Tech UPTechnologyWhy does matter (and not antimatter) dominate the universe?

Why does matter (and not antimatter) dominate the universe?

In the first moments of life of the universe , equal amounts of matter and antimatter were created, at least according to our most complex and profound theories. This incredibly hot, small and dense primordial soup contained all kinds of particles but they were perfectly balanced with each other. If there were many positively charged particles on the one hand, there would be as many negatively charged ones. If there were many particles with one of the other charges of the different fundamental interactions, such as color charge, baryon number or lepton number for example, there would be as many with the opposite charge. This initial equilibrium should have been followed by a mutual annihilation, in which matter and antimatter interacted to produce, ultimately, photons, light. Given that initial composition and our current understanding of the most fundamental laws of the universe, everything should have been reduced to light.

But this hot, small, dense universe expanded, cooled, and thinned out , giving rise after billions of years of evolution to the universe we see today. And yet, the matter that we observe today seems to be composed only of particles, without any presence of the corresponding antiparticles . All the stars, nebulae, and galaxies we see appear to be made of the same stuff that makes up everything around us here on Earth. Therefore, something must have happened for that balance between matter and antimatter to be broken , for matter to win that phase of annihilation. How this happened is one of the big questions of modern particle physics and this problem has been given the name of the matter-antimatter asymmetry problem.

Antimatter is one of those concepts in modern physics that has a certain aura of mysticism. This is partly due to the licenses of the cinema when using it in their scripts. Antimatter is actually equivalent to “ordinary” matter , which your body is made of for example. If with a snap of the fingers we could change all the particles that make up our universe for their corresponding antiparticle , we would not notice anything at all . Antimatter is just as capable of forming atoms, molecules, cells, and any other structure as matter. The main difference between both types of particles is that they have some opposite fundamental charges . An electron, for example, is a particle with a certain mass, a spin, an electric charge and a specific lepton number. The positron , its antiparticle, will have the same mass and spin, but opposite electric charge and lepton number.

The exotic behavior of antimatter arises when it interacts with matter. If a positron and an electron meet and interact, assuming they have enough energy, they will tend to create other particles . But in this reaction the fundamental charges must be conserved , those of the positron and electron but also those of any other particle that can be created. This is why, as the pair has a total electric charge of zero (because those of the electron and positron are opposite and therefore cancel) and as it has a lepton number equal to zero (for the same reason), the result of their interaction must also have electric charge and lepton number equal to zero. Therefore, when they interact, they will create particles that have these or other opposite properties (that is, they will create a particle-antiparticle pair) or they will directly create particles that do not have these properties, such as two photons , which do not have an electric charge or a lepton, baryon or alpha number. no type.

In the long run, if we have enough particle-antiparticle pairs, no matter what type they are, they will end up giving rise to pairs of photons, leaving behind radiation, light, where there was matter before . This should have happened in the early universe, according to our theories. Currently there are some proposals that try to explain this asymmetry between matter and antimatter, although none of them manages to do it completely . The most promising proposal has to do with the violation of two of the most fundamental symmetries in the universe and is known as “CP symmetry violation” . This CP symmetry tells us that the properties of a particle should not change if we change it to its antiparticle and at the same time invert its spatial coordinates (as if it were a mirror). This symmetry is not perfect and we know of mechanisms that break it naturally, although these would not be able to explain what happened in those first moments of the universe. That is, the asymmetry is greater than what these mechanisms predict.

The problem we have in investigating these matters is that they require much higher energies than we are capable of in our particle accelerators, sometimes thousands or even millions of times larger. This is why a team of researchers has proposed using the incredible energies of the early universe to study these very processes. Of course we do not have direct access to this time in the history of the universe, but we can study how those fundamental interactions have left their mark on the large-scale structure of the universe , studying the distribution of galaxies and galaxy clusters on the largest scales and studying the cosmic microwave background .


Yanou Cui, Zhong-Zhi Xianyu. Probing Leptogenesis with the Cosmological Collider. Physical Review Letters, 2022; 129 (11) DOI: 10.1103/PhysRevLett.129.111301

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