Dark Matter | The overwhelming majority of theoretician physicists agree that the universe is not just composed of billions and billions of galaxies, which are the ordinary matter whose interactions are studied by physics and chemistry, but also by a mysterious substance over five times more abundant than ordinary matter, called dark matter, according to LiveScience.

For a long time, there have been two main theories about how the Universe will end. These were Big Freeze and Big Crunch.
In short, Big Crunch (Great Implosion or Great Collapse) claims that the Universe will stop growing and would collapse.
This would lead to implosion. Think of it as something completely opposed to the “Big Bang” event.
Essentially, it would be a Big Crunch. On the other hand, Big Freeze argues that the Universe will continue its expansion forever until it becomes a frozen wasteland.
This theory states that the stars will be more and more removed from each other, they will burn and (as there will be no other stars to form) the universe will freeze and will be forever black.

Dark Energy

In January 1998, the Supernova Cosmology Project announced at a press conference in Washington that, following the measurements of 40 type I supernovae, it was concluded that we were witnessing an expansion of the accelerated Universe. A surprising result for that time, and for this reason was received with some mistrust. Something, a terrible mysterious force, makes the expansion of the Universe, rather than slow (as expected), is accelerated!

A real shock in the world of cosmologists, but, as is always the case in science, facts prevail over ideas strongly rooted in the minds of scientists. This bizarre matter would be composed of one or more subatomic particles of an unknown type that do not interact just like ordinary matter particles, electromagnetism or strong (strong) nuclear power and weak nuclear power. Though dark matter is now a completely unknown type of matter, whose existence can not be proven experimentally, physicists estimate that it accounts for 83% of the matter in the Universe and 23% of its mass-energy.

Although dark matter remains an undemonstrable hypothesis, according to physicists, it is a very strong hypothesis. Any scientific theory starts from some predictions, and if confirmed, then the experimental measurements must align with those predictions. The same applies to dark matter. For example, dark matter theories advance predictions about the rotation speed of galaxies, but so far the measurements made on the distribution of dark matter at the centre of the mass galaxies did not coincide with the predictions formulated.

A recent calculation has changed this. This calculation contributes to solving the mystery of the Tully-Fisher mathematical relationship, which reports the visible or ordinary matter of a galaxy at its rotation speed. So scientists have learned that the more spiral galaxy it is (and thus brighter) spiral, the faster it spins through space.

But if dark matter exists, then the calculations about the size of a galaxy should also take into account this mysterious matter and not just the usual one. And how the mathematical relation Tully-Fisher lacks an extremely important variable – the amount of dark matter – then this relationship should not make the right predictions. And yet it does.

How did we find out about dark matter?

The first clues about the existence of dark matter date back to 1932. Dutch astronomer Jan Oort measured the orbital velocities of stars in the Milky Way and concluded that they move too fast for their movement to be explained by the observable mass of the galaxy. The real “hunt” of dark matter, however, began more than 50 years after Jan Oort’s research into the late 1970s when astronomers Vera Rubin and Kent Ford measured the rotation rates of galaxies near the Milky Way as a function of the distance of the stars in the respective galaxies from the galactic centres. They compared these measurements with the predictions formulated according to the Newtonian theory of gravity.

The stars move around the galactic nuclei in almost circular orbits governed by gravity. One of the predictions that escape from the gravity equations is that the force that makes the stars move in circular orbits, F (circular), should be equal to the force of gravity exerted on the star, F (gravity), otherwise the star either collapses in the galactic centre or leaves the galaxy and wanders alone in space. Near the galactic centres, Rubin and Ford have found that F (circularly) is roughly equal to F (gravity), according to predictions of the Newtonian theory.

But at great distances from the galactic centres, the two sides of the equation are no longer equally balanced. This discrepancy must be explained. Near the galactic centres, the measurements made by Rubin and Ford confirmed the theory, while the discrepancy that occurs at great orbital distances to the galactic centre indicates the existence of at least one variable that the classic theory of gravity omits. The results of their studies have indicated that scientists do not yet understand how inertia works in the case of F (circular), or do not understand how gravity F (gravity) works.

A third possibility is that between the two terms of the equation the equal sign must not be given, with at least another force or variable not included in the equation. In the 40 years since the publication of the study by Rubin and Ford, scientists have tested many theories to try to explain the discrepancies they have identified. Physicist Mordehai Milgrom proposed a change of inertia called “Newtonian modified dynamics” or “MOND”. In its initial form, Milgrom’s theory postulated that at very low accelerations Newton’s equation, force (F) = mass (m) or acceleration (a), does not work.

Other physicists have suggested changes to the laws of gravity. Einstein’s general relativity does not make any contribution here because in this field his predictions are identical to those of Newton’s theory. Also, quantum theories, which have attempted to describe gravity by inventing atomic subgraphs called “gravitons,” have failed in an attempt to provide an explanation. Then some physicists formulated predictions about the existence of an undiscovered force that would act in such circumstances – a fifth force that manifests in nature beyond gravity, electromagnetism, strong and weak nuclear power.

Then came the theory of dark matter: which sustains the counterintuitive existence of a form of matter that does not interact with light at all and yet develops a strong enough gravitational force to influence the galaxies and swarms of galaxies and is spread throughout the Universe. If measurements of the rotation speed of galaxies were the only data available to physicists, then it would be very difficult to show which of these theories is the best one. There are, however, many observations on different astronomical phenomena that help physicists identify the most plausible of theories, and this is that of dark matter. Such a phenomenon is the velocity of individual galaxies within large galactic swarms. These galaxies are moving too fast, and galactic swathes should fall apart.

And yet the galaxies remain together. Another phenomenon observed is that of light behaviour from far distant galaxies. Observations on these very distant galaxies demonstrate that their light is distorted, deviated when it passes through the gravitational field of other galaxies interposed between the Earth and the light source. To all this, there are also some small differences in cosmic background radiation, “fossil” radiation that is part of the energy produced during the Big Bang, kept in the form of photons.

The theory of dark matter provides answers to many of these issues, which is why it is also highly respected within the scientific community, even if it could not be demonstrated directly, by identifying a possible particle of dark matter. But the absence of direct evidence is a matter for science. If dark matter exists, we should be able to measure its possible interactions, however weak, when passing through the Earth, or we should obtain such matter in large particle accelerators, such as the Large Hadron Collider (LHC). And yet no such approach has been successful. In addition, if there is indeed dark matter, it should be compatible with all observed astronomical phenomena, not just with some of them. Although the theoretical model of dark matter is the most successful model, it can not answer all questions.

For example, the theoretical models of dark matter indicate that around large galaxies, such as the Milky Way, there should be more satellite dwarf galaxies than there really is. Another important question that remains unanswered is about how dark matter influences the relationship between the galaxy’s brightness and its rotation speed.

This relationship, called the Tully-Fisher relationship, was first introduced in 1977 and has since been confirmed many times. She argues that the observable mass of a galaxy is perfectly correlated with its rotation speed. Thus, the Tully-Fisher relationship represents a heavy test stone for the theoretical models of dark matter. The rotational speed of a galaxy is determined by the total mass of matter it contains. If the dark matter really existed, then the total mass of matter of a galaxy, the variable that determines its rotation speed, is the sum of the mass of matter and the dark matter mass.

But the existing theories about dark matter indicate that any galaxy may contain more or less dark matter. Thus, when someone measures the visible mass of that galaxy, it may omit much of its total mass.
Therefore, the mass of ordinary matter should be a very weak indication of the total galactic mass and implicitly of the rotational speed of that galaxy. The total mass of a galaxy might be similar to ordinary matter, visible, or it could be much larger. And yet the mass of visible matter, as a stand-alone variable, is an excellent indication of the rotational speed of any galaxy. In a study published in June, scientists give even more “weight” to dark matter theory, managing to find a solution within this paradigm and for the Tully-Fisher relationship.

The new study proposes a “semi-analytic” model, based on a mix of analytical equations and computerized simulations. This model simulates clumps of dark matter from the childhood of the Universe, clustering that would have made possible the formation of the first galaxy, but also includes possible interactions with ordinary matter, explaining phenomena such as the collapse of ordinary matter under the gravity of a massive cosmic body, star formation and explosions the supernova stage. By adjusting the parameters, the researchers could also explain Tully-Fisher’s relationship.

The key to this explanation is that the expected rotation speed of a galaxy comprises a value of the ratio of the baryons (the heaviest known subatomic particles, consisting of three-quarters) and the dark matter in that galaxy. The new study is an important step towards validating the theory of dark matter, but there is still much to do, provided that any successful theory must be able to explain all the experimentally measured values. Until then, dark matter remains an important theoretical construct with regard to the structure of the Universe. It is far from being a complete model and still needs empirical validation by discovering and isolating a particle of dark matter. Scientists still have a lot of work to do in this area, but recent advances bring us closer to finding answers to the question of whether the structure of the Universe is truly dominated by a dark side.

References:

1. https://www.space.com
2. https://science.nasa.gov