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How do we know that anti-matter has disappeared and not just in a far off galaxy?

  1. Nov 18, 2009 #1
    Is it possible that a far off galaxies that we are unable to reach are made up of anti-matter? How do we know that they are made up of matter?
     
  2. jcsd
  3. Nov 18, 2009 #2

    mathman

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    We can't be certain. However cosmic rays (mostly high energy protons) travel throughout intergalactic space. If there was any significant anti-matter around, the annhilation radiation would be observed from interactions.
     
  4. Nov 19, 2009 #3

    Chalnoth

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    We're pretty darned sure this can't be the case. As mathman noted, if this were the case we would see a lot of matter/anti-matter annihilations. We have looked for these, by the way, and don't see them.

    But perhaps even more compelling is the existence and smoothness of the cosmic microwave background: this stuff was emitted when our universe cooled from a plasma. At the time, our universe was uniform to one part in 100,000. It was almost perfectly smooth. Because of this, there just wasn't anywhere for the anti-matter to "hide" from the matter to avoid annihilation. So there really can't be much of any anti-matter left around.
     
  5. Nov 19, 2009 #4

    Vanadium 50

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    The lack of annihilation radiation, like mathman points out, provides a powerful constraint. Any antigalaxies must be very far away indeed. However, eventually the annihilation radiation is washed out in the diffuse x-ray background, so there is a limit to this technique. A better method is to launch a particle detector into space and look for anti-helium and anti-iron nuclei in cosmic rays produced in distant anti-galaxies. Only test runs have been made so far, but when the whole experiment is run, the limit will be pushed close to the edge of the visible universe.
     
  6. Nov 19, 2009 #5
    Annihilation rate can be very low now, but not at the CMB era when all matter was spread almost evenly in space. So CMB is a very strong evidence that there were no clouds of antimatter.
     
  7. Nov 19, 2009 #6
    Just to clarify, we don't see matter/anti-matter radiation from distant galaxies. There is however a rather large cloud of anti-matter in the center of the milky way

    http://apod.nasa.gov/apod/ap970501.html

    Also lightning storms on earth appear to produce anti-matter

    http://www.sciencenews.org/view/generic/id/49288/title/Signature_of_antimatter_detected_in_lightning
     
  8. Nov 19, 2009 #7

    Chalnoth

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    Well, it's not a cloud of anti-matter. It's a cloud where matter-antimatter annihilation is occurring. Though apparently its direct causes aren't well understood, this doesn't surprise me a whole lot because there are a lot of extremely energetic interactions going on in and near the galactic core. One possible explanation might be that it's just a cloud of normal matter caught in a jet of almost equal parts matter and anti-matter.
     
  9. Nov 19, 2009 #8
    Due to the universe being 13.7 billion years old means that we can only see light that has taken that long to reach us. Could matter-antimatter annihilations occur further away than that so the gamma radiation hasnt reach us yet to detect?
     
  10. Nov 19, 2009 #9

    Chalnoth

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    Well, no, not really. It just comes back to the CMB: our early universe was extremely uniform, so that there really wasn't any place for the anti-matter to avoid annihilation early-on. This would have been the same for every place in the universe that stemmed from our inflation event.
     
  11. Dec 3, 2009 #10


    how are we so sure theres no bumpy spots i mean sure our area is pretty calm but if this were the ocean we were talking about you wouldn't see a glassy calm spot and think the whole ocean were glassy calm the universe is 156billion lightyears across http://www.msnbc.msn.com/id/5051818/ and is 15 billion years old assuming we can see 15 billion lightyears in both directions this leaves 126billion lightyears of stuff we cannot see.. so whose to say matter isnt the rare item in another 30 billion lightyear swath of the cosmos i know it says that we can see those objects due to expansion but we only have a really young snapshot of the other side of the universe no idea what it looks like now and definantly no radiation can get here from there anytime soon (soon being the next 100billion years)
     
  12. Dec 3, 2009 #11

    Chalnoth

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    Er, the 156 billion light years is what we can see. The curvature of space-time allows us to see further than you might otherwise expect from naively looking at the age of our universe.

    We know that the universe is smooth out to the edges of our vision. For obvious reasons, we can't say much about what lies beyond. However, what we can say is that the smoothness of the part of the universe that we can observe stems from the physics of what went on at early times, and we expect those same physics to cause many other regions to be smooth as well.
     
  13. Dec 4, 2009 #12

    Ich

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    That's being misreported in the article.
    The radius of the observable universe, as defined in the article, is about 46.5 Gly, not 78 Gly.
    Cornish et al. actually reported a minimum diameter (minimum translational distance) of the whole universe of 78 Gly.
    There are some more glitches in those articles, obviously the press release was not really well formulated.
     
  14. Dec 4, 2009 #13

    Chalnoth

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    Sorry, yeah, silly mistake on my part.
     
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