The Initial Asymmetry (or Bias) in the Universe

[Xonas Pitfall]

This imbalance is referred to as baryon asymmetry. The word “baryon” refers to heavy subatomic particles like protons and neutrons, the building blocks of atomic nuclei. Experiments and observations suggest that for every billion particle-antiparticle pairs created in the early universe, there was one extra matter particle. That one-in-a-billion surplus survived the mass annihilation event and became the matter of our cosmos. But how did this happen? The process that might have caused it is known as baryogenesis. The Russian physicist Andrei Sakharov outlined three necessary conditions for baryogenesis: violation of baryon number (meaning some reactions need to produce more baryons than antibaryons), violation of charge and parity symmetries (C and CP violation), and conditions that prevent the system from returning to equilibrium. Without all three, the symmetrical conditions of the early universe would have persisted.

Now, the key suspect in this mystery is the weak force - one of the four fundamental forces of nature. Unlike gravity, electromagnetism, or the strong force, the weak force behaves differently when it comes to matter and antimatter. It shows CP violation. This has been directly observed in certain decays of subatomic particles like kaons and B mesons. The weak force also exhibits what’s called parity violation, meaning it doesn’t treat left and right the same. For example, neutrinos - ghostly particles that only interact via the weak force - are always left-handed, while antineutrinos are always right-handed. This strange handedness, or chirality, breaks mirror symmetry and hints at a deeper imbalance in nature that could explain why matter won out.

Neutrinos are especially interesting here. They might hold the secret to understanding how the universe tipped the scales in favor of matter. Experiments like T2K in Japan and NOvA in the US are examining how neutrinos oscillate - that is, change type - as they travel. These oscillations seem to happen differently for neutrinos and antineutrinos, which is a form of CP violation. If confirmed, it could be the mechanism responsible for the matter-antimatter asymmetry. Neutrinos are also unique in that they might be their own antiparticles (a concept known as Majorana particles), which would allow certain rare processes to occur that violate the conservation of lepton number, another potential pathway to baryon asymmetry through a process called leptogenesis.

In parallel, other experiments like LHCb at CERN have found CP violation in particles containing bottom quarks, while experiments like ALPHA and BASE at CERN trap and study antimatter directly to see how it compares to regular matter under different forces, especially gravity. These tests have so far not shown any obvious differences beyond the weak interaction, which seems to be uniquely asymmetric.

Theoretically, physicists have proposed various extensions to the Standard Model to explain these observations. Some of them invoke supersymmetry, extra dimensions, or new particles that might have existed briefly in the early universe. One such idea is the Affleck-Dine mechanism, which suggests that scalar fields tied to supersymmetric particles could have caused matter to dominate over antimatter. Others look at how quantum fluctuations during the inflationary phase of the universe might have set up slightly different conditions in different regions, leading to small imbalances that had big consequences.

Despite these advances, the full explanation is still out of reach. We have experimental evidence of CP violation, but the amount observed so far in the known particle interactions is not enough to account for the enormous asymmetry in the universe. This suggests that there must be other, yet undiscovered, sources of asymmetry. That’s why research in particle physics and cosmology is so active in this area. Scientists are hoping that more precise measurements of neutrino behavior, the discovery of new particles, or anomalies in antimatter experiments will eventually reveal where the universe broke the perfect symmetry it should have had - and why.

[LastThursday]

Interesting stuff.

As a thought experiment, how does any sort of asymmetry come about in the first place? Unless the asymmetry has always been present, then a symmetry had to be broken somewhere along the line. What is that mechanism?

In terms of the universe as a whole, then you could take the most symmetrical thing possible and start from there. A symmetry is just some attribute that stays the same after some sort of transformation. What is the most symmetrical thing possible? I'd say a complete void: whatever you do to it it stays the same. So to create a baby universe from a void, you have to break some of its symmetries. Note that this is a bit paradoxical, because there are potentially an infinite number of possible symmetries for a void. On the other hand breaking any symmetry here would produce "something" from "nothing". Conversely, the fact that there is anything at all means that original symmetry of the void has been violated.

The asymmetry of the Weak force then is a consequence of there being something at all. Those asymmetries could have come about in an infinite number of potential ways, the egg just so happened to crack the way it did, so to speak. However, it doesn't mean that all those symmetry violations are not connected to each other. In some sense a "force" is not separable from the particles it acts on: they form one system. So I would say the Weak force and the particles it acts on have to be taken together. And together they break a symmetry. 

The question then is not why is the Weak force asymmetric, but more like, is the Weak force asymmetry connected to other asymmetries in nature?