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DaveDauria
2002-Feb-02, 11:41 PM
Hi all,

Sorry if this question has been answered before. I know I've read that the universe is almost completely made of matter and there's very little anti-matter left. I'm just curious how this was determined. What I mean is what do we detect from distant galaxies that let scientists determine that they're not composed completely of anti-matter? Is it because we've never seen a huge explosion from say an anti-matter galaxy hitting a matter one?

Kaptain K
2002-Feb-03, 12:07 AM
I'll give this a shot.

1) Although space is a better vacuum than we can produce on Earth, it is far from empty.

2) Anytime an electron meets a positron, they annihilate each other. This releases a gamma ray with an energy of 511 mev (proton-antiproton and neutron-antineutron reactions also have their specific signature spectra).
3) If you assume that there exists (somewhere in the observable universe) a galaxy of antimatter, then there would have to be a region where the antimatter ends and normal matter begins. At the interface of the two regions matter and antimatter would be continuously reacting. The result would be the brightest source of gamma rays (with a unique spectrum that would shout "antimatter is here").

4) We find no such signatures.

Therefore, there are no large concentrations of antimatter in the observable universe.

edited for typos



<font size=-1>[ This Message was edited by: Kaptain K on 2002-02-02 19:13 ]</font>

James
2002-Feb-03, 12:19 PM
On 2002-02-02 19:07, Kaptain K wrote:

Therefore, there are no large concentrations of antimatter in the observable universe.

Maybe there is an antimatter galaxy, but it justs moves around behind our backs whenever we look. /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_smile.gif

John Kierein
2002-Feb-03, 01:22 PM
The Compton Gamma Ray Observatory found the tell tale signature of positron electron annihilation above our own galaxy's galactic plane. It's not at all certain that there is really an excess of matter over anti-matter in the universe. The process may be an ongoing low level process with the gammas absorbed or converted back to electrons and positrons when encountering other particles. I disagree that the source need be really bright. (I also remain unconvinced that gamma ray bursters are so distant. They can have an intrinsic Compton Effect Red shift.)