Why is there more matter than antimatter?
Answers
In the first few moments of the Universe, enormous amounts of both matter and antimatter were created, and then moments later combined and annihilated generating the energy that drove the expansion of the Universe. But for some reason, there was an infinitesimal amount more matter than anti matter. Everything that we see today was that tiny fraction of matter that remained.
But why? Why was there more matter than antimatter right after the Big Bang? Researchers from the University of Melbourne think they might have an insight.
Just to give you an idea of the scale of the mystery facing researchers, here’s Associate Professor Martin Sevior of the University of Melborne’s School of Physics:
“Our universe is made up almost completely of matter. While we’re entirely used to this idea, this does not agree with our ideas of how mass and energy interact. According to these theories there should not be enough mass to enable the formation of stars and hence life.”
“In our standard model of particle physics, matter and antimatter are almost identical. Accordingly as they mix in the early universe they annihilate one another leaving very little to form stars and galaxies. The model does not come close to explaining the difference between matter and antimatter we see in the nature. The imbalance is a trillion times bigger than the model predicts.”
If the model predicts that matter and antimatter should have completely annihilated one another, why is there something, and not nothing?
The researchers have been using the KEK particle accelerator in Japan to create special particles called B-mesons. And it’s these particles which might provide the answer.
Mesons are particles which are made up of one quark, and one antiquark. They’re bound together by the strong nuclear force, and orbit one another, like the Earth and the moon. Because of quantum mechanics, the quark and antiquark can only orbit each other in very specific ways depending on the mass of the particles.
A B-meson is a particularly heavy particle, with more than 5 times the mass of a proton, due almost entirely to the mass of the B-quark. And it’s these B-mesons which require the most powerful particle accelerators to generate them.
In the KEK accelerator, the researchers were able to create both regular matter B-mesons and anti-B-mesons, and watch how they decayed.
“We looked at how the B-mesons decay as opposed to how the anti-B-mesons decay. What we find is that there are small differences in these processes. While most of our measurements confirm predictions of the Standard Model of Particle Physics, this new result appears to be in disagreement.
In the first few moments of the Universe, the anti-B-mesons might have decayed differently than their regular matter counterparts. By the time all the annihilations were complete, there was still enough matter left over to give us all the stars, planets and galaxies we see today.
Antimatter is identical to normal matter but with opposite charge, spin and other quantum numbers. Mesons are a type of particle made up of a quark and an antiquark. Quarks are the particles that make up the protons and neutrons found in atomic nuclei, and come in six ‘flavours’ – known as ‘up’, ‘down’, ‘strange’, ‘charm’, ‘bottom’ and ‘top’.
The D-mesons in the CERN experiment are made up of one charm quark and one charm antiquark. The physicists have witnessed the D-mesons oscillating between being a normal particle and an antiparticle, a process that has previously been observed in K-mesons (composed of a strange quark and an up or down antiquark) and B-mesons (a bottom antiquark and any of an up, down, strange or charm quark). When this happens, the constituent quark becomes an antiquark and vice-versa, so for example the antimatter partner to the K-meson is made up of a strange antiquark and a ‘normal’ up or down quark.
But in some cases this flip-flopping happens at different rates depending on whether a meson is transforming into an antimeson or the reverse is happening. Experiments in the 1960s showed that K-mesons are more likely to change from their antiparticles to their normal particles than the other way round, and some observations at Fermilab up to 2010 have suggested that the same is true of B-mesons.
This is an example of what is known as CP violation – an exception to the principle that physical laws should be the same for a particle as they are for an antiparticle with its direction reversed. This may help to explain why the universe appears to be made entirely of matter with no antimatter except that which is created in high-energy particle collisions.
One would expect the Big Bang to produce equal amounts of matter and antimatter, and, since the two annihilate one another on contact, this should have led to a universe with no particles, filled only with radiation.
This problem can be solved if there exists some process that favours matter over antimatter, leading to the excess that we see today.
Alternative explanations include the possibility that there are regions of the universe made of antimatter – which is thought to be unlikely since any overlap with matter regions would produce easily detectable radiation – and the suggestion that antimatter also exhibits gravitational repulsion, which would keep such regions separate.
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