The universe should be "empty", why does it have matter?
Have you ever thought about why there is matter in the universe instead of nothing? Why is there you, me, and her? This question is the biggest problem in cosmology, and even physics as a whole.
Then what are we? We are positive matter, because we are composed of protons, neutrons and electrons, so the question is why there are protons, neutrons and electrons in the universe?
Then the reason for this problem is the conservation of baryon number and lepton number. Here I explain that protons and neutrons are baryons, and electrons are leptons. Because one is heavy and the other is light, they are called baryons and leptons.
The conservation of baryon number and the conservation of lepton number mean that the number of baryons and the number of leptons in the universe remain constant and will not change.
You see, chemical reactions in daily life only involve the breaking and bonding of chemical bonds. Even nuclear reactions are just the splitting and recombination of atomic nuclei. They will not affect baryons and leptons, nor will their numbers occur.Change.
The second is β decay. After β? decay, neutrons will become protons, electrons and anti-electron neutrinos. From this reaction, it can be seen that the number of baryons remains unchanged, just a neutron changeIt becomes a proton. Although an electron is generated, an anti-electron neutrino is also generated. This is also called a lepton, but its lepton number is opposite to the electron's lepton number, so they cancel each other out, and the lepton number remains unchanged.The same is true for the β+ decay from protons to neutrons. Changing back and forth does not change the number of baryons and leptons.
So as far as we know, in today’s low-energy universe, we cannot change the number of baryons and leptons. Then some students will ask, then I will take an anti-proton or an anti-electron, which is the same as protons and electrons.Annihilation, doesn't this destroy their number conservation?
Yes! It is indeed possible! But the question is, where do you go to find antiprotons and antielectrons? You know, all our experiments and observations have shown that when you want to create an antimatter, you will also create an antimatter.Corresponding positive matter, when you want to destroy a positive matter, you must destroy a corresponding antimatter.
In the case of electrons, Dirac’s equation predicts that electrons have negative energy, that is, below the ground state of the atom, there are negative infinite energy levels, and these energy levels are filled with electrons with negative energy..
So the electrons with positive energy did not fall into the negative energy level. This is because of Pauli’s incompatibility principle. Therefore, in our view, the vacuum is an ocean of negative energy. The sea level is the zero point of the energy.The above is the world of positive energy. This is the sea of Dirac that we often hear.
Since there are electrons in the energy level below 0, these electrons should be excited and jump to a positive energy world, right, so we only need to inject energy into the vacuum, for example, use a gammaWhen light hits the nucleus, when the energy of this gamma photon is equal to 2mc^2, where m is the mass of the electron, we can excite the electron in the vacuum, and the vacuum that loses the electron will leave a hole, thisHoles will exhibit a positive charge, which is what we call positrons.
So, as long as we create a positive particle, it will leave a corresponding hole in the vacuum. This hole is its antiparticle. The positive and negative particles will soon be annihilated and then released.The energy of excitation, to put it plainly, is that the excited positive particle will quickly return to the hole, that is, it will jump to a lower energy state, and then release energy.
So it is impossible for us to change the number of baryons and leptons in the current universe, so we believe that the number of baryons and leptons in this low-energy universe is conserved.
And from the high-energy state, we can also see that the baryon number and the lepton number are conserved. You see, as long as the energy excites a positive matter, an antimatter will come out. The two cancel each other out.The number of children does not increase or decrease.
If you apply the above knowledge to the early universe, you will find that when the universe was born, the energy in it should have created equal amounts of matter and antimatter to ensure that the number of baryons and leptons at the beginning of the universeThe number is conserved and remains at 0.
Then, as the universe expands and the temperature decreases, the creation of positive and negative matter stops, and the produced matter and antimatter will annihilate each other and become photons.
But there are still some positive and negative particles that are too late to be annihilated, which will be pulled a long distance by the expansion of the universe and retained. So the final result is that there will only be a small amount of protons, antiprotons, electrons, and positives in our universe.Electrons, neutrinos and antineutrinos. A large number of photons will be left, so the ratio of photons to matter particles is about 10-20:1. Such a universe will not give birth to any matter structure.
But the real situation is that there is a lot of positive matter left in our universe. The ratio of photons to matter is about 10-9:1, and the amount of matter is more than ten orders of magnitude higher than the above value. This shows that the universeFrom the beginning, the law of conservation was broken.
For a long period of time, we thought this was impossible, so we just wished that in the universe, there should be stars, galaxies, and galaxy clusters composed of antimatter, the number of which is equal to the number of our positive matter, but weNo trace of antimatter was found.
So now it is believed that after the birth of positive and negative matter, although they were equal at the beginning, in the process of cooling the universe, the law of the conservation of baryon number and lepton number was destroyed, which led to the occurrence of the number of positive and negative matter.Asymmetry.
Then the quantity of positive matter is 1 billionth more than that of antimatter. After the annihilation of positive matter and antimatter, the extra positive matter remains to this day, forming all the material structures we see, including you and of course.Me and her.
Because antimatter has disappeared, and in today's low-energy state, we have not found any physical phenomena that can cause the asymmetry of positive and negative matter, so today's baryon number and lepton number are also conserved.
But the extra positive matter also shows that there have been non-conservative phenomena in the universe, so we must explain what happened in the process of cooling the universe?
Obviously, this is a big problem of the century, but it doesn’t matter. We still made some possible explanations for this problem. In 1967, Soviet scientist Sakharov put forward three conditions, saying that the universe only needs to meetThese three conditions can cause an asymmetry in the quantity of positive and negative substances.
First, the universe must deviate from the state of thermal equilibrium in the process of expansion. Second, there must be two kinds of breaks in the three basic symmetries we believe in as truth, including C break and CP break. Third, There should be interactions that destroy the conservation of baryon numbers in the universe.
These three conditions are called the Sakharov condition. They must happen at the same time to explain why there is so much more matter than antimatter, and it won’t work without one.
Ok, let me explain these three conditions below. The first one to destroy the thermal equilibrium is very simple for the universe, because the expansion rate in the early universe was very fast, which would lead to matter particles in one place.Before it can exchange information with the material particles in another place, they are drawn to a long distance, so the temperature of the two cannot always be the same. So our universe is in a non-thermal equilibrium state. Then the temperature is different, it may causeThe physical process of asymmetry between positive and negative matter.
The second condition, C broken and CP broken. There are three basic symmetries in our universe, C symmetry, that is, charge conjugate symmetry. It means that a particle and its antiparticle have the sameThe physical properties of positrons, such as positrons and electrons, have the same mass and spin. The difference is that the charge and the number of leptons are opposite.
This means that anti-particles can also form an anti-matter world. The physical laws in the anti-matter world are exactly the same as those in the positive material world. You cannot distinguish them. This is called C symmetry.
For example, in the Dirac Sea, people in the anti-matter world would think that we are below sea level, and they apply energy to the vacuum, which will excite positrons, and then form a hole. This hole is seen by others.It’s electrons. It’s exactly the same as what we saw, but the other way around.
P symmetry, also called mirror symmetry or conservation of parity, refers to the mirror image of a particle, which has the same physical laws as it, except that the spin is opposite. The rest are like mass, charge, lepton number, and baryonThe numbers are the same.
To put it bluntly, it means the world in the mirror is indistinguishable from our real world, that is, we cannot define absolute left and right. This is why when you look in the mirror every day, you don’t feel that you are in the mirror.Strange place, this embodies the conservation of parity.
T symmetry, that is, time reversal symmetry, it means that the physical laws experienced by a particle will not change whether it is time forward or backward.
The simple point is that the laws of physics will not change with time. The laws of physics we summarize today are applicable to the past and future universes.
In addition to the above single symmetry, there is also a compound symmetry, that is, CP symmetry. It is said that the mirror image particle of a particle’s antiparticle has the same physical properties as it, and we say that this particle satisfies CP symmetry.
Then what are C breaking and CP breaking? Let me give you a particle. If there is a meson, it spins upwards. If it is rotating counterclockwise, then its antiparticle is from the top.Looking down, it should also be rotating counterclockwise. If the meson now decays and an electron is ejected from above it, then as long as its antiparticle also maintains the same decay law as it, it will also be ejected from above when it decays.An electron, we say that they satisfy C symmetry.
If its antiparticle decays, it sometimes ejects electrons from above and sometimes ejects electrons from below, we think that the two satisfy different decay laws, which is called C breaking.
Then the CP is broken, the same is true. If the mirror image of the antiparticle of this meson also shoots electrons upwards when it decays, we say that the two CPs are symmetrical. As long as the mirror image of its antiparticle appears once and downThe phenomenon of ejecting electrons, we say that the CP is broken.
Generally speaking, the break of C and CP will cause the weak interaction of positive and negative particles to satisfy different laws, so they may undergo different decays during the cooling process of the universe, leading to positive and negative matterThe number is not symmetrical.
At present, we have found that three types of mesons have C breaking and CP breaking. Mesons are actually composed of quark and antiquark pairs. These three types of mesons include three quarks, strange quarks and bottom quarks.And charm quark.
But this is far from explaining the asymmetric quantity of positive and negative matter, so we need to discover more particles that violate the symmetry of C and CP when they experience weak interactions in the future.
Finally, let’s talk about the third condition, destroying the interaction of baryon number conservation. As mentioned earlier, we have not found any phenomenon that can destroy the baryon number conservation, so this has become the largest uncertainty in the asymmetry of positive and negative matter.Factor.
So according to the current knowledge we have, it is indeed possible to cause the asymmetry of positive and negative matter through the weak interaction of C breaking and CP breaking, but it cannot explain the quantitative difference between the two. To put it bluntly, it needs moreMany broken.
This also indicates that the particles in the standard model we know now are not all of the universe. For example, under the scale of electroweak force, that is, when the energy of the early universe reaches a certain level, electromagnetic force will be combined withWeak forces are combined into one and become electroweak. At this time, there are only three basic forces in the universe, electroweak force, strong force, and universal gravitation. In this case, new particles may appear. When the universe cools down, When the electroweak force becomes electromagnetic force and weak force, this kind of particle may cause the number of baryons to be broken.
There is also the current grand unified theory that the universe before the electroweak force has higher energy, and the strong force at this time will also be added to the electroweak force. At this time, the universe has only two forces, the grand unity force and the gravitational force.
We believe that there was a kind of massive particle at that time, and its decay directly caused the asymmetry of the positive and negative quarks, which in turn caused the break of the baryon number.
There is another theory that started with neutrinos, because we found that all neutrinos are left-handed, there are no right-handed neutrinos, and all anti-neutrinos are right-handed, and there is no left-handedness.Antineutrinos, which is different from other particles.
Here I explain what is left-handed neutrino and right-handedness. Now you stretch out your left hand, bend your four fingers, and give a thumbs up. The direction of your thumb represents the direction of movement of the neutrino, so the four fingers are bentThe direction represents the spin direction of neutrinos. Neutrinos only satisfy left-handed chirality, and antineutrinos only satisfy right-handed chirality.
So people guess that in the early universe, there should be a large number of very heavy right-handed neutrinos, and very heavy left-handed antineutrinos, because their decay caused the asymmetry of the number of leptons, andThe asymmetry of the lepton number, which in turn leads to the asymmetry of the baryon number.
Finally, there is a guess, that is, the prediction of supersymmetry made by string theory. It believes that in the early universe, all particles in the Standard Model had a supersymmetric partner particle, this supersymmetric partnerThe spin of the particle is 1/2 different from it, but the mass is equal. This is a very strange group of particles.
For example, if the spin of an electron is 1/2, then the spin of its supersymmetric partner is 1, and 1 is a boson, which is a photon, but the mass of this photon is the same as that of the electron. So itInstability requires decay. Then the supersymmetry theory believes that the decay of these particles has led to the asymmetry of positive and negative matter.
The above-mentioned processes are called baryon generation, which is the preface of today's physics and the boundary of human knowledge. Of course, they are all guesses. In the future, more experiments are needed to discover and verify more.Theories and new particles, and explaining why the universe is not empty, but full of matter.
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