Looking around, there are various forms of matter on our earth, from the most hydrogen elements to the heaviest elements in the periodic table, full of rich diversity, but we know that these elements all came from the previous generations of the solar system The stars of are not from the universe big bang itself. After the universe was born, only provided us with 92% of hydrogen atoms and 8% of helium atoms, as well as a small amount of lithium and beryllium. Then the question comes, after the birth of the universe, the temperature is so high and the density is so high, why didn’t heavy elements fuse? Today we will talk about: What is the nucleosynthesis of the big bang?
The period of annihilation of matter and antimatter
In the early days of the birth of the universe, there was only plasma radiation composed of photons and matter elementary particles (including matter and antimatter ), and these things were "running away" at a crazy speed close to the speed of light ", can be called "original soup". Collisions between particles are happening almost all the time. With the support of abundant energy, a large number of "particle-antiparticle pairs" are also created and annihilated at any time. With all this chaotic activity, this hot, compact universe is also expanding at an incredible rate and cooling down as a result.
After only about 1 second passed, the temperature of the universe dropped to "only" about 11 billion degrees Celsius. The degree of "coldness" has made the "particle-antiparticle pair" no longer continue to form. Since matter and antimatter have opposite charges, they annihilate each other when they collide. In view of the fact that the galaxies in the current universe are mainly composed of matter (not antimatter), we believe that a large amount of antimatter has been annihilated in contact with an equal amount of matter, and the remaining matter is only in the original radiation particles (including photons) , neutrinos , etc.) survived the "bombing" a very small part, and its surviving ratio may be only one in a billion.
Proton and neutron change in ratio
The remaining material particles include protons, neutrons and electrons, but because the temperature of the universe is still high at this time, they cannot be connected with each other for the time being. However, if a proton of sufficient energy collides with an electron, and the collision is of sufficient energy, they can transform into neutrons or neutrinos. The opposite process is also possible: neutrons collide with neutrinos, which can also morph into protons and electrons.
When the temperature of the universe is still high enough, these two reactions will occur with basically equal probability, so we get an initial material universe in which protons and neutrons each account for about 50% of matter (of course, in this universe A corresponding number of electrons are also required to balance the charge of the protons to ensure overall electrical neutrality). This must make sense, since protons and neutrons have about the same mass and contain almost equal amounts of energy: a resting neutron is only 0.138% more massive than a resting proton.
These facts bring up some interesting situations. First, about 1/3 of a second after the above-mentioned time node, the temperature subtly drops below the level of the above-mentioned mass difference (this sentence seems confusing, but it is not wrong, see later), at this time the proton and The reaction where the electrons collide into neutrons and neutrinos gets harder and starts to happen less frequently than the other reactions. Why is there such a phenomenon?
Because the temperature of the universe continues to drop as it expands, the kinetic energy of various particles will also decrease. When the "proton-electron pair" collides, the energy will not be high enough to "make up" more neutrons than protons That bit of quality. But if neutrons and neutrinos collide at this time, it is enough to become protons and electrons for the time being (although this reaction will eventually become more difficult as the temperature continues to drop). The above differences lead to a ratio of 50:50 protons to neutrons in the initial material universe, which becomes about 85:15 after the age of the universe reaches a few seconds, with nearly 6 times the number of protons and neutrons.
deuterium bottleneck
After this, the second important thing happened: the temperature of the universe dropped enough to stop the reaction of neutrons and neutrinos to form protons and electrons (the reverse reaction has also been stopped before), but this time temperature is still high enough for protons and neutrons to fuse together inTogether. Yes, the temperature and density of the universe at this time can still lead to nuclear fusion and reactions, but the intensive radiation "bombing" will bring about a problem called "deuterium bottleneck" (deuterium is also called "heavy hydrogen", is an isotope of hydrogen).
deuterium's atomic nucleus contains a proton and a neutron, and the formation of a deuteron is the first link in the nuclear fusion reaction chain. If this chain is not activated, heavier elements cannot be produced. To form a deuteron, a proton and a neutron are combined, and the combined mass of the two is about 0.2% less than before the combination. However, in the chaotic radiation "bullet rain" in the universe at this time, deuterium nuclei will be hit by radiation particles just after they are formed. are beaten back into individual protons and neutrons. Even if the average energy of radiation particles is far lower than the binding energy required by gas nuclei, the speed of deuteron destruction is still higher than the speed of its formation (don't forget that for every proton in the universe, there are no less than a billion photons), so the universe at this stage is still filled with free protons and free neutrons. Although
free protons cannot form heavier atomic nuclei through aggregation for the time being, at least they will not be destroyed, so they can wait. However, free neutrons are unstable! Although the free neutron is already the longest-lived of various unstable monomer particles, its average lifetime is less than 15 minutes.
Although it only took about 3 seconds for the ratio of protons to neutrons in the universe to change from 50:50 to 85:15, but if the temperature of the radiation drops to such an extent that it will no longer break up the newly formed deuterons into protons and neutrons level, but it took no less than 3 minutes. During this time, quite a few free neutrons decay: a free neutron breaks down into a proton, an electron, and a neutrino (or, in the more unusual case, an antielectron neutrino). By the time deuterons could be stably produced from protons and neutrons, nearly 88 percent of the matter in the universe was protons, and just over 12 percent existed in the form of neutrons.
You may be wondering: why do I focus so much on talking about these details about the formation of protons and neutrons in the universe? After all, they seemed insignificant in the super-hot, rapidly expanding sea of radiation at the time. But don't forget that protons and neutrons are the "bricks" of all kinds of atomic nuclei, and only by understanding those processes can we understand how (and in what quantities) elements came into existence before the first stars were born of. This is the basis for your further reading.
Nuclear fusion began after the Big Bang
When the temperature of the universe dropped to "only" about 8 million degrees Celsius, deuterium nuclei could finally stabilize after formation. After that, protons and neutrons combine in large numbers to become new deuterons at an amazing speed. After another 4 minutes or so, the free neutrons died off quickly. But the changes in the universe obviously didn't stop here! Because the temperature is still high and the density is still very high, some deuterons are combined with a new proton to form an isotope of helium element— helium-3 (its nucleus contains two protons and one neutron) nuclear. Other deuterons combine with a neutron to form the largely stable triton, an atomic nucleus with one proton and two neutrons. Either a helium-3 nucleus or a triton can interact with another deuteron to become helium-4 (that is, two protons, two neutrons).
If helium-3 and deuterium combine to form helium-4, there will be one proton left; if tritium and deuterium combine to form helium-4, then one neutron will remain. These temporarily lonely protons and neutrons return to the beginning of the reaction chain. But what about elements heavier than helium-4? People have tried adding a proton or a neutron to form lithium-5 or helium-5. Although this can indeed get the expected nucleus, it can only exist for less than 10^-21 seconds, which will decay into helium-4 almost instantly. As a result, none of the atomic nuclei made up of protons and neutrons with atomic masses up to 5 are stable.
people have also tried making two helium-4s come together to form a beryllium-8, which also works, and the new core lasts a bit longer, but it doesn't take more than 10^-16 seconds before turning back Helium-4. This fleeting process makes beryllium-8And become a heavier, stable nucleus (even if you add another neutron to become beryllium-9, it is too late). Since this stage of nuclear polymerization takes almost 4 minutes, the universe has become cooler and more diffuse, so it can no longer form any meaningful nuclei heavier than helium.
By the end of this period, all the surviving neutrons have basically joined the helium-4 nuclei, so the nuclei of the No. 3 element lithium and the No. 4 element beryllium in the periodic table that can be formed here are pitifully few (in the " "trace" levels), elements above No. 4 are not retained at all.
Summary
This fusion of protons and neutrons into helium in a few seconds left the universe with 75% to 76% protons (i.e. hydrogen nuclei) and 24% to 25% helium-4 nuclei. This ratio is calculated by mass, and if calculated by the number of nuclei, there are 92% protons and 8% helium-4 nuclei. About 0.001% each of deuterium and helium-3 remained, and lithium also remained, but as little as 0.0000001%. (As for the beryllium nuclei, most of them are beryllium-7, which has a half-life of 53 days and will decay into lithium-7.) Since the temperature and energy are low enough at this time, all kinds of nuclei are no longer It will be destroyed again, and at the same time, no new types of nuclei will be produced, and this situation will continue for millions of years.
The above-mentioned whole process is the birth process of the lightest elements in the universe, also known as "prime nucleosynthesis", and the top contemporary observers almost all agree with this deduction. And the ratio of the various elements left by this process remained the same for the next millions of years, until the first stars were born.
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