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Why we have 4-5% matter, given most m/anti-m annihilate to photons

  1. May 16, 2013 #1
    For every one billion anti-matters in the universe, there needs only one billion and one matter particle counter-parts to create the universe today.

    Don't we also know that they annihilate into photons?

    Following that logic, can't we assume 2billion/(2billion+1) is the percentage of energies carried by photons in the universe? Photon's can't decay can't they? They do collide and create matter and anti-matter pairs, but what are the chances of that and the percentage of that happening. The universe is also expanding, so as time goes by, the likely hood of colliding photons are decreasing not increasing. Moreover, pairs of matters and anti-matters created this way would be likely be annihilated again, into photons.

    Now, there is a model saying there is 4-5% of visible matters that we understand, and 23% are dark matters while the rest are dark energies we don't understand. Does this 4-5% include photons?
    If yes, why is it so low when we knew matters and anti-matter annihilated each other into photons in the early universe? Isn't energies carried by photos included into this 4-5% of known universe?
    If no, how is it even possible to have 4% given only 1/2,000,000,001 matters didn't annihilated long time ago.
  2. jcsd
  3. May 16, 2013 #2


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    The latest recipe as reported by the AMS experiment is 4.9 % normal matter, 26.8 % dark matter, 68.3 % dark energy.

    The energy density due to photons is much smaller - maybe 0.005 %
  4. May 16, 2013 #3
    I do not know the detailed answer to this, but it is my understanding that the particles in the primordial plasma were believed to be in something close to thermal equilibrium. The matter/energy density numbers reported based on the CMB measurements make this assumption I believe. So if a vast excess of energy was dumped into photons then the photons would quickly distribute this energy amongst the rest of the plasma. You would thus not expect there to be some giant number of primordial photons still flying around today.
  5. May 17, 2013 #4


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    At the time of matter/antimatter-annihilations, photons dominated the energy content of the universe.
    They do not decay, but they lose energy as the universe expands, as this increases the wavelength of those photons. The energy of matter is basically the rest energy of those particles, and stays constant during expansion. The energy density of dark energy is (probably) constant, while matter and photons get spread out more and more.
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