From the Register, talking about the recent CERN antimatter experiment. I'm not concerned with the experiment itself, but rather this claim

This, in turn, would help us understand how come our universe is asymmetrical, home to vastly more matter than anti-matter.

I'm very skeptical of this claim because it seems to be making a definitive statement about something that appears unmeasurable. Is there legitimate scientific backing to this statement?

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    If they had specified "... how come our observed/observable universe ..." would that be the end of your question? – ChrisW Jun 7 '11 at 13:16
  • @ChrisW -- I don't know. Even of what we can observe, I'm not clear on how one would go about measuring the dark matter content of areas of spaces light years away. We can observe a lot of the universe (from what little i know) but that doesn't automatically imply the ability to accurately measure. My hope is that someone with more knowledge of physics and astronomy can explain it. – Russell Steen Jun 7 '11 at 13:38
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    If you want someone to explain physics and astronomy then perhaps the physics.stackexchange.com site would be a good place to ask that. – ChrisW Jun 7 '11 at 13:41
  • @Russell: Dark matter is observable (albeit indirectly): Dark matter has mass, which means it affects gravitational fields, which is observable in deformations of the orbits of objects we can see. – ESultanik Jun 7 '11 at 14:16
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    Please note: The original claim was about the ratio between matter and antimatter. This should not be confused with dark matter. – Oddthinking Jun 7 '11 at 15:12

Matter/antimatter annihilation produces gamma rays at specific frequencies. That means we can detect regions of space where matter and antimatter are interacting. The logic showing matter antimatter asymmetry thus goes something like this:

  1. Obviously in our local area of the Universe (solar system, Milky Way) consists of matter.
  2. Can’t tell if distant galaxies consist of matter or antimatter– spectra etc. all the same. Universe could consist of domains of matter and antimatter, with net baryon asymmetry.
  3. If matter/antimatter domains are in contact, gamma rays produced at boundary from annihilation. Cosmic gamma ray background indicates domains must be at least ~Gpc in size.
  4. Voids between domains would show up in the CMB.

(list from The Origin of Matter-Antimatter Asymmetry - pdf)

The gamma ray background doesn't reveal domain borders. WMAP does not show voids between large domains.

-- So if there's a lot of antimatter out there, it's not in contact with matter anywhere that we can see, and it's not separated from normal matter by cosmic voids either. That doesn't leave much room for antimatter in the observable universe.

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