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Originally Posted by Profpat  Being an accountant something doesn't add up. |
Sorry, I didn't say what I meant to say above. What I meant to say was that the annihilation created two billion photons per particle we have in the universe now.
You can think of this as follows: there were one part in a billion more particles than antiparticles in the early universe. Since particles and antiparticles annihilate, this means that, for every particle we see nowadays there must have been one billion particles and one billion antiparticles that annihilated, creating two billion photons.
N.B. I don't claim the numbers are exact, but the are about right to an order of magnitude.