Scientists continue to puzzle over dark matter. There are actually many open questions: the first, and most important, concerns its actual existence – because at the moment we have not yet been able to observe it directly – and the others, in no particular order, the need of its existence, its nature, its behavior, its interactions with everything else. Questions whose answers are, in fact, still obscure, or if you want hypothetical, and above all still susceptible to upheavals: on the basis of a bold reformulation of the theory of gravity, for example, last March two scientists deduced that perhaps the Universe might not need dark matter to exist as we observe it; and today another work, just published on Monthly Notices of the Royal Astronomical Society from Richard Lieuphysical to University of Alabama in Huntsville (Wow), he arrives at more or less similar conclusions by following a slightly different path. Lieu also worked on gravity, showing mathematically that the latter could exist even in the absence of mass (it seems like heresy, but the scientist’s calculations add up) which, among other things, would exclude the presence of dark matter in the Universe .
Why dark matter is (would be) necessary
The fact that at the moment the most accredited cosmological models also “contain” dark matter is linked to experimental results that can only be explained with this assumption. It all begins with the so-called Standard model of particle physics, a theoretical framework that describes the nature and behavior of ordinary matter (i.e. non-dark matter, so to speak): at the moment, this model is the most solid and experimentally verified for representing the world around us and making accurate predictions about natural phenomena. But it is not yet perfect: over the last century, in fact, astronomers and cosmologists have begun to accumulate various experimental observations, mainly linked to phenomena having to do with gravity, which could not be explained or predicted by the Standard Model. Among these, for example, the rotation speed of stars and galaxies, some characteristics of cosmic background radiation and the curvature of light due to the so-called gravitational lenses.
If we consider only the particles of ordinary matterIndeed, these phenomena are inexplicable. For this reason, already in 1932, the Dutch astronomer Jan Oort proposed the presence of another type of matter – dark matter, precisely – with which to reconcile experimental observations and theoretical models. The problem, as we mentioned, is that at the moment this type of matter has never been directly observed, and we still don’t have a precise idea of what its characteristics should be. Oort’s hypothesis, then refined in the following decades, is that dark matter interacts with ordinary matter via gravity; According to current estimates, this elusive entity actually represents the majority of the Universe – around 85% of all existing matter and around 27% of the total mass.
Flaws of the Universe
“My inspiration” Lieu explains “comes from the search for an alternative solution to the gravitational field equations of general relativity by Albert Einstein, whose simplified version, applicable to the behavior of stars, galaxies and galaxy clusters, is known as Poisson equation, according to which a non-zero gravitational force can exist even in the absence of detectable mass. But not only that: I was also driven by frustration at the difficulty of finding direct evidence of the existence of dark matter”. In Lieu’s idea, the matter (i.e. gravity) “necessary” to hold galaxies or clusters of galaxies together would have nothing to do with hypothetical dark matter, but could be due to what he calls “topological defects similar to ‘shells’ present in structures commonly present throughout the cosmos and probably created in the first moments of life of the Universe”during a so-called cosmological phase transitioni.e. a physical process in which the general state of matter changes throughout the Universe (an example of phase transition is the freezing of liquid water, which passes into a solid state: imagine a similar phenomenon but extended to the entire Universe ).
Zero mass shells
In simpler words, Lieu’s hypothesis is that in the first (turbulent) moments of life of the Universe a sort of ripplesor folds – what scientists call “topological defects” –, i.e. regions in which enormous quantities of matter have accumulated. “At the moment – continues the scientist – However, it is not yet clear what type of cosmological phase transition can give rise to topological defects of this type. These are very compact regions of space with a very high density of matter, which usually appear as linear structures, the so-called cosmic strings; but different types of structures are also possible, for example two-dimensional shells. In my work I examined, precisely, spherical shells made up of an internal layer of positive mass and a thin external layer of negative mass, so that the total mass of the shell is exactly zero. When a star ‘lies’ on one of these shells, although the (net) mass is zero, it actually experiences a gravitational force that pulls it towards the center.”.
Goodbye dark matter?
If Lieu’s model were correct, it could have the consequence that dark matter would no longer be “necessary” as previously believed. But it is still very early to set aside dark matter models – which at the moment still remain the most accredited – because the model just formulated is still incomplete: Lieu says nothing, for example, about as these structures should be formed, simply observing that if they existed could actually give rise to the behavior described. The scientist knows it well: “Naturally” concludes “my hypothesis, however suggestive, is not in itself sufficient to discredit the existence of dark matter. At best [sic] it could be an interesting mathematical exercise. But it is still the first proof that gravity can actually exist even without mass.”.
