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Welcome to our Physics lesson on End of the Fiery Stage, this is the fourth lesson of our suite of physics lessons covering the topic of Chronology of the Universe, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
At 10-6 s after the Big Bang, the energy of quarks and gluons present in the primordial plasma decreased to 1 GeV and as a result, they began to combine with each other in suitable ways. Hence, protons and neutrons were produced. Initially their number varied with time until the end of this era, where the total number of nucleons in the Universe "froze". This does not mean protons and neutrons have had the same number since then, rather, protons and neutrons are combined in such ways that the number of protons increases at the expense of neutrons. The term "freezes" used here, means that the corresponding antiparticles disappear and only the particles remain in the Universe.
This stage of the Universe that has lasted from 10-6 s to 1 s after the Big Bang is known as the nuclear era. This is because thermal energy was so high that is prevented nucleons from getting closer to form atomic nuclei. The whole primordial plasma looked like a giant atomic nucleus where protons and neutrons moved by colliding with each other. This plasma also contained electrons, positrons, neutrinos, antineutrinos as well as a large number of photons formed during the particle-antiparticle annihilation process.
The value of 1 GeV energy was a characteristic of the beginning of nucleons formation process. But at the beginning of this process, the formation of nucleons occurred very slowly because the energy of quarks was still very high. The culmination of nucleons formation process in the primordial plasma occurred at a temperature T < 1013 K. The time corresponding to this temperature is more than 1 μs after the Big Bang. For the same reason, all temperatures in the subsequent processes are much smaller than those given by the relation
At t = 1 s, electrons and positrons interacted and as a result, they annihilated each other producing photons. At the same time, a certain number of electrons "froze", i.e. they no longer annihilated. Moreover, neutrinos and antineutrinos (known as elementary particles that have a very low rate of interaction with matter) detached from primordial plasma and began moving freely, no longer colliding with other particles.
When the temperature of the Universe dropped to T = 3 × 109 K (a few seconds after the Big Bang), the process of helium nuclei formation began. Hence, the Universe entered the nuclei-formation stage. The main feature of this stage consists on the thermonuclear reaction that unites two protons and two neutrons in a single helium nucleus, accompanied with a large amount of energy release as photons. This process is the same as that actually occurring at the Suns core. Hence, this era is also known as the fireworks era, as a large number of sparks were present in the Universe during photons emission. All free neutrons in the Universe that had survived to fission process due to weak interaction, at this point combined in groups of two to form helium nuclei.
At first glance, it seems like this process must have continued further for the formation of heavier nuclei. However, such nuclear reactions cannot occur, as they require very high temperatures and the Universe cooled down continuously with the increase in volume after the Big Bang. Hence, in the primordial universe only hydrogen (i.e. protons) and helium nuclei were produced. This stage lasted for about 3 minutes after the Big Bang. After this time, the reaction in which one photon produces one particle and its corresponding antiparticle did not take place very frequently; only the reverse reaction was regularly taking place.
The next stage that began 3 minutes after the Big Bang and lasted for several thousand years it is known as the era of atomic plasma. During this time, the entire space was full of electrons, protons, photons and helium nuclei, looking like a giant atom. Thermal energy was still very high which allowed nuclei get close to electrons and form atoms. No special events occured at this stage of the Universe where photons were colliding with electrons being reflected several times in many electrons. They cannot escape but are blocked inside matter. Such a movement is similar to that of actual photons inside the Suns core; it is a kind of Brownian motion.
The process of protons and helium nuclei - electrons combination to form the hydrogen and helium atoms occurred at t = 1013 s, which corresponds to 300 000 years after the Big Bang. The temperature of the universe dropped to 3000 K - a value that allows the stability of such combinations, because thermal collisions are too weak to destroy these light atoms. At this point, the Universe entered into the era of chemical processes, an era that persists to this date.
After the era of atomic plasma, electrons were not free anymore; photons were no longer hampered so they could move freely in space. Matter has since became transparent to photons; both these components of the universe become independent and photons therefore are able to cover more distance than before. The free photons present in the universe form the cosmic radiation that we have mentioned in previous guides. The process of cosmic radiation occured because of the separation of photons from matter, this is similar to that of the process where neutrinos detach from primordial plasma which we explained earlier (at t = 1 s after Big Bang). The only difference is that unlike neutrinos which interact very little with matter, photons detach much later from matter because they have mutual interaction.
Hubble's Law is not the only method used to confirm the veracity of Big Bang theory. There are also two other facts obtained through observations that confirm this theory. The first is the amount of primordial helium in the Universe and the second is cosmic radiation in the sky. Let's take a look at both of these in more detail.
You have reached the end of Physics lesson 22.11.4 End of the Fiery Stage. There are 6 lessons in this physics tutorial covering Chronology of the Universe, you can access all the lessons from this tutorial below.
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