Where we left off the fundamental forces of the universe were being defined and the universe continued to be an extremely high energy soup of constant collision. The universe at this point was in a constant state of high energy photons converting into matter-antimatter pairs only to annihilate with each other back into photons (Das & Ferbel, 2013). Antimatter has the same mass but opposite electric charge of their matter component. So you can have a quark and an antiquark, or an electron and an antielectron (also called a positron). The universe was basically a photon gas, as well as a "quark-gluon soup" of tightly packed matter and antimatter continuing this process.
Diagram of pair production. γ is “gamma” or the term for a high energy photon. The energy it contains is so high that it converts into a pair of an electron and an antielectron (positron). At this point, the universe was so hot that photons would also convert into pairs of quarks and antiquarks. Note, you would actually need a pair of photons, not just one, to collide with one another and produce this pair.
Image by Christian Nölleke / CC BY-SA 3.0
That would change. Sometime in the early universe (we don't exactly know when this happened) the temperature of the universe dropped to a point where this photon matter creation process would happen en masse for one last time. This critical temperature was 2x10^12 K, leading to the final annihilation which is called Baryogenesis (baryo- meaning baryon). Mysteriously, even though a particle and antiparticle are seemingly always created together in pairs, this final annihilation left us with mostly matter and almost no antimatter (Cline, 2004). This is the great mystery of Baryogenesis, the asymmetry between matter and antimatter. It has been calculated that there must have been an extra particle for every 5 billion particle-antiparticle pair.
Note: This baryonic asymmetry has actually been observed in our own experiments. In 2010, Fermilab managed to generate 1% more matter than antimatter. The reason why this happens is still not known. To learn more about the experiment and other modern quantum experiments, see here.
Diagram of annihilation. The matter and antimatter attract each other and collide, converting back into high energy photons. Note again, there would not be a single photon in this transition, but a pair. This should happen, but for some reason a small amount of matter is left over.
Image by Iamion / CC BY-SA 4.0
There are many ideas, though unverified, for how this could have occured. Within the Standard Model, it is possible that when the electroweak force was splitting into the weak and electromagnetic forces symmetry breaking occured. Symmetry breaking famously occurs during phase transitions (as in the phases of matter). When an object decreases in temperature it reaches a critical point where it stops decreasing in temperature and instead transitions to a new phase of matter. Objects of lower temperature phases are less symmetrical than objects of higher energy phases. So as the universe cooled and the symmetry of the electroweak force broke, then it would lead to the change in how matter and antimatter are created and eventually annihilated. This could have led to the matter-antimatter asymmetry that we observe today.
Note: The idea of symmetry is very important in physics. Basically, symmetry describes any situation in which certain features remain unchanged during a sort of transformation, such as a phase change. Sometimes there is a symmetry breaking, meaning these features change. This is what could have happened after the Electroweak Epoch, changing the features of the creation and annihilation of matter and antimatter, possibly allowing for the matter asymmetry we observe today. For more information on symmetry in physics, see here.
A second idea involves this occurring after the Grand Unified Epoch. It states that in order for this baryogenesis asymmetry to occur, there must be a not only a violation (breaking) of the baryon number (a mathematical constant) and violation of charge-symmetry (C Symmetry) and parity-symmetry (P Symmetry), but also thermal equilibrium (Today’s Science, 2015). This means there would be no flow of heat from one part of space to another, that all parts of space had equal amounts of heat. This is how heat acts, it wants to flow from an area of greater heat to an area of lower heat so that both areas have the same amount of heat. In thermal equilibrium the flow of heat does not occur because this state is already reached. This too, has not been verified.
A third, much more daring idea is that there are equal amounts of matter and antimatter, yet they somehow separated without annihilating each other. They would create their own regions, meaning supposedly there are antimatter stars and even antimatter galaxies somewhere out there (Thomsen, 1973). However, this has never been observationally verified.
At this point all matter for the universe has been produced. In this truly Hadron Epoch, quark production had ceased. The remaining quarks would form into various types of hadrons, of which there are baryons and mesons. A baryon is made up of 3 quarks, and they include protons and neutrons, whilst a meson is made up of 2 quarks. These hadrons, and especially the baryons, would become the foundation of nuclear chemistry.
The following Lepton Epoch (t=1 to t=180 or 3 minutes) was very similar. For leptons, a type of fermion just like the quarks but with much smaller masses, the required amount of energy needed for constant photon conversion into lepton and antilepton pairs was much lower. So the universe had to cool for even longer before this too would end. The leptons include the muon, tau, and of course the famous electron. Leptons interact with all the fundamental interactions except for the strong force. So they did not form into heavier hadrons like the quarks did. These leptons have not only corresponding antimatter versions but also neutrino, meaning it does not interact with the electromagnetic force. So there is such a thing as a neutrino electron, though the vast majority of electrons are charged. During the Electroweak Epoch, when electromagnetism and the weak force acted the same, neutrinos were able to interact with the other fermions. This is neutrino decoupling, and it occurred at around this time, as evident of the Cosmic Neutrino Background.
Note: The Cosmic Neutrino Background (CNB), has very recently been directly observed for the first time. For more information on this discovery, see here.
The famous diagram of the Standard Model outlining all the elementary particles of the universe. Note, this diagram is dated and the Higgs boson has been confirmed.
Image by Andres Rojas / CC BY-SA 4
At this point the temperature of the universe was so low that photons no longer had the energy to constantly convert into various quarks and leptons like it used to. These photons made up the majority of the universe and would continue to exist as the catalyst of the electromagnetic force. The quarks and leptons would now be able to exist without constant annihilation, and antimatter would become the ultimate rarity. As the universe continued to cool and space continued to expand, more familiar processes could take hold. That will be next week. For now, the Standard Model is complete.
References:
Das, A., & Ferbel, T. (2013). Introduction to nuclear and particle physics (2nd ed.). World Scientific.
Cline, J. (2004). The Origin of Matter. American Scientist, 92(2), 148. https://doi.org/10.1511/2004.46.926
“Why More Matter Than Antimatter?” Today's Science, Infobase, 2015. Science Online, online.infobase.com/Auth/Index?aid=103612&itemid=WE40&articleId=1009338. Accessed 13 Nov. 2020.
Thomsen, D. E. (1973). Does the Universe Need Antimatter? Science News, 103(13), 211. https://doi.org/10.2307/3958135
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