Assumption that Anti-Matter Galaxies Would Emit Detectable Gamma Ray Bursts

Cadaei

In the hunt for antimatter galaxies, the tools that we are currently using look for gamma rays that would be emitted due to annihilation events, because everything else about these galaxies is predicted to be the same emission-wise. The hypothesis is that these annihilation events would occur along matter-antimatter boundaries.

However, why would we even assume that these boundaries exist? If there are entire galaxies composed of antimatter, how do we know that there would be these events at all? It could be true that these galaxies exist in virtual isolation from matter due to the vast distances between galaxies.

Furthermore, the very same annihilation events should theoretically occur in a mostly matter galaxies. For example, there are large clouds of antimatter within our own galaxy. Would not the number of annihilation events in a galaxy that were say 90% matter and 10% antimatter be *exactly* the same as in a galaxy that is 90% antimatter and 10% matter? Wouldn't there be no difference in the emitted gamma rays?

Whovian

True, but one would expect that at least one matter-antimatter pair would be close enough together to "try" to merge. There are at least a few pairs of matter-matter galaxies that seem to have some overlap

I'm most probably wrong here, but I can't seem to remember anything about clouds of antimatter in the Milky Way.

Chronos

For an in depth discussion of this issue, see "A Matter-Antimatter Universe?"
http://arxiv.org/abs/astro-ph/9707087. Note that Sheldon Glashow is one of the authors, so the paper has some 'star' power.

mfb

Even without overlap, the space between galaxies is not completely empty. There would be some boundary regions, with matter and antimatter at the same time.

mrspeedybob

Maybe there used to be, but those regions are now empty because the matter-antimatter reactions have already taken place.

The other possibility is that there do exist boundary regions which are beyond our cosmic horizon, 40+Gly away.

twofish-quant

Except that they aren't empty. We can see hydrogen lines.

Also, cosmology gives us the amount of stuff in the universe.

That's possible, but getting the physics to do that is tricky. The problem is that if you have a mix of matter and anti-matter there's not any known way of making them "clump". You also run into "speed of light" problems. The problem is that the important reactions that produce matter/anti-matter differences happened after the universe had inflated and was expanding slowing.

So if one part of the universe was "matter" it's hard to tell the rest of the universe that it should also be "matter".

Right now the assumption is that there is some asymmetry that we don't know about in high energy particle reactions. Even though there are only two reactions that we do know about that aren't symmetric, that at least tells us that it's possible.

mfb

Which reactions do you mean? CP-violation was observed both in the kaon and B-meson system, with multiple different observables. At D-mesons, there are first hints for CP-violation, too (with 0 excluded at 3.8 standard deviations).

And what prevents matter and antimatter from (relatively...) nearby galaxies to go into this region again? The global structure of the visible matter/antimatter does not contain isolated regions, it is like a large "network" with some large voids in between (nearly without galaxies - neither matter nor antimatter).

twofish-quant

OK. Now three :-) :-)

The other thing is that large amounts of anti-matter wouldn't produce "bursts". They'd produce this diffuse gamma ray background. Now we do see a gamma ray background, but it's 10^-5 lower than what we'd see if there were matter/anti-matter interactions in the intergalactic media.