Anti-Matter Galaxies

Aspchizo

As far as I know, matter and anti-matter are only different in charge and spin. Therefore a star undergoing fusion reactions with anti-matter should give off the same light spectrum we see with fusion reactions in a star comprised of matter, so it would show the same absorption lines.

Would we be able to tell the difference between light that comes from a star/galaxy that is made of anti-matter as opposed to one that is made of matter?

cwilkins

I don't think you can distinguish the two through emission spectra. However, I believe the decay of positrons--either by interaction with normal matter or by some other means--has a unique gamma ray signature. So, you'd probably have to see a collision between the star and some other large normal matter object to be able to tell.

Drakkith

By the visible light emitted itself, no. However we would see the gamma ray emissions from the annihilation of normal matter with this antimatter, as all of space is filled with a low density gas of hydrogen, protons, electrons, etc. The border of this normal gas with the antimatter galaxy would give off radiation we should be able to easily see.

Chronos

There could be subtle clues hidden in the spectrum of an antimatter star, but, we don't have enough antimatter to play with to know yet. Although, if such a star or galaxy happened to come into contact with normal matter, it would be fairly obvious. There would be high energy gamma ray emissions that would at frequencies characteristic of matter - antimatter annihilations.

Bandersnatch

I remember reading a book of some obscure Polish author, with highly misguided ideas about what a big bang is etc., in which he argued that an antimatter star/galaxy would not be detectable as you've described, due to an effect similar to what happens when you pour water on a hot plate. An isolating boundary forms that keeps the bulk of the water drop from touching the plate. Similarly, he argued that a boundary would form where interstellar hydrogen/antihydrogen came in contact, preventing the annihilation reactions from generating enough energy to be detectable.
I can not for the love of me remember the guy's name or the book's title, nor can I repeat his calculations to see whether the energy emitted from the boundary would be high enough to notice. Still, the idea itself seems sensible.

Vanadium 50

Drakkith's argument is correct, out to a few hundred million parsecs. Beyond that, the diffuse x-ray background makes such detection impossible.

Drakkith

But a boundary of what? If it's matter or antimatter then there is no actual boundary, and if it's the gamma rays themselves, then I think we'd see them. Plus I don't think gamma rays react very strongly with interstellar gas since the density is so low. But I'm not sure.

Bandersnatch

The idea was that in the area where two clouds of matter and anti-matter dust collide, the initial annihilation produces enough radiation pressure to push the remaining particles of dust away from the region of annihilation, slowing down the reaction to an undetectable ratio and producing a de facto boundary separating regular matter from anit-matter.

Just to be clear on that, I'm not endorsing the idea in general, and indeed I have no credentials to be able to say anything substantial about it. But this particular objection you've raised seemed not so difficult to circumvent.

Drakkith

I think it's a valid argument IF the radiation pressure is high enough AND not enough gets through for us to see. Since I don't know how to find either of those out then I cannot say anything more.

Aspchizo

So the answer is, as far as we know thus far, there would be no difference in the emmision spectrum. However we could expect to see specific high energy gamma rays from annihilation of matter and anti-matter particles, unless there is a mechanism we do not yet fully understand like one Bandersnatch suggests.

Thanks for the replies. I was just curious, because physicists appear to me to be quite sure that the universe is mostly matter. Yet it makes much more sense for their to be equal amounts of both, a symmetry between the two. Not that the universe always makes sense! Just thought I would explore the possibility.

Drakkith

It's important to realize that if there had been equal amounts of matter and antimatter in the very early universe, right after particle creation stopped, it would all have annihilated, leaving practically nothing to form galaxies, stars, and planets.

Bandersnatch

Yeah, that mechanism I mentioned, supposing for the moment that it's plausible, is hardly enough to support the idea of coexistence of matter and antimatter in our universe.
For one, as Drakkith mentioned, there's a question of how could possibly the two types of particles separate from the primordial soup without annihilating and go on to form large scale structures.
And even if you consider only the now of the universe, for the sake of the argument, that mechanism would only work if everything was close to static. If the two stars/galaxies just sat there calmly forever never getting too close to each other.
Any sort of galactic collision, or a supernova ejecting gas into the interstellar medium, or one star wandering close by another, would trigger violent annihilation. And since these interactions occur all the time, everywhere, with no observable signatures of annihilation, we might just as well discard the hypothesis.

Aspchizo

I don't think that can be said with certainty that it would have been impossible for such a thing to happen but I do agree the chances seem slim.

Drakkith

It's not impossible, and I would indeed expect a very very small percentage of particles to not have annihilated by sheer chance, but the amount of normal matter we see far exceeds what we would expect to see without something like CP violation occurring. Even with current CP violation added in, the amount of matter seen in the universe is far in excess of what it can explain. It only accounts for about one galaxies worth of matter, so we are obviously missing something.

Aspchizo

I saw an experiment where they were trying to determine if Antimatter fell down or up in a gravitational field. They don't have results yet. It got me thinking, we say matter and antimatter have opposite values for everything except mass. What if the mass is somehow different? negative mass or something. What if that causes a different form of curvature in spacetime, and that pushes matter and anti-matter galaxies apart? preventing annihilation from occurring at the borders.

Just a thought.

PAllen

I believe the scientist Bandersnatch is referring to is Hannes Alfven. His book was:

Worlds-Antiworlds: Antimatter in Cosmology (1966)

See: http://en.wikipedia.org/wiki/Hannes_Alfv%C3%A9n

I was quite intrigued with his theories in high school (circa 1970).

Bandersnatch

Hey, that's the book! For some reason I thought the author was Polish.

If anybody wants to have a look at it, it's available from here:
http://archive.org/stream/WorldsAnti...ge/n0/mode/1up