Axion: The ultimate solution to the universe?

Physicists love the word “radical”. They tend to answer even the simplest of the questions with solutions that try to alter our perception of the universe. Just the thought of penning a theory that will topple our understandings and make us wonder the very reason for our existence is enough to give them goosebumps. Most of the times, such theories go in vain, with the discrepancy they were trying to explain ending up being an experimental error. But here and there, a few theories end up being the correct ones and end up widening our perspective of the universe. The axion is one such radical theory, unproven yet, but has the potential to solve three major puzzles in physics.

Puzzle 1: The Strong CP Problem…

The Standard Model (SM) does an amazing job in comprehending the universe. But it is filled with loopholes that are hard to explain. The standard model is a constant battle between physicists and inconsistencies. One of the most underrated, but very principal puzzles of physics, which is not yet resolved is called the Strong CP problem. 

The SM talks about particles in terms of mathematical ‘groups’. In everyday mathematics, we see that most of the stuff commutes, A+B = B+A or A*B = B*A. But that is not the general case as you know. The case with the groups is also the same. So fundamentally, the groups can be divided into two groups, the abelian – which commute and the non-abelian – that does not commute. For example, the group associated with translation is abelian, but the one associated with rotation is non-abelian. 

In the SM, Electromagnetism is abelian, Strong and Weak interactions are non-abelian. The difference between these is visible in its symmetries. Abelian theories must be symmetric under:
C (Charge conjugation): If the particle is replaced by anti-particle and vice-versa.
P (Parity): If the particle is replaced by its mirror image.
T (Time reversal): If the direction of time is reversed.

For abelian theories, interactions have C, P and T individually conserved, and the groups CP (C and P together), CT (C and T together), PT (P and T together) and CPT (All C, P and T together) are also conserved. 
This is not the case with non-abelian theories. These do not require individual conservation of C, P and T, and also the combinations CP, CT and PT can be violated. But CPT should always be conserved. 
This means that for electromagnetism, C, P, T, CP, CT, PT and CPT all are conserved. But for Strong and Weak forces, every symmetry except the CPT is allowed to be broken.

[Images: Forbes]

Murray Gell-Mann has coined a principle called The Totalitarian Principle: Everything that is not forbidden is compulsory, which means that processes which are not forbidden should have a finite non-zero probability of occurring.
This should mean that CP violation which is not forbidden in Strong and Weak interactions should occur with non-zero probability. In Weak interactions, CP violation is plentily observed. But in Strong interaction, no CP violation is observed at all, not even once in a billion interactions. This is called the Strong CP problem and is one of the problems that question the very fundamentals of physics.

In 1977, physicists Roberto Peccei and Helen Quinn proposed a possible solution to this problem. They stated that there exists a scalar field called the axion field, that permeates throughout the universe. This scalar field influences the strong interactions and is the reason for the forbiddance of CP violation. If this is true, there should be a particle associated with the scalar field, and this particle was called the AXION.

Puzzle 2: Dark matter…

The above proposed Axion should be extremely light – about a billion times lighter than the proton, should have no electric charge and should be so huge in number that they should be floating everywhere literally, but rarely interacting with anything. All these properties make it the perfect candidate for dark matter. Due to its low interaction and omnipresence, it can clump together and form regions of high mass and low interaction, just like what is required for a dark matter candidate. Many experiments are currently working to find this Axion and effort is going on in the scientific community. The major experiment being the ADMX(Axion Dark Matter eXperiment), which uses a strong magnetic field to convert dark matter axions to detectable to microwave photons. 

Puzzle 3: Baryon Asymmetry…

Matter and antimatter particles are always produced in pairs, and also annihilate each other once they come in contact, leaving behind pure energy. During the first few moments after the big bang, the universe was a nursery of particle-antiparticle pairs, with them popping in and out of existence continuously. This should mean that matter and antimatter should have been created in equal amounts and also should have been destroyed soon, leaving behind a universe with only energy. But this is not what is observed, we see matter present in this universe, and also it is way huger in amount than the amount of antimatter. The reason for this being still unknown. Scientists theorize that something happened in the early universe leading to an asymmetry in matter-antimatter annihilations causing a tiny portion of matter to survive, and this is what we see in the universe today.
Imagine a coin spinning on a table. The coin has a 50-50 chance of falling either heads up or tails up. But if there were some kind of special marbles that roll on the table, influencing the coin to fall more on heads than tails, the probability distribution would completely change.
According to a team of scientists, axions might be the answer for this too. They postulate that at the beginning of the universe, the axion field started oscillating, and this caused the matter to form a bit more in amount than antimatter. They have named this theory Axiogenesis, and this seems to be a potential explanation for the baryon asymmetry problem if axions are found.

The fact that axions have the potential to solve the three greatest puzzles in physics is truly amazing. This has caused a stir among the scientific community has pressurized the experimentalists to further pursue the search for these axions. These puzzles have also promoted a new field of physics beyond the standard model. In case the axion is found, then it can be termed as the saviour of physics, with three successes in its bag. Also, it could mean that all the puzzles in physics could be solved if thought of as being influenced by axions. In short, if the axion is to be found, then it might potentially solve all the puzzles in the world of physics.