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Given that antimatter galaxies are theoretically possible, how would they be distinguishable from regular matter galaxies?

That is, antimatter is equal in atomic weight and all properties, except for the opposite reverse charge of the particles, identical to regular matter. Hence a star composed of antimatter hydrogen would fuse to anti-helium in an analogous way to our own Sun, and it would emit light and radiation at the same wavelengths as any regular matter star and would cause the same gravitational forces for planetary systems to form as in any other star system.

Hence, what would be a telltale sign if you were observing a galaxy made up entirely of antimatter?

Also, is there any evidence for that half of all galaxies are not made of antimatter -- while general theories currently assume that there is an imbalance of matter over antimatter in the universe, then what is the rationale for not assuming that there is in fact an even balance between the two?

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An antigalaxy could have negative gravitational mass in theory - leading the question whether that has been modelled/simulated somewhere –  Tobias Kienzler Nov 4 '13 at 15:08

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You're right - for isolated galaxies, there is no obvious way of discerning whether they are made of matter or antimatter, since we only observe the light from them. But if there are regions of matter and antimatter in the universe, we would expect to see HUGE amounts of radiation from annihilation at the edges of these regions. But we don't. You could also make the case that galaxies are well-separated in space, and there's not much interaction between them. But there are plenty of observed galaxy collisions even in our own small region of the universe, and even annihilation between dust and antidust in the intergalactic medium would (probably) be observable.

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The intergalactic medium is sparse by any terrestrial definition of matter, but given the large volume of it, there's a lot of it. Like Ben said, if there were any region of space dominated by anti-matter, the IGM would have to switch "polarity" as well. The boundary or transition area between matter and anti-matter regions would produce spectaaaaaaaaacular fireworks, which we just don't see anywhere in our visible universe. –  Andrew Jul 11 '11 at 14:44
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Would spectral absorptions give any indication that you're looking at antimatter? ie, does anti-hydrogen have a different absorption line to regular hydrogen? –  Rogue Jul 11 '11 at 16:54
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@Rogue We have yet to measure it directly, but every theory says that anti-Hydrogen should have indistinguishable spectral emissions and absorptions to ordinary Hydrogen (or for any other element/anti-element comparison, for that matter). –  Wedge Jul 12 '11 at 4:18
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Maybe much of the intergalactic dust in these transition regions has already annihilated, and the space between the matter/antimatter regions is getting larger due to the expansion of the universe. –  Carson Myers Jul 12 '11 at 6:01
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An interesting idea would be negative-mass antimatter, such that galaxies consisting out of antimatter would repel ordinary matter, drastically reducing the likelihood of such boundary annihilations... –  Tobias Kienzler Nov 4 '13 at 15:15

Let's assume that the anti-matter galaxy is well isolated from galaxies consisting of ordinary matter (you could assume that at the boundary the annihilation reactions would have proceeded very fast and matter and anti-matter don't come into contact at the time we see the anti-mater galaxy). Then the telltale sign would come from supernova neutrinos, although at present we can only detect such neutrinos from nearby galaxies.

When the gravitational collapse happens, the electrons of Iron atoms are pushed into the Iron nucleus and all the protons get converted to neutrons leading to an immediate burst of electronneutrino emissions.. In case of a star made out of anti-matter you would have a burst of electron anti-neutrino emissions, and this leads to a different detection signal in detectors on Earth. There are other processes that give rise to both neutrinos and anti-neutrinos, but the entire process is not symmetrical w.r.t. interchanging matter with anti-mater.

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That's generally the answer (other than the more observationally significant comment about the IGM). You have to observe some sort of process that's mediated by the weak force, since this is the lowest-energy force that violates C symmetry. –  Jerry Schirmer Sep 9 at 20:05

Because we can only speculate and guess (although they are very educated speculations and guesses) we don't know anything for certain. In the Universe, anything can happen. Anything. Which is why you never rule something out until you've actually ruled it out- if that makes sense. I'm going to go with Tobias' response that Anti-Matter Galaxies could possibly repel Matter Galaxies. We only know Anti-Matter as seen in a lab and on very very very small scales and from the mysterious floating masses we've detected in space. But an entire Anti-Matter Galaxy? How about narrowing that thinking to just an Anti-Matter Planet? Could it be possible that there's a such thing as a Anti-Graviton? (Assuming Gravitons exist, which I am) How would a Anti-Graviton behave? Would that repel a Graviton? The Anti-Gravity of an entire Galaxy could be more then enough to repel regular non-anti-matter that floats in such abondunce in space. Meaning, in my opinion, that Anti-Matter Galaxies are not outside the realm of possibility. They could exist, although us lowly non-anti-matter beings will never be allowed to explore them first hand. Being barred from such experiences like running into a Galaxtic "No Girls Allowed" type "No Non-Matter Allowed" sign posted hundreds if not millions of light years from the border of such anti-matter galaxies. Maybe we could build anti-matter probes or build a Star Trek style Force Feild that repels the anti-Gravitons and later Anti-Matter particles (much like the magnetic fields we use to store anti-particles now) allowing us access to such forbiddon places. Although, if the force field were to deactivate, I believe that the explosion that would insue would be so catostrophic it could possibly consume the entire Galaxy creating a Hiroshima bomb the size of an entire galaxy. That could take out a chunk of the known universe! Perhaps that's what's happening now, that mysterious force called the "Great Attractor" coming from the other side of the universe could be the result of a Anti-Matter Galaxy colliding with a Matter Galaxy. Anyway, to answer your question, no evidence is not evidence. We would expect to see fantastic explosions for when matter collides with antimatter and see the tremendous amount of radiation coming from the direction of those interactions but we don't know what anti-gravitons are like and how they react and it's possible that Anti-galaxies could very well have an impenetrable anti-graviton barrier from regular matter therefore it's possible that there could not be any visible sign from a distance that such galaxies exist. You have to assume that if anti-galaxies exist that they have some form of protection from being destroyed by regular matter. Or, if they did not have some form of natural protection against regular matter they could have all been destroyed by now since the universe has been around for so long. That's my take.

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Note that the graviton is supposed to be its own antiparticle, much like an anti-photon is just... a photon. –  Kyle Aug 19 at 16:13

protected by Qmechanic Aug 19 at 16:08

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