MATERIA OSCURA- Pt.1 #darkmatter

Good morning, welcome to my second video on the YouTube channel, today we talk about dark matter. This time I'm happy because the topic we're going to deal with is not a topic that can be found every day here on YouTube. This will be the first of a long series and to get to the focal point of our discussion, that is, to analyze the actions well, we will have to make digressions on what dark matter is, on why it has been hypothesized. We will delve into the field theory to understand what the field theory is, largely and intuitively, and then we will talk about the actions as dark matter. This video will be introductory, so we will remember what dark matter is in general, how it was discovered. In the following episodes we will go into more detail on all the extremely fascinating aspects that led me to conclude my master's thesis in physics. A month ago I recommend you to subscribe if you are interested in physics and to come and find me also on Instagram because I post daily Reels and space news, so see you there. Let's start talking about this fascinating matter, one of the most interesting mysteries of our universe, one of the unsolved problems of contemporary physics that also pushed me to enroll in a faculty as a physicist because I was extremely curious. Today in particular we will deal with understanding how dark matter has been hypothesized in the past and then try to understand at the cosmological level how this contribution of matter is necessary to explain some experimental measures that are made. So let's start from the origins. Already in 1933, an astronomer called Zwicky theorized a new component of matter that he called dark matter. This is because observing masses of galaxies and trying to calculate the mass contained within these masses of galaxies he realized that in reality there was something wrong. In particular, this astronomer, from the total luminosity that he observed in the masses of this galaxy, he tried to obtain the amount of matter present inside. Moreover, from the measurement of the Doppler shift of the emitted light, he determined the radial velocity of these galaxies and from his studies it turned out that these galaxies were moving too fast because the mass remained compact. And with the observed velocities, these galaxies belonging to the mass would have to disperse, would have to evaporate and would not have been seen grouped, but would have been seen in flight from each other. To solve this paradoxical situation, Zwicky hypothesized the existence of a new type of matter, a new type of invisible matter, and that this mass contained it and called this new matter dark matter. Obviously, it does not emit light and is detectable through its gravitational effects on the objects that surround it. And this new form of matter would have contributed to make the galaxies remain compact. The fascinating thing is that even cosmology needs to introduce this new form of matter. To explain, for example, the structures that we observe today in the universe, the cosmic structure that we observe that is formed as a result of cosmic inflation. In fact, the evolution of galaxies, of the masses of galaxies, is strongly connected to the existence of dark matter. The most standard hypothesis and the most accredited is that dark matter is formed by particles, particles that are not present in the standard model. The standard model, I remember, is the theory that currently describes the microscopic world, that describes the particles belonging to the microscopic world. So all that spectrum of particles that make up us humans, the planet we live on, the trees, but also the stars, the light described by the standard model. While this dark matter seems to be composed of particles that are not described by the standard model. To understand how dark matter is necessary to explain some theories, you have to start from the beginning of time, you have to start from the Big Bang. In fact, immediately after the Big Bang, all the particles we know, also the particles that make up me, that make up you, that make up the phone that is recording me, the computer, belonged to a primordial cosmic bath, at extremely high temperatures. So all these particles that, as I said, in addition to making up us, also make up the whole universe, were condensed in this primordial bath, very hot, in which there was also radiation, electromagnetic radiation. As time passed, the universe cooled down, because as it expanded, the temperatures decreased more and more. And that there were inside this primordial thermal bath also particles of dark matter, well, this is not certain. To recalculate, however, the current abundance of dark matter present in the universe, it is necessary to try to understand the mechanism of production of these particles. The mechanisms of production of particles are described with thermal mechanisms and non-thermal mechanisms. The thermal mechanisms are linked to this primordial thermal bath, because, as I said, being the particles all contained in this plasma, they went to hit each other, they clashed, they interacted, and as long as they interact enough, these particles decay into other particles, they transform. In fact, as long as the interaction of a particular type of particle is enough to interact with the other particles, then it will remain in thermodynamic equilibrium. When this interaction becomes less, so when this interaction is no longer enough to keep it in thermodynamic equilibrium with the rest of the plasma, then this particle will be decoupled. And this mechanism is called freeze-out, until the expansion rate of the universe, therefore, which is linked to the constant of Hubble, is less than the interaction rate given by gamma, then our particles will interact with the others. When, instead, the interaction rate will become comparable with the rate of expansion of the universe, because I remember that this universe continues to expand, then the particles will become automatically decoupled and will freeze. From that moment on, the number of particles per moving volume, it is said, will remain fixed, and today we can therefore detect the total number of particles present in our universe of a particular species. And this particle production mechanism is called thermal mechanism. And it is important to try to understand dark matter, because this mechanism is associated with dark matter candidates, such as WIMPs. WIMPs have been for years the main candidate of dark matter, and their acronym stands for Weak Interactive Massive Particle. As the name suggests, they are weakly interacting massive particles, produced, in fact, thermally in the primordial universe. The candidate I took care of, which is action, which is another of the possible extremely important candidates, which solves another very fascinating theoretical problem that we will talk about later, is not a thermal mechanism, it has another mechanism that we will deal with in the following chapters. So, I remember that the fundamental interactions that we currently know are four. They are electromagnetic interaction, weak interaction, strong interaction and gravitational interaction. Dark matter, as I said, is hypothesized to interact almost entirely only gravitationally, but there are also hypotheses that it can also interact through weak interaction. In fact, WIMPs could interact weakly. But let's go back to cosmology. So, what is cosmology? Cosmology is the study of the global properties of the universe and deals with its evolution. Cosmology derives from Einstein's field equation, because through the metric of Friedman, Robinson and Walker, we can determine some properties of the universe in which we live, such as the geometry of the universe in which we live, connected to the matter and the energy that are inside. What interests us today is to understand what are these contributions that make up the universe. In particular, three contributions are defined, one related to the luminous matter, one related to the non-relativistic matter and one related to the vacuum energy, to the so-called dark energy. And the study of the stellar evolution in the galaxies, together with the study of the distribution of the luminosity present in the universe, allows us to determine the contribution of the luminous matter present in the cosmos. It is estimated to be much, much smaller than the non-relativistic matter, even an order of magnitude less. From other independent estimates, however, it is found that the total matter present in our universe is 20-30%, and only 5% of this contribution is related to baryonic matter, as I said before, the matter that is described by the Standard Model. From these observations, therefore, two extremely important information arise. The first is that galaxies are mainly formed by dark matter, and also that the main contribution does not come from our galaxies, but from another contribution that we call vacuum energy, due to the so-called cosmological constant. This brings us back to the graph that you will have already seen. About 90% of ordinary matter, baryonic matter, does not emit radiation in the optical band, so everything we see with our eyes and with telescopes in the optical band is really a point compared to everything that is hidden in our universe. All the rest of the matter that we are unable to detect is this new component of dark matter. You will have your brain cooked for all this information that I have just given you, so I put you a summary scheme in which you can look at the various contributions, and it turns out that this contribution of matter has two possibilities, being cold dark matter or hot dark matter. Hot dark matter means relativistic dark matter, while cold dark matter is dark matter that is not relativistic, dark matter that has velocities not close to those of light, so there is a common consensus that dark matter must be cold, so hot dark matter has been excluded, or at least it does not seem to be present in small quantities. Let's move on to dark matter candidates. One of the most prominent candidates is WIMP, which we have already talked about before, they are weakly interactive massive particles that come from speeches on supersymmetry, which I will not discuss here because they are complicated topics. It is worth emphasizing, and I hope not to lose too many people, that there are fermionic and bosonic dark matter candidates, as the particles of the standard model are divided between fermions and bosons, also the dark matter candidates are divided between fermions and bosons, and these two subdivisions concern intrinsic properties of the particles that, for the most studied, may be associated with a property called spin. The fact of distinguishing between fermions and bosons is very important because it allows us to determine limits on the masses of these particle candidates, or on how they are arranged, let's say, in the structure of the universe. For example, this leads us to exclude dark matter candidates such as neutrinos, because they are too light, neutrinos are fermions, and they are excluded because fermions have limits of mass... have more stringent mass limits, due to Pauli's principle of exclusion, compared to dark matter candidates, such as bosons, which can all be found in the same quantum state, but this was a digression. Or even magnetic monopoles, which derive from the hypothesis that Maxwell's equations, which present a symmetry regarding electric field and magnetic field, can be somehow symmetrized, introducing the presence of these magnetic monopoles, which have not yet been observed, so these are perhaps even more speculative theories, so I would not go further. The next dark matter candidate I wanted to introduce you to, let's say, is action, which will be the focus of my discussion. Actions are particles, dark matter candidates, bosonic, however. They were introduced in the case of a problem in strong interactions, around the 80s, they are introduced for an anomaly that occurs in the theory of strong interactions, which, for very complicated reasons, linked to the system's big bang, have problems that I will not go into. Just know that in strong interactions, the theory would impose, on an experimental level, to observe a phenomenon that, however, is not observed. This phenomenon is called the neutron dipole moment. We, through experiments, should observe this dipole moment of the neutron's electric field, but this is not verified, this is not verified. So, the scientists of the time did not understand what the solution to this problem was. The solution was found thanks to the introduction of this particle of dark matter, which is called action, and is described mathematically as a field, and for this reason we will have to enter into the field theory mechanism to better understand what action is mathematically, this field is introduced, which solves this problem of strong interactions. Well, we have come to the conclusion of this second video, perhaps a little more complicated than the previous video, I hope I have put images that help to understand the concepts, I hope you were interested in this video, I hope I was precise enough, I thank you, I wish you a good day, a good night, or all the good you will do later, a big kiss, I remind you to follow me also on Instagram if you want to follow daily updates on dark matter. Bye!