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View Full Version : There's a Lopsided Halo of Antimatter Surrounding the Centre of the Milky Way



Fraser
2008-Jan-09, 10:00 PM
Bring matter and anti-matter together, and you get a potent explosion. Since antimatter is annihilated almost as soon as it forms, you wouldn't think you could find any out there in the Universe. But you'd be wrong. There's a giant cloud of the anti-stuff in the central regions of the Milky Way. Oh, and the [...]

More... (http://feeds.feedburner.com/~r/universetoday/pYdq/~3/214013514/)

01101001
2008-Jan-09, 10:45 PM
More... (http://feeds.feedburner.com/~r/universetoday/pYdq/~3/214013514/)


Perhaps there's a more exotic process going on. Other astronomers have theorized that the shape and position of the antimatter cloud matches the expected distribution of dark matter in the centre of the galaxy. Perhaps dark matter is somehow being annihilated or decaying into other particles - including antimatter.

That would be cool.

iantresman
2008-Jan-12, 12:12 AM
Hannes Alfvén created the Alpha-Centauri medal to be given to the first person who is able to prove whether the star Alpha-Centauri is made of either normal matter or anti-matter.

How do they know that the gamma rays detected are due to matter-anti-matter annihilation, and not some other source?

01101001
2008-Jan-12, 02:58 AM
How do they know that the gamma rays detected are due to matter-anti-matter annihilation, and not some other source?

Yeah, it could be anti-matter-matter annihilation... Oh, wait...

Well, the ESA Integral: three years of insight into the violent cosmos (http://www.esa.int/esaMI/Integral/SEMY146Y3EE_0.html) says this:


Integral is also keeping its promise to dig into the behaviour of ‘antimatter’. When electrons meet their antimatter counterparts (‘positrons’), they annihilate each other and produce a distinctive gamma-ray emission.

So their PR person, at least, seems to think there is something distinctive about some of the gamma-ray emissions Integral has observed.

I'm afraid if I take it farther, I'll have to delve into technical papers that don't interest me.

iantresman
2008-Jan-12, 01:13 PM
So their PR person, at least, seems to think there is something distinctive about some of the gamma-ray emissions Integral has observed.

Papers seem to suggest that a line emission at 511 keV is a "signature of a compact source of annihilation radiation". See:


Early SPI/INTEGRAL measurements of galactic 511 keV line emission from positron annihilation (http://arxiv.org/abs/astro-ph/0309484), P. Jean, et al., arXiv:astro-ph/0309484v1 (2003) & Astron.Astrophys. 407 (2003) L55.

antoniseb
2008-Jan-12, 03:05 PM
So their PR person, at least, seems to think there is something distinctive about some of the gamma-ray emissions Integral has observed.

When an electron and positron annihilate they release two 511 KeV gammas.
The apparent energy can vary a little if the pair are moving relative to us, or are deep in a gravity well. For example these gammas have been measured at about 400 KeV from the surface of a nearby neutron star.

When protons and anti-protons annihilate they give off something a little more variable, but still distinctive if enough are detected.

iantresman
2008-Jan-13, 11:20 PM
When an electron and positron annihilate they release two 511 KeV gammas.
The apparent energy can vary a little if the pair are moving relative to us, or are deep in a gravity well. For example these gammas have been measured at about 400 KeV from the surface of a nearby neutron star.

When protons and anti-protons annihilate they give off something a little more variable, but still distinctive if enough are detected.

Presumably there are more gamma rays detected at around 511 KeV compared to other energies; otherwise it would just suggest a mechanism which produces gamma rays at a range of energies.

Is it not also possible that it could suggest a mechanism that produces antimatter but which is subsequently annihilated.

antoniseb
2008-Jan-13, 11:35 PM
Presumably there are more gamma rays detected at around 511 KeV compared to other energies; otherwise it would just suggest a mechanism which produces gamma rays at a range of energies.
Well, yes. That is what they've observed.


Is it not also possible that it could suggest a mechanism that produces antimatter but which is subsequently annihilated.
Unless I am misunderstanding what you are saying, that IS what they are suggesting. Positrons are created in a sufficiently empty space that they have a long mean free path, and then get annihilated. The size of the lobe is suggestive of the length of the mean free path.