Assuming matter and antimatter in the universe are in equal amount, if they would annihilate,would the universe be left filled only with photons, resembling the state of the early universe? Furthermore, is it theoretically possible for these photons to converge at a single point, and if so, could this trigger a new Big Bang-like event?
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Ordinary particles make up about 5% of the universe. The rest is dark matter and dark energy. We don't know enough about these to say whether or not there is anything with which they could annihilate.
As for ordinary matter, something like what you suggest already did occur. In the very early universe, matter and antimatter were present in very nearly equal amounts. There was a 1 part per billion excess of matter. The rest annihilated, leaving the matter we see. The matter-antimatter asymmetry problem
mmesser314
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It's usually problematic to answer a what if question when the "if" is so far from reality.
There a problems: where's the 1/2 of antimatter coming into existence? Are we just randomly flipping 1/2 of it, for a uniform mixture? Then stars and planets explode instantly.
Nebula...idk, what is the mean free path of interstellar gas?
Note that electron/positron annihilation is dominated by 2 and 3 photon final states, so they're mostly making gammas. With so many reactions you're going to probe the rarer events, though. Any number of photons is possible.
Baryon/antibaryon annihilation is a little different. You're going straight to mesons, which eventually weak decay through muons to electrons (and positrons), which will also annihilate. It will make lots of neutrinos, though.
The baryons are going to be hot. A neutron star w/ half antineutrons...I am reticent to estimate the temperature, but:
$$ E = 10^{33}{\rm g/mol}\times N_A \times m_Nc^2 $$
is a big number. Around a kilo FOE.
JEB
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