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Matter/antimatter asymmetry

  1. Oct 8, 2008 #1
    I had a thought the other day regarding this "mystery". First, assume MWI is true (for the sake of the argument). If MWI is true, then all universes that have some probability of existing given the initial conditions of the big bang do exist. Now imagine that the symmetry between matter and anti-matter exists as well. That is, the expected amount of matter in the universe is equal to the expected amount of anti-matter.

    Now assume there is some process that allows for the creation of only matter or anti-matter, with a 50/50 chance of either. Although a vast majority of the universes will have equal amounts of matter and anti-matter, some small percentage will be dominated by either matter or anti-matter.

    In a universe with equal parts matter/anti-matter, life can't exist since they would annihilate and there would be no matter or anti-matter. Since we can only exist in a universe where one is dominated by the other, that is the class of universe we live in. If this were true, asking "why is there so much matter and no anti-matter?" would be meaningless since the only way to ask why is if its true.

    I made two assumptions here.
    1) MWI is true. However, this can be replaced with the anthropic principle. Whether or not those other universes exist, the only way for us to exist is if the universe is dominated by 1 type of matter.

    2) there exists some process that creates either only matter or only anti-matter. This one Im less sure about but I thought of a process that has no immediate problems that I can see. gamma -> 4 anti-neutrinos + 1 electron + 1 proton. Im not sure if this is valid, but it does preserve the lepton, strange, energy, momentum, charge, and spin conservation laws (if it doesn't preserve energy and momentum that adding more photons at the beginning would). Although this does produce anti-matter, it produces it in the form of neutrinos and allows an electron and a proton to be created without their anti-matter counter-parts.

    Im pretty sure my argument is consistent with the assumptions, and I'm pretty confident in the first assumption (MWI or anthropic principle). Is the second assumption correct (for some process, not necessarily the example I gave)? And if it is doesn't this provide a reasonable explanation for why the universe is primarily matter??
  2. jcsd
  3. Oct 15, 2008 #2
    rly? nothing? I figured someone would at least either call me retarded or tell me its been proposed already...
  4. Oct 15, 2008 #3


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    Actually, somebody DID, but I deleted it right-away and drop-kicked that person.


  5. Oct 15, 2008 #4
    1.I see no reason life cannot exist in a universe with equal ammounts of matter and antimatter. You may have some galaxies consisting of matter, some of antimatter. They anihilate only during collisions but such events are rare enough for life to have time to develop.

    2. I don't think the anthropic principle explains anything. Anyway, you cannot use it because of 1. above.
  6. Oct 20, 2008 #5
    1) ok its true that life COULD potentially exist in a universe with equal amounts of matter and anti-matter, but it would be significantly more difficult. If the matter/anti matter were close to one another, there would be an intense amount of radiation due to annihilations that would make life more difficult. If, on the other hand, the matter/anti matter were far apart, then any localized part of the universe would look just like ours, with one dominating the other. Who's to say matter ACTUALLY dominates anti-matter in our universe? We can't see the entire thing. While this could be argued and is really impossible to know, it seems to me like life is much more probable in some area of the universe that is dominated by either matter or anti-matter.

    2) Are you saying the anthropic principle isn't valid? Because if it is, ignoring your first objection, it WOULD explain it. Heres a more concrete example (that I am not supporting, nor do I necessarily believe it):

    Lets say life is only possible if the constants of the universe are exactly what they are in our universe. This WOULD explain why the constants are what they are. The only way for life to exist and to ask the question "why did the constants take these values?" is if the constants DID take those values. If you don't accept this as a valid explanation (in this hypothetical situation that Im not claiming is true), I don't know what else to say to you...
  7. Oct 20, 2008 #6
    haha thanks zapper.
  8. Oct 21, 2008 #7
    I think this postulate alone is enough to justify why there's some matter in the universe. You don't need many worlds really. This is a well known problem. The fact is that what we observe is pretty much all matter. The other fact is that experimentally, matter and anti-matter are created in equal amounts. This is backed up by CPT symmetry. If CPT symmetry is wrong then so is the Lorentz symmetry. So only in places where this Lorentz symmetry is broken can there be any baryogenesis. I'd expect this breaking of symmetry to occur in huge gravitational fields... like at the beginning of the universe where everything was believed to be highly compacted... So a good quantum gravitational theory should predict some form of asymmetry between matter and anti-matter creation at (mega?) high gravity...

    Either that or we're just not looking hard enough for this antimatter which may abound in the universe outside of our field of view...
  9. Oct 21, 2008 #8


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    YOu don't need CPT breaking to have matter/antimatter symmetry breaking. Instead you just need CP breaking (its one of three prerequisites). Thats fine, in principle, we know plenty of processes already in the standard model that break CP (Kaon mixing, etc)

    The problem is that those processes are very small in the early universe, and theres absolutely no way under normal conditions, that you can get the observed abundances.

    More problematic is that you also need baryon number nonconservation in order to generate an antimatter/matter split. We know of a few nonperturbative effects that can satisfy this, but there too they're completely tiny. It seems instead that new physics is required, and presumably this must occur at very early times in the universe unless you wish to get into anthropic explanations.

    Completely open question, and they typically fall into the field of baryogenesis etc
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