Have we observed antimatter in the early universe?

  • #1

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I have heard before that for every 1 billion antiparticles there are 1 billion and 1 particles of normal matter. Has this been observed directly or just predicted? Have we only observed antimatter through its creation on earth?
 

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  • #2
Nugatory
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I have heard before that for every 1 billion antiparticles there are 1 billion and 1 particles of normal matter. Has this been observed directly or just predicted? Have we only observed antimatter through its creation on earth?
That estimate comes from comparing the total amount of energy in the universe with the total amount of matter. If we assume that matter and antimatter were created in approximately equal amounts at the big bang, and then all the antimatter annihilated with matter to produce energy... we conclude that the universe started out with one billion and one particles for every billion antiparticles (which certainly qualifies as "approximately equal amounts") and then the antiparticles and particles paired off and annihilated leaving behind only the extra one in a billion particle - and these make up all the matter we observe.
 
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  • #3
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Why wouldn't the matter / antimatter production be exactly equal?
And if wasn't exactly equal, why would we presume that it was approximately equal?
 
  • #4
Nugatory
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Why wouldn't the matter / antimatter production be exactly equal?
That is a good question for which, as far as I know, science doesn't have a completely satisfying answer. However, you'll find better answers over in the Cosmology subforum where the people who are up to date on this stuff hang out.
And if wasn't exactly equal, why would we presume that it was approximately equal?
We don't assume it, we infer it from the amount of energy that we observe. If for every particle we find energy sufficient for one billion particle/anti-particle pairs, that leads to the billion-plus-one to billion ratio OP is asking about.
 
  • #5
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But why do we think the energy came from particle/antiparticle annihilation? Why does it make more sense to say there were 2 billion + 1 particles and 2 billion of them annihilated to become energy, instead of saying there was 2 billion +1 units of energy and 1 of them condensed into a particle? Or maybe 3 of them condensed and 2 annihilated, which would make matter/antimatter production 2 to 1. Or really pick any ratio?
 
  • #6
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But why do we think the energy came from particle/antiparticle annihilation? Why does it make more sense to say there were 2 billion + 1 particles and 2 billion of them annihilated to become energy, instead of saying there was 2 billion +1 units of energy and 1 of them condensed into a particle? Or maybe 3 of them condensed and 2 annihilated, which would make matter/antimatter production 2 to 1. Or really pick any ratio?
We have calculated rates of particle and anti-particle creation shortly after the big bang, so we know (very roughly - that "one billion" number is not exactly a rigorously specified quantity) how many particles had to have disappeared in annihilations.

But the real point of the pop-sci claim that OP is making is that only a very small asymmetry is needed to explain the universe that we find ourselves in. "One billion and one for every one billion" is just a striking way of describing just how small that asymmetry need be.
 

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