To understand the universe around us we have come up with and proven many different laws of physics, with one of the most fundamental being the different conservation laws. From these conservation laws, it follows that whenever matter is created an equal amount of antimatter must be created, but when matter and antimatter meet, they annihilate each other leaving only energy. According to this, the big bang should have created an equal amount of matter and antimatter, where most would annihilate almost instantaneously, leaving behind mainly energy and an equal amount of matter and antimatter. But if we look around us in the universe today, we only see matter. So, where did all the antimatter go?
The building blocks of our universe
To get a better understanding of this topic let us first look at what matter actually is and how it is created. For that, we take a look at the standard model of particle physics.
This theory does not only describe (anti)matter, but also how (anti)matter interacts with each other. Together with Einstein’s theory of general relativity, these two theories describe the vast majority of how anything in our universe works. One of the issues with these models is however, that it does not explain the origin of matter and antimatter, nor does it describe how our current universe is practically entirely made up of matter with barely any antimatter.
The creation of (anti)matter
The question of how something, like matter, can be created out of nothing, like empty space, was something that scientists had never been able to answer till quantum mechanics. If we consider a space without any particles or radiation one might say that this space contains nothing. However, the laws of quantum mechanics are still valid in this space, meaning that there are still, so-called quantum fluctuations. These quantum fluctuations implicate that matter and corresponding antimatter can come into existence and annihilate each other in a very short period of time. To simplify, empty space has energy and as Einstein’s famous equation $E=mc^2$ tells us, energy can be converted into mass. This is also supported by Heisenberg’s time-energy uncertainty-principle $\Delta E \Delta t \geq \frac{\hbar}{2}$, which basically tells us that in the vacuum of space, energy can be created as long as it is returned very quickly. An important thing to keep in mind is that with all experiments ever done where matter has been created, an equal amount of antimatter has been created simultaneously. Combining this knowledge with the conservation law of energy we understand how (anti)matter can come into existence, but we would expect the amount of matter and antimatter to be equal throughout the universe.
The conditions for the matter/antimatter imbalance
In 1967 Andrei Sakharov proved mathematically that there are three conditions how a universe that starts with equal amounts of matter and antimatter can end up in its current state:
- The universe must be out of equilibrium
- The universe must exhibit C and CP symmetry violation
- The universe must have baryon-number-violating interactions
The first condition is easily met since we live in an expanding and cooling universe, meaning that the universe in its current state is actually out of equilibrium. C symmetry, which stands for charge conjugation symmetry, says that the laws of physics must be applied equally to matter and antimatter. For example, this implies that if a certain particle spins clockwise, then the antiparticle also spins clockwise, which is possible for all particles that can be in two different spin states. Neutrinos however do only have one spin state, causing the particle and antiparticle to spin the other way around, violating C symmetry.
To obtain an antineutrino spinning the same way around as a neutrino, a so-called charge-parity (CP) transformation is needed. In quantum mechanics, a parity transformation flips the sign of one (or all) spatial coordinate(s), for example spin. However, during the decay of K mesons, a particle made up of one down quark and one strange antiquark, CP symmetry can be violated. Even though it has been proven that CP symmetry can be violated, this is not yet enough to say that the second condition is met. There are just not enough CP violating interactions to explain the difference we see in the amount of matter and antimatter in the universe.
The limited number of CP violations in our universe is however not the greatest constraint in theorizing the current imbalance between matter and antimatter in our universe. The third condition is one that has not (yet) been observed in any experiment. A baryon is a particle composed of three quarks, for example a proton or neutron. Baryons can be changed, for example by beta decay where a proton becomes a neutron, but the number of baryons stays the same. Currently, there is no evidence that baryons can be created or destroyed to satisfy baryon-number-violating interactions.
Alternative theories
In the theory described above, we assume that an equal amount of matter and antimatter was created during the big bang and that we are sure of our current findings of the amount of matter and antimatter in our universe. There are however some other interesting theories that do not rely on this.
One fascinating theory arose from Richard Feynman’s observation that antimatter is mathematically equivalent to ordinary matter moving back in time. This theory proposes that at the big bang there were two universes created, one moving forward in time containing only matter, the universe that we live in, and one moving backwards in time only containing antimatter. Since these matter particles and antimatter particles were never in the same place at the same time, they did not annihilate each other and a multiverse, consisting of two universes was created. The big problem with this theory, however, is that in our universe when we create matter, and thus also antimatter, we observe the antimatter going forwards in time, not backwards. Making the theory unlikely, but nevertheless a fascinating possibility.
Another theory is that at the big bang matter and antimatter were separated so quickly that they did not annihilate each other. If this theory is true, then there must be very distant galaxies made of antimatter. The problem with this theory is however that at the edge of the region containing galaxies made from matter and the region of galaxies made from antimatter, we should see a very bright phenomenon caused by the annihilation of matter and antimatter creating photons. There are no indications that such a phenomenon is happening anywhere in our universe.
Are we stuck?
From the analysis above, one might conclude that we are stuck and that a theory for the imbalance between matter and antimatter cannot be conceived, or at least not in the foreseeable future, but there is hope. In 2015 the LIGO and Virgo detectors documented the very first gravitational wave ever observed. Just like we have a cosmic microwave background that tells us a lot about the universe, the hope is that by improving our gravitational wave detectors we can one day understand how our universe is as it is by studying the cosmic gravitational wave background.
This article is written by Quinten Huisman
