to bind quarks into protons and neutrons. Imagine that these topological effects are very compact regions of space with a very high density of matter, usually in the form of spherical shells. These shells would be composed of two layers: a thin layer of positive mass on the inside and a thin layer of negative mass on the outside. The total mass of these layers would therefore be zero, which means that they would have no direct measurable mass.My own inspiration came from my search for an alternative solution to the gravitational field equations of general relativity – the simplified version of which, applicable to conditions in galaxies and galaxy clusters, is known as Poisson’s equation – that yields a finite gravitational force in the absence of any detectable mass.“, explains the researcher. “This initiative is in turn motivated by my frustration with the status quo, namely the notion of the existence of dark matter despite the absence of any direct evidence for a century.”

Similar effects

Spherical shells resulting from such transitions could create effects similar to those attributed to dark matter. A star resting on such a shell, for example, would experience a strong gravitational force pulling it toward the center of the shell. Another example is gravitational lenses, phenomena in which starlight is bent by the gravity of massive objects. If such topological shells exist, then they could bend light in the same way, creating gravitational lenses. More precisely, the gravitational bending of light by a set of concentric singular shells comprising a galaxy or cluster would in this case be due to the fact that a ray of light is slightly deflected inward, that is, toward the center of the large-scale structure, or set of shells, when it passes through a shell. Future research questions will likely focus on how a galaxy or cluster is formed by the alignment of these shells, as well as how these structures evolve over time. It will also be necessary to determine whether the shells were initially planes or straight strings that curled up over time due to angular momentum. Finally, specific observations will need to be made to confirm or refute the existence of these topological shells. Source: Notices of the Royal Astronomical Society