When we look at the Universe, at all its planets and stars, galaxies and clusters, gas, dust, plasma, we see the same signatures everywhere. We see lines of atomic absorption and emission, we see that matter interacts with other forms of matter, we see star formation and death of stars, collisions, X-rays and much more. There is an obvious question that requires explanation: why are we seeing all this? If the laws of physics dictate symmetry between matter and antimatter, the universe we observe should not exist.
But we are here and no one knows why.
Why is there no antimatter in the Universe?
Think about these two seemingly conflicting facts:
- every time we create a quark or lepton, we also create an antiquark and antilepton;
- every time a quark or lepton is destroyed, an antiquark or antilepton is also destroyed;
- created or destroyed leptons and antileptons must be in balance across the entire summerpon family, and every time a quark or lepton interacts, collides or decays, the total number of quarks and leptons at the end of the reaction (quarks minus antiquarks, leptons minus antileptons) should and will be the same as it was at the beginning.
The only way to change the amount of matter in the universe was also to change the amount of antimatter by the same amount.
And yet, there is a second fact.
But we do not see any signs of destruction of matter by antimatter on the largest scale. We see no sign that some of the stars, galaxies, or planets we observe are made of antimatter. We do not see the characteristic gamma rays that one would expect to see if antimatter collided with matter and annihilated. Instead, we see only matter everywhere we look.
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And it seems impossible. On the one hand, there is no known way to make more matter than antimatter by looking at particles and their interactions in the universe. On the other hand, everything we see is definitely made of matter, not antimatter.
In fact, we have observed the annihilation of matter and antimatter under some extreme astrophysical conditions, but only near hyperenergetic sources that produce matter and antimatter in equal amounts - black holes, for example. When antimatter collides with matter in the universe, it produces gamma rays of very specific frequencies, which we can then detect. The interstellar intergalactic medium is full of material, and the complete absence of these gamma rays is a strong signal that there will never be much more antimatter particles, as the signature of antimatter matter would then be discovered.
If you throw one particle of antimatter into our galaxy, it will last about 300 years before being destroyed by a particle of matter. This limitation tells us that the amount of antimatter in the Milky Way cannot exceed 1 particle per quadrillion (1015), relative to the total amount of matter.
On a large scale - the scale of satellite galaxies, large galaxies the size of the Milky Way, and even clusters of galaxies - the constraints are less stringent, but still very strong. Observing distances ranging from a few million light years to three billion light years, we have observed a dearth of X-rays and gamma rays that could indicate the annihilation of matter and antimatter. Even on a large cosmological scale, 99.999% of what exists in our universe will definitely be represented by matter (as we are) and not antimatter.
How did we end up in such a situation that the Universe consists of a large amount of matter and practically does not contain antimatter, if the laws of nature are absolutely symmetric between matter and antimatter? Well, there are two options: either the Universe was born with more matter than antimatter, or something happened at an early stage, when the Universe was very hot and dense, and gave rise to an asymmetry of matter and antimatter, which originally did not exist.
The first idea cannot be scientifically tested without recreating the entire Universe, but the second is very convincing. If our Universe somehow created an asymmetry of matter and antimatter where it was not originally, then the rules that worked then will remain unchanged today. If we are smart enough, we can develop experimental tests that reveal the origin of matter in our universe.
In the late 1960s, physicist Andrei Sakharov identified three conditions required for baryogenesis, or the creation of more baryons (protons and neutrons) than antibaryons. Here they are:
- The universe must be a nonequilibrium system.
- It must have a C- and CP-violation.
- There must be interactions that violate the baryon number.
The first is easy to observe, since an expanding and cooling Universe with unstable particles in it (and antiparticles), by definition, will be out of equilibrium. The second is also simple, because C-symmetry (replacing particles with antiparticles) and CP-symmetry (replacing particles with specularly reflected antiparticles) are broken in many weak interactions involving strange, charmed, and beautiful quarks.
The question remains how to break the baryon number. We have observed experimentally that the balance of quarks to antiquarks and leptons to antileptons is clearly preserved. But in the Standard Model of particle physics there is no explicit conservation law for any of these quantities separately.
It takes three quarks to make a baryon, so for every three quarks we assign a baryon number (B) 1. Likewise, each lepton will receive a lepton number (L) 1. Antiquarks, antibaryons and antileptons will have negative B and L numbers.
But according to the rules of the Standard Model, only the difference between baryons and leptons remains. Under the right circumstances, you can not only create additional protons, but electrons to them. The exact circumstances are unknown, but the Big Bang gave them the opportunity to be realized.
The very first stages of the existence of the Universe are described by incredibly high energies: high enough to create every known particle and antiparticle in large quantities according to Einstein's famous formula E = mc2. If creating and destroying particles works as we think, the early universe would have to be filled with an equal number of particles of matter and antimatter, which mutually transformed into each other, since the available energy remained extremely high.
As the universe expands and cools, unstable particles, once created in abundance, will collapse. If the right conditions are met - in particular, the three conditions of sugars - this can lead to an excess of matter over antimatter, even if initially there was none. The challenge for physicists is to create a viable scenario, consistent with observation and experimentation, that can give you enough excess matter over antimatter.
There are three main possibilities for this excess of matter over antimatter:
- New physics on the electroweak scale could significantly increase the amount of C- and CP-violation in the Universe, which will lead to asymmetries between matter and antimatter. SM interactions (via the sphaleron process) that violate B and L individually (but retain B - L) can create the desired volumes of baryons and leptons.
- The new high-energy neutrino physics that the universe is hinting at could create a fundamental asymmetry of leptons: leptogenesis. Sphalerons conserving B - L could then use lepton asymmetry to create baryon asymmetry.
- Or baryogenesis on the grand unification scale, if the new physics (and new particles) exist on the grand unification scale, when the electroweak force is combined with the strong one.
These scenarios have common elements, so let's take a look at the last one, just for the sake of example, to understand what might have happened.
If the grand unification theory is correct, there must be new, superheavy particles called X and Y that have both baryon-like and lepton-like properties. There should also be their partners from antimatter: anti-X and anti-Y, with opposite B - L numbers and opposite charges, but with the same mass and lifetime. These particle-antiparticle pairs can be created in large quantities at energies high enough to subsequently decay.
So we fill the universe with them, and then they disintegrate. If we have C- and CP-violations, there may be slight differences in how particles and antiparticles (X, Y and anti-X, anti-Y) decay.
If the X particle has two paths: decay into two up quarks or into two anti-down quarks and a positron, then anti-X must go through two corresponding paths: two anti-up quarks or a down quark and an electron. There is an important difference that is allowed when C- and CP are broken: X may be more likely to decay into two up quarks than anti-X into two anti-up quarks, while anti-X is more likely to decay into down quark and an electron than X - into an anti-up quark and a positron.
If you have enough pairs and decay in this way, you can easily get an excess of baryons over antibaryons (and leptons over antileptons) where there was none before.
This is just one example to illustrate our understanding of what happened. We started with a completely symmetrical universe, obeying all known laws of physics, and with a hot, dense, rich state, filled with matter and antimatter in equal amounts. Through a mechanism that we have yet to determine, obeying Sakharov's three conditions, these natural processes ultimately created an excess of matter over antimatter.
The fact that we exist and are made of matter is undeniable; the question is why our Universe contains something (matter) and not nothing (after all, matter and antimatter were equally divided). Perhaps in this century we will find the answer to this question.
Ilya Khel