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Where did the mass go?

  1. Nov 10, 2011 #1
    Briefly into the big bang, matter anti-matter annihilation which lead to an excess of matter that is what is our current universe, left us with roughly 20 billion photon for each proton.

    From what I understand, the rough scenario is that CP violation meant that for every 10 billion matter anti-matter pair, we got an extra proton, which is the left over.

    The 10 billion matter, and 10 billion anti-matter proton then annihilated and produced the 20 billion photon of gamma energy that is now the micro-wave background radiation.

    So, my question is, does this process mean that in theory, had the big bang created 20 billion normal matter proton instead of 10 billion matter and 10 billion anti-matter proton, that the current universe would be 20 billion time more massive, and have 20 billion time more matter?

    I mean where am I wrong? Because it seems that a proton anti-proton annihilation simply makes the mass of the both of them convert into a massless gamma ray, that then cools off for billions of years... Which sounds as a weird way to get rid of mass.

    That also begs the question, how does a gamma ray cool down and loses all that energy?
  2. jcsd
  3. Nov 10, 2011 #2
    The radiation field is not massless. In fact the mass doesn't change during the annihilation.

    By the expansion of the universe. The wavelength has been stretched from gamma to microwave.
  4. Nov 10, 2011 #3
    Ok, so, the universe then has 20 billion time its mass in floating micro-wave photons?

    Or is this mass actually taken into account when they measure the mass of the universe?
  5. Nov 10, 2011 #4
    Yes, the mass density of the universe is equivalent to its energy density and radiation contributes to the energy.
  6. Nov 10, 2011 #5


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    Worth noting that radiation energy constitutes only a negligibly small fraction of the total energy density at the present epoch. This is because, as was mentioned implicitly above, radiation (photons) dilute like 1/a^4, while normal nonrelativistic matter dilutes only like 1/a^3 under the expansion of the universe. This means that as time goes by, photons make up less and less of the total energy density as compared to matter. Conversely, going back in time, there was a point where radiation was actually the dominant constituent (~70,000 years after the bb).
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