# Matter/antimatter, mass questions

by Dav333
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 P: 89 I heard for every (think it was) billion antimatter there was 1 extra matter. Does this mean before the annihilation after the big bang the universe was billions of times more heavy? So all that antimatter is now photons zipping around the universe? Other question. If there is all the matter & energy ever created, then why do particles pop in & out from the vacuum & annihilate each other? Looking for a simple answer if possible as wikipedia is over my head.
 Sci Advisor P: 6,057 The total mass, including energy, was the same. Annihilation converts mass to energy (E=mc2).
 PF Gold P: 184 There is no global energy conservation rule, and during the energy-dominated era, the BB photons lost energy, and their energy per photon was on average proportional to t-1/2. At a microsecond, when baryon pair production was stopping, there were ~a billion photons per excess matter baryon, and the average photon energy was roughly the same as the baryon energy (~1000 MeV). Now there is the same ~billion ratio, but the average photon energy is well below 0.01 eV. Also the photon energy is now decreasing more rapidly than t-1/2.
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Matter/antimatter, mass questions

 Quote by mathman The total mass, including energy, was the same. Annihilation converts mass to energy (E=mc2).
This is incorrect. BillSaltLake has it right.
 P: 2,456 Correct, energy is not conserved. However, you can isolate small flat region where it is conserved. In the case energy was the same after the annihilation. What had happened to the rest mass is much more complicated story. Photons dont have rest mass, other particles (say, protons) have it, but it is not clear what is it. If we look at photon at whole it is one story, if we look at it as bound quark system we get different number, if we go back to hot vacuum without Higgs condensate we get the 3rd number - probably 0.
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 Quote by Dmitry67 Correct, energy is not conserved. However, you can isolate small flat region where it is conserved. In the case energy was the same after the annihilation. What had happened to the rest mass is much more complicated story. Photons dont have rest mass, other particles (say, protons) have it, but it is not clear what is it. If we look at photon at whole it is one story, if we look at it as bound quark system we get different number, if we go back to hot vacuum without Higgs condensate we get the 3rd number - probably 0.
Well, at the time that there was a lot of anti-matter around, the rest mass was pretty much irrelevant. The particles themselves typically had a lot more kinetic energy than rest mass energy (this is why there was lots of anti-matter still around: if your particle has a lot more kinetic energy than rest mass energy, then collisions will often produce new matter/anti-matter pairs, to replace the ones that annihilate). The expansion cooled the universe until the typical kinetic energy became much smaller than the rest mass energy, and so the normal matter condensed out of the matter/anti-matter mix.

The result of this condensation is that the rest mass energy in the field in question got dumped into radiation. For example, when the temperature dropped much below the mass of the proton, anti-protons disappeared and their energy became radiation. Same thing happened when the temperature dropped much below the mass of the electron.

So you've sort of got two effects going on. Local interactions always conserve energy, in a sense. But globally the expansion was cooling everything down, meaning the universe lost energy as it expanded.
 Sci Advisor P: 6,057 My point was that the annihilation process does not result in any loss of total mass plus energy. The effect of expansion is another matter.
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 Quote by mathman My point was that the annihilation process does not result in any loss of total mass plus energy. The effect of expansion is another matter.
Ah, okay, good point.

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