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How did the universe make the first element?

   We've all heard the story of the Big Bang at one point or another. It all started in an incredibly high temperature and dense state, and then the universe continued to expand. This expansion spreads everything out in the universe while reducing its energy and temperature and forcing particles to interact, decay and freeze...

  4 seconds after the big bang, there are no free quarks in the universe , no more antimatter, and no more neutrinos to collide or interact with any remaining particles. At this time, there is more matter than antimatter in the universe, each proton or neutron corresponds to 1 billion photons, and the temperature of the universe is only slightly lower than 1010 K at this time. However, it can't make any elements yet.

  How was the first element of the universe created? This needs to start with what happened in the 3 seconds after the big bang, which is especially important for the production of the following elements.

unstable protons and neutrons


  The universe at this time is full of protons and neutrons, which collide with electrons or neutrinos and transform into each other, changing from one type to another. These reactions all obey the law of conservation of energy, that is, the number of baryons (the total number of protons and neutrons, set to 10) and the total amount of charge are constant. So, this means that in the initial stage, the ratio of protons to neutrons is 5:5, and the number of electrons is equal to the number of protons.

  However, since neutrons are heavier than protons, according to Einstein's mass-energy formula E=mc2, it can be known that producing neutrons requires more energy than producing protons. Under such conditions, the collision of neutrons and electrons (or neutrinos) can still be converted into protons, but the collision of protons and electrons (or neutrinos) cannot be converted into neutrons (because more energy is required), So it can only remain a proton. In this way, neutrons continue to be converted into protons over time, with fewer and fewer neutrons and more and more protons. By the end of the third second, protons — and an equal number of electrons — make up about 70 percent of the universe, and neutrons about 30 percent.

  Protons, neutrons, and electrons are all flying around in a very hot, very dense environment, much like what's happening at the center of the sun today. Yes, it is natural to think that protons and neutrons will fuse together, creating a nucleus, and releasing energy (following Einstein's E=mc2). And in turn, those nuclei combine with electrons and start producing a steady stream of those stable, neutral elements in our periodic table.

  George Gamow - the founder of the Big Bang theory, claimed that all these elements were formed at the Big Bang - that is, in the hottest and densest places. Unfortunately, his view is not correct. The universe did produce elements in the highest, hottest places during the Big Bang, but only in very small numbers.

The earliest atomic nucleus - the deuteron


  We know that in order to make an element, there needs to be enough energy to fuse these protons, neutrons, electrons, etc. together. And in order to keep them and use them to make something heavier, first make sure they don't get damaged. But in the early days of the universe, there was little that could be done—they would soon be destroyed.

  Suppose in the 3rd second after the Big Bang, the universe is filled with 70% protons (and an equal number of electrons), 30% neutrons, and each proton or neutron corresponds to 1 billion to 2 billion photons. To make heavy elements, the first step must be to collide a proton with a neutron, or a proton with another proton. The purpose of this is to create an atomic nucleus that combines two nucleons, such as protons and neutrons, as the first step in building more complex elements.

  The part is simple, the universe makes a lot of deuterons without a problem. But the problem is that after these deuterons are created, they are quickly eliminated.

easily destroyed deuteron


  When the universe was at its hottest, photons far outnumbered protons and neutrons, so the probability of it colliding with a deuteron was extremely high. How big is it? It's almost 100 percent -- conversely, it's less than one in a billion chance that it's not a photon colliding with a deuteron.

  At high heat, photons have enough energy to instantly split deuterons into protons and neutrons. So, here's the situation - the universe keeps making deuterons, but it gets destroyed quickly (if not, it would be a lot easier for the universe to produce elements).



  As long as the universe is hot enough, this will always be the case. This is why cosmologists say that "deuterons are the bottleneck of the universe's production of elements": the universe is willing and able to make elements, but must go through a stage where the deuterons are easily destroyed.

  So the only way is to wait until the universe cools down, until its temperature drops to a temperature at which photons have insufficient energy to break down deuterons. But it takes more than 3 minutes, and in the meantime, other things are going on in the universe. As long as neutrons are free and unconstrained, they become unstable and begin to decay.

production of other nucleons


  The actual time it takes for the universe to expand and cool until the deuterons are not broken down immediately is about 3.5 minutes. During this time, about 20% of the neutrons will become protons. Protons and neutrons are in a 5:5 ratio early on, and after 3 seconds it becomes a 7:3 ratio. Now, after more than 3 minutes, it has become about 9:1.

  At this time, the universe has cooled to a suitable temperature - the deuteron is no longer destroyed. The deuteron was finally made, and soon after, other nucleons on our periodic table were made. For example, adding another proton to a deuteron gives helium-3. Add one more neutron to the deuteron and you get hydrogen-3, or tritium. If you then add a deuteron to helium-3 or tritium, you get helium-4 and a proton or a neutron, respectively. By the time the universe was 3 minutes and 45 seconds old, almost all the neutrons had been used to form helium-4.

  In terms of mass, the universe at this time looked like this: 76% hydrogen (protons), 24% helium-4 (2 protons and 2 neutrons), and deuterium (1 proton and 1 neutron) 0.01%, tritium and helium-3 (tritium is unstable and decays into helium-3 with 2 protons and 1 neutron) 0.003%, lithium-7 and beryllium-7 (by Tritium or helium-3 and helium-4 are fused together to form, with 4 protons and 3 neutrons) accounting for 0.00000006%.

  When the universe has expanded and cooled to a density that is only one billionth the density of the core of the sun, nuclear fusion can no longer take place, and there is no way to stably combine a proton with helium-4 or two helium-4 nuclei fused, so both lithium-5 (composed of protons with helium-4) and beryllium-8 (composed of two helium-4s) are very unstable and disappear after a fraction of a second.

The earliest elements in the universe - hydrogen and helium


  The universe did form elements immediately after the Big Bang, but almost all of the elements formed were nuclei (without electrons) of hydrogen or helium. After the Big Bang, a small amount of lithium remained in the universe because beryllium-7 would be broken down into lithium, but it was less than one part in a billion by mass.

  When the temperature of the universe dropped enough for these nuclei to trap electrons, we had our first elements—the ones from which the first stars would be made. However, at this time, the universe was not able to produce the elements we think are essential for survival, such as carbon, nitrogen, oxygen, silicon, etc. At that time, there were only hydrogen and helium in the universe, and it reached the level of 99.9999999%. It took less than 4 minutes from the start of the Big Bang to the formation of the first stable nuclei, a process that took place in a hot, dense, expanding and cooling environment. From then on, the story of matter in our universe begins.


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