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Nov 24
2009
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Soon after the second symmetry breaking (the GUT era), there is still
lots of 
energy available to produce matter by pair production,
rather than quark confinement. However, the densities are so high
that every matter and anti-matter particle produced is soon destroyed
by collisions with other particles, in a cycle of
equilibrium.
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Note that this process (and quark confinement) produces an equal
number of matter and anti-matter particles, and that any particular
time, if the process of pair production or quark confinement were to
stop, then all matter and anti-matter would eventual collide and the
Universe will be composed only of photons. In other words, since
there are equal numbers of matter and anti-matter particles created
by pair production, then why is the Universe made mostly of matter?
Anti-matter is extremely rare at the present time, yet matter is very
abundant.
This asymmetry is called the matter/anti-matter puzzle. Why if particles are created symmetrically as matter and anti-matter does matter dominate the Universe today. In theory, all the matter and anti-matter should have canceled out and the Universe should be a ocean of photons. |
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It is not the case that the Universe is only filled with photons (look
around the room). And it is not the case that 1/2 the Universe is
matter and the other half is anti-matter (there would be alot of
explosions).
Therefore, some mechanism produced more matter particle than anti-matter particles. How strong was this asymmetry? We can't go back in time and count the number of matter/anti-matter pairs, but we can count the number of cosmic background photons that remain after the annihilations. That counting yields a value of 1 matter particle for every 1010 photons, which means the asymmetry between matter and anti-matter was only 1 part in 10,000,000,000. This means that for every 10,000,000,000 anti-matter particles there are 10,000,000,001 matter particles, an asymmetry of 1 particle out of 10 billion. And the endresult is that every 10 billion matter/anti-matter pairs annihilated each other leaving behind 1 matter particle and 10 billion photons that make up the cosmic background radiation, the echo of the Big Bang we measure today. This ratio of matter to photons is called the baryon number. |
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Even though the baryon number is extremely small (10-10) why isn't it zero? In Nature, there are
only three natural numbers, 0, 1 and infinity. All other numbers
require explanation. What caused the asymmetry of even one extra
matter particle for every 10 billion matter/anti-matter pairs?
One answer is that the asymmetry occurs because the Universe is out of equilibrium. This is clearly true because the Universe is expanding, and a dynamic thing is out of equilibrium (only static things are stable). And there are particular points in the history of the Universe when the system is out of equilibrium, the symmetry breaking moments. Notice also that during the inflation era, any asymmetries in the microscopic world would be magnified into the macroscopic world. One such quantum asymmetry is CP violation. |
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