Ask Ethan No. 31: why are we made of matter?

If at the beginning of the existence of the Universe there was the same amount of matter and antimatter in it, why does matter dominate in today's cosmos?

You may not feel very powerful, but if you are a medium-sized adult, your humble body contains at least 7 × 10 ¹⁸ J of potential energy - it would be enough so that you could explode with the power of thirty very large hydrogen bombs, if would you know how to free her and if you really needed it.
- Bill Bryson

At the end of the week, I choose the best of your questions for the “Ask Ethan” column, and this week the honor goes to Justin Starr, who asks:

As far as I understand, in the nascent Universe there were equal parts of matter and antimatter, as a result of which there was serious annihilation. And how did matter win as a result?

Justin asks about one of the greatest unsolved mysteries of the universe.

Think about these two seemingly contradictory facts:


a) pairing; b) annihilation

1) None of the particle interactions we observed with any energy level created or destroyed a single particle of matter without creating or destroying an equal number of particles of antimatter. The physical symmetry between matter and antimatter is even more stringent:

• every time we create a quark, we create an antiquark
• every time the quark is destroyed, the antiquark is also destroyed
• every time we create or destroy a lepton, we create or destroy an antilepton of the same family, and
• every time a quark or lepton experiences interaction, collision or decay, the total number of quarks and leptons at the end of the reaction (quarks minus antiquarks, leptons minus antileptons) remains the same as at the beginning

The only way to add or reduce matter in the Universe is a method that reduces or adds antimatter in the same quantities. But there is a second fact:



2) When we study the Universe, stars, galaxies, gas clouds, clusters, superclusters, structures of the largest scale - everything looks created from matter, and not from antimatter. Every time matter and antimatter meet in the Universe, an incredible release of energy occurs due to the annihilation of particles.

In some places, we observe this annihilation, but only near hyper-energy sources that produce matter and antimatter in equal amounts. When antimatter collides with matter in the Universe, it produces gamma radiation of very specific frequencies, which we can detect.

But if we look at the space between stars and between galaxies, even on a large scale, we find that there is a lot of material there, even if there are no stars there. The cosmos is large and the density of matter is low, so you may wonder - if you throw one particle of antimatter (say, an anti-proton), how long will it last on average before it crashes into a particle of matter and annihilates.



In the interstellar space of our galaxy, the average lifetime of a particle would be 300 years, which is very small compared with the age of the galaxy. This limitation suggests that at least within the Milky Way, the amount of antimatter that can fit among matter is one part in 10 ¹⁵ ! ..

On large scales, on the order of galaxies and galactic clusters, the restrictions are not so strong, but still quite serious. Observing space at distances from several million to three billion light years, we found a deficiency of X-rays and gamma rays, which should arise from the annihilation of matter and antimatter. We see that even on a cosmic scale, 99.999% of everything that exists is made up of matter like us, and not antimatter.



And this is only a lower estimate of the level of dominance of matter over antimatter in the Universe, according to observations.

So, on the one hand, we have the results of experiments demonstrating the impossibility of creating and destroying matter without the simultaneous destruction and creation of the same amount of antimatter, and on the other, we have the Universe, which, as far as we can judge, consists of almost 100% of matter, and almost 0% antimatter. What the heck?

If we want to understand how this happened, we need to return to the very origins of the Universe, to the moment immediately after inflation and the Big Bang: to the time when the Universe was hot, dense and full of matter, antimatter and radiation.



In the early stages of the universe, everything was hot and dense. That part of it, which now makes up the observable part of the Universe, consisted of 10 ⁹⁰particles of matter, antimatter and radiation, while the amount of matter and antimatter was the same. There was so much energy that in any collision of particles they could spontaneously generate the same amount of matter and antimatter, and when matter and antimatter collided, they turned into radiation. And this happened constantly and everywhere.



If the Universe could only create pairs of matter and antimatter so that they again annihilate, our Universe today would look very different. Theoretically, if there were no symmetry of matter and antimatter, with the cooling and expansion of the Universe, we would reach the point where it would become impossible to create a new particle. Existing pairs of matter and antimatter would annihilate until space became so sparse that the particles could not find each other, and we would have a universe filled with photons and a small amount of matter and antimatter.

How many would be left? As far as we know, approximately 10 ⁷⁰ particles of matter and antimatter, and there would be 10 ²⁰ photonstimes more than protons. That is, for each proton there would be 100,000,000,000,000,000,000 photons, and there would be as many antiprotons as protons.

But we can measure the ratio of the number of photons to protons.


Vertical: the number of elements relative to hydrogen; horizontal: the density of ordinary matter relative to protons.

Not at all such a terrible asymmetry. Of course, there are many, many times more photons than protons, but the ratio is more like a couple of billion to one (and almost without antimatter), which indicates some kind of incident in the early Universe that created the fundamental asymmetry of matter and antimatter. As far as we can see, asymmetry has happened everywhere and with the same power.



If you want to know exactly how it happened - welcome to the club. This is the problem of baryogenesis, and this is one of the greatest unsolved problems of fundamental physics. But just from the fact that we do not know exactly how this could happen, does it not follow that we do not have a worthy general idea of ​​how this happened. Specifically, Andrei Sakharov showed that if three conditions are satisfied, then it is possible to create an asymmetry of matter and antimatter from an initially symmetric state:

• nonequilibrium conditions
• violation of C and CP symmetry
• interactions with violation of the law of conservation of baryon numbers

That's all. Only three things. And as far as we know, in the Universe they should all be.



Nonequilibrium conditions. It's simple. If you have a large, hot, expanding and cooling Universe, governed by the laws of general theory of gravity and quantum field theory, congratulations: your conditions are not balanced. Equilibrium is when all particles in the system have the ability to communicate with each other, or exchange information. But in an expanding cooling universe, particles from one edge do not have causal relationships with particles from another. In a very early Universe, there were 10 ⁵⁰ unconnected regions where even the light would not have enough time to pass from one region to another.

The early Universe was not only in a nonequilibrium state, it would be very difficult for you to construct a system that would be in a less equilibrium state.


The parity of elementary particles is determined by the direction in which they emit decay particles. A muon usually emits an electron to the right. With reverse parity, the muon should emit an electron to the left. Once out of a thousand, the anti-muon decays to the right, which violates the CP symmetry

Violation of C- and CP-symmetry. C-transformation or charge conjugation (the operation of replacing a particle with the corresponding antiparticle), and P is parity (mirror reflection of everything). Roughly speaking, C and P are preserved if you fix each of these symmetries, while the laws of physics are preserved; and CP symmetry is preserved if you simultaneously fix both symmetries, and all the laws of physics remain unchanged.

In our universe, gravitational, electromagnetic, and strong interactions conserve C, P, and CP. But the weak break them! Specifically, the decay of mesons containing strange quarks and lovely quarks greatly violates C, P and CP, that is, there are fundamental differences in the behavior of particles and their antiparticles. Two of the three conditions are.

And finally ...


Protonchik grandchildren and proton grandfather:
- Grandfather, tell us about the great war in which you participated!
- Sit down, kids, it's a long story. Today it is called baryogenesis, and we call it Hell. It began almost fourteen billion years ago. Shortly after the Big Bang, the young Universe cooled to such an extent that particles appeared. First leptons appeared. They survived in their war-leptonogenesis. During these nanoseconds, baryons and anti-baryons formed in equal amounts ...
To be continued ...

Interactions with violation of the law of conservation of baryon numbers. This thing is complicated, since we never observed the creation of a quark without an antiquark (and a baryon is any particle consisting of three quarks, such as a proton or neutron. Quarks exist in nature only in bound states!). But if we look at the Standard Model, we conclude that it should contain interactions of this type.

And now I'm going to show you the field equations governing the Standard Model of particle physics (don't worry about the details).



It is important here that this equation has a mathematical “anomaly” necessary for a certain amount of decay of particles - for example, for decay of a neutral pion - and also allowing violation of the law of conservation of the baryon number. Moreover, it allows violations of the conservation law of both baryon (for example, proton) and lepton (for example, electron) numbers to occur, but they must be violated together, that is, in the Universe there must be a constant total number of baryons and leptons! This very successfully explains why there are equal numbers of protons and electrons, and, therefore, why the Universe, where both protons and electrons exist, is electrically neutral.



Of course, the main question arises when we start to make calculations. If we take into account

how much the Universe is disequilibrium
the number of observed violations of C- and CP-symmetries;
how much

does the Standard Model violate the law of conservation of baryon numbers ? Do we get a sufficiently strong violation of the law of conservation of baryon numbers?



As far as we know today, no, not really. So far, there is a ten order discrepancy. Of course, in the Standard Model, at higher energy levels, there can be much more CP-symmetry-breaking interactions that we have not yet discovered, but most often assume that there is physics outside the Standard Model that allows more CP-symmetry or law violations to occur conservation of baryon numbers.



Features include, but are not limited to, the following options:

• Affleck-Dyne mechanism based on supersymmetry
• additions to the Standard Model on electroweak scales
• leptogenesis, in which the fundamental asymmetry of leptons is created (possibly from the new neutrino physics), and from it the asymmetry of baryons arises
• large-scale baryogenesis of the Theory of Grand Unification, where the new physics of the scales of electroweak unification with strong interaction allows us to create more matter than antimatter

Most likely, the meaning of these words is not clear to you, so let me tell you as an example about the possibility of the Great Unification Theory. Note that this is most likely not happening at all, and the described scenario is used for illustration only.



Imagine an early Universe filled with radiation and various particles and antiparticles, the latter of which exist in equal quantities. Some are quarks and antiquarks, some are leptons and antileptons, some are bosons (and their antiparticles, where applicable; many bosons are antiparticles for themselves), etc.

Imagine that there is a new kind of charged particles, pairing with both quarks and leptons. We call them Q-particles.



Initially - just like in the case of particles of matter and antimatter - they are created in a hot, early Universe in pairs. Sometimes Q +, a version of a particle from matter, finds Q-, a version from antimatter, and they annihilate, and sometimes other particles encounter enough energy to create Q + / Q- pairs.

So all this happens (within a small fraction of a second), and then the Universe cools down. Suddenly, new Q + / Q- pairs cease to appear, and after some of the Q + / Q- pairs annihilate and turn into radiation, the remaining unstable particles decay.



According to the laws of elementary particle physics (even allowing extensions to the Standard Model), some symmetries must be preserved. Particles Q + and Q- should be the same:

• lifetime
• decay paths
• charge, mass and number of “baryons minus leptons”

In this example, the particles Q + and Q- have the same average lifetime, the number of “baryons minus leptons” is zero, and although Q + can decay into a proton and neutrino, or into an anti-neutron and anti-electron, Q- can decay into an antiproton and antineutrino, or neutron and electron. These processes violate the law of conservation of baryon numbers and lepton numbers, but not “baryons minus leptons”. This scenario is possible and reasonable, but will not create baryon asymmetry unless we introduce violation of CP symmetry.



Without this violation, what is called a branching ratio, or proportions in which Q + and Q- decay along each path, will be the same. If 60% Q + decays into protons and neutrons, then 60% Q- decays into antiprotons and antineutrinos. Another way can be 40% for both Q + and Q-, which also preserves CP symmetry.

But if its violation is allowed, the relations of branching into particles and antiparticles can be different! As long as the general decay proportions for Q + and Q- are the same, the laws of physics allow this behavior. Let's introduce a violation of CP symmetry.



A small difference: Q + decays again, as before, but Q- now decays more into neutrons and electrons, and less into antiprotons and antineutrinos!

When all the particles Q + and Q- decay (ignore the leptons for simplicity), what do we get?



A bunch of protons, neutrons, antiprotons and antineutrons. Over time, antiprotons and protons will meet and annihilate, as will neutrons and antineutrons. But due to the asymmetry of the decay of Q + and Q- particles, there are more protons than antiprotons, and neutrons than antineutrons.

After annihilation of all pairs of particles / antiparticles, baryon asymmetry remains. If we follow the lepton asymmetry, we will find that there are exactly as many electrons as protons, and the number of neutrinos exceeds the number of antineutrinos exactly by the number of neutrons.

And although this is probably not an accurate description of the mechanism of baryogenesis, something similar most likely happened and created our today's Universe!



These three conditions are Sakharov:

• nonequilibrium conditions
• violation of C and CP symmetry
• interactions with violation of the law of conservation of baryon numbers

unequivocally exist in the Universe, and the only question that remains to be answered is: “how did such an asymmetry of matter and antimatter arise that we observe today?” My answer reflects our current views, and I am not ashamed of the fact that it is not complete. But of all the great puzzles on the topic “where did everything come from, this, most likely, we can solve during my lifetime.”

Thanks for the great question, Justin, and I hope you enjoyed it. If you have questions, ask them to me, and you may appear in the next article.