How the universe created atoms

　　Everything we see on the earth, the solar system, and even the universe, they all consist of the same thing-atoms. Electrons and atomic nuclei interact and connect to form a single atom. The atom then forms simple or complex molecules. The molecules then produce the macroscopic structure of the entire universe today (and also life).
　　The universe is too hot, could have atomic
　　structure of the universe today, We are really shocked. But from the moment the Big Bang occurred, in the entire universe, after hundreds of thousands of years, no atom appeared. In order to create them, the universe took a long time and went through a complicated evolution.
　　Four minutes after the birth of the universe, the nucleus has appeared. There are no free neutrons at this time, because they have been incorporated into the heavier nucleus. These heavier nuclei are: helium-4 (two protons and two neutrons), deuterium (one proton and one neutron), helium-3 (two protons and one neutron) and hydrogen-3 (one proton) And two neutrons), lithium-7 (three protons and four neutrons) and beryllium-7 (four protons and three neutrons).
　　At this time, there are also many free electrons in the universe, and their number is equivalent to the number of protons, so that our universe can remain neutral (uncharged). The universe at this time is too hot and too much energy, and photons are continuously scattered from the nucleus and electrons, so that nothing can be formed.
　　The reason is simple: these nuclei do not have enough energy to fuse with each other to form a heavier combination, but the electrons cannot be bound to the nucleus due to too much energy, so they cannot form atoms.
　　In fact, for the stable creation of neutral atoms, there was too much energy in the universe at that time-the temperature was still hundreds of millions of degrees a few minutes after the universe was born. In order for the created neutral atoms to be stable, the temperature must fall below several thousand degrees.
　　Generating space to cool the atoms to create conditions
　　of course, the universe is expanding, which means that the wavelength of light in which all the stretching being constantly cooled. But it needs to be stretched to about 100,000 times, which will take a lot of time.
　　Over time, the universe has been expanding and cooling. At this time, beryllium-7 began to produce radioactive decay. By trapping electrons, it slowly transforms into lithium-7. After a year or two, beryllium-7 almost completely disappeared (all converted to lithium-7). As time continued to elapse, hydrogen-3 also began to produce radioactive decay (by emitting electrons), and finally became helium-3.
　　All these transformations took about 100 years.
　　However, the universe is still too hot to form stable atoms. Therefore, the universe continues to expand, cools and its density is decreasing.
　　Hundreds of years later, the redshift of photons became more and more severe-more than 1 billion times that of other particles, until they almost lost all their energy.
　　After tens of thousands of years, the radiation density dropped below the density of matter. At this time, the universe was transformed from being controlled by radiation moving at the speed of light to being dominated by slow moving matter. Based on this change, universal gravitation began to pull dark matter into clumps, and these clumps became larger and larger, attracting more matter. There is no radiation to break down and destroy these clumps, so the universe began to form a structure-the prototype of our universe structure appeared at that time.
　　But then the universe was still too hot to form neutral atoms. Every time an electron successfully combines with an atomic nucleus, there will be two situations: one is to emit ultraviolet photons, because the atom will naturally transition to a lower energy state (spontaneous radiation); the other is to be bombarded by other particles, one of which is a photon ( There are about a billion photons around every electron in the universe). Even after tens of thousands of years after the Big Bang, there are still enough photons with enough energy. Once electrons combine with atomic nuclei (whether free protons or heavier nuclei), it will immediately explode them.
　　The situation began to change about 300,000 years after the Big Bang. The background photons left behind in the big bang began to cool, so that there was not enough energy to immediately kick the electrons out of its core. At this time, there are very few photons with extremely high energy, much less than electrons in the universe, and only less than one billionth of a billion photons can ionize neutral atoms.
　　This means that neutral atoms can begin to form from then on, but how to retain them after they are formed is a problem.
　　Two-photon transition
　　Causing the universe to obtain a neutral atom
　　when forming a stable neutral atoms, which itself emits ultraviolet photons. These photons will continue to travel in a straight line until they encounter another neutral atom, and then ionize it. Therefore, even if we can make a small number of neutral atoms, they cannot maintain this state because they will soon be destroyed.
　　You might think that if these ultraviolet photons travel long enough in space and no longer interact with neutral atoms (there is no chance to touch the next neutral atom), then they will be redshifted (the energy becomes smaller) ), so that the neutral atoms can no longer be excited and ionized.
　　Indeed, this is a possible situation, but this is only true for a few percent of the neutral atoms initially formed in the universe. In addition, there is the following situation-
　　usually, in a hydrogen atom, when the electron is in the first excited state, it will simply drop to the lowest energy state and emit a specific energy ultraviolet photon-Lehman alpha photon. However, in 100 million transitions, there will be a drop that occurs through a different path, that is, emitting two lower-energy photons. This is called two-photon decay, or two-photon transition. It is the main reason that causes the universe to become neutral-when a neutral atom that is forming emits two photons, after the first photon hits and ionizes another nearby neutral atom, the second photon will ionize The latter nucleus and electrons recombine to produce a new neutral atom. At this time, the universe gets an additional neutral atom.
　　This is the story of how the universe created a neutral atom. The process of generating neutral atoms is very long, requiring more than 100,000 years, but this is what the universe has to do. Although this kind of two-photon transition is rare, it is the process of first forming a neutral atom. It changes our universe from being full of plasma to being full of 100% neutral atoms.
　　Although we say that the universe began to form these atoms 380,000 years after the Big Bang, in fact, the formation of atoms is a slow and gradual process, which took about 100,000 years.
　　Once the atom is neutral, the light of the Big Bang will not disperse. This is the origin of the “cosmic microwave background radiation”. Scientists have detected this light for the first time since 1964, further confirming the correctness of the “big bang” theory and entering the era of modern cosmology. Two-photon transitions have been confirmed in laboratories on Earth, and the phenomena we have observed represent an amazing agreement between our theoretical predictions and what actually happened in the distant past of the universe. It took about 500,000 years for the universe to finally create neutral atoms. Then, universal gravitation began to condense the universe into clusters. Finally, our universe story is ready to continue to the next stage.