Can there be stars and planets made out of antimatter?
From a distance it is impossible to tell the difference. It's not observationally ruled out that distance galaxies can be antimatter.
Where the AM from those galaxies met the real matter form ours there would be some gamma ray signal but intergalactic space is so empty that this would be very small.
If it's not observationally ruled out, why is there consensus that the universe is primarily made of matter?
There's no presence of anti-matter in the universe. Any production of antimatter (say in the proton proton chain) usually interacts with regular matter and produces gamma ray energy, annihilating both particles.
Obviously there aren't any anti-matter objects nearby but if you had a distant galaxy composed of antimatter it wouldn't have any matter to react with.
There would be a matter-antimatter front where interactions would occur but the particle density of intergalactic space is only 1atom/few 10s of m^3 so you wouldn't expect much signal.
I can't remember what the observable limits are, but all sky X-ray surveys aren't all that sensitive.
I hear you. It is theoretically possible that if there is no matter within a reasonable gravitationally bound distance, then an antimatter star/galaxy could exist. The only qualm with this is the origin of this anti-matter. At one point very very early in the universe's lifetime there was regular and anti-matter. However it was completely annihilated (the specific term is baryogenesis) as the universe exited the lepton epoch, big bang nucleosynthesis had occurred and the universe had reached the photon-dominated era (photon-dominated vastly due to annihilation). Obviously the reason why matter (baryons) dominated over anti-baryons is still a big mystery as this is peering well before the surface of last scattering and is mostly theoretical, but there's no observational evidence currently for any undisturbed anti-matter in the present universe we all know and love.
I agree that the idea of distant anti-matter galaxies is 'pushing it a bit'.
However claiming that there is no observational evidence for anti-matter, therefore there must be some not-understood event in the early universe to remove all the anti-matter, therefore there can't be any anti-matter galaxies because all the anti-matter was removed - has a certain circularity to it !
To answer the OP more clearly. There is believed to be no large amounts of anti-matter in the universe however it is not observationally ruled out for distant objects and cannot be ruled out for objects beyond the observational universe.
Exactly. We can only see billions of light years, which is a pretty tiny part of a perhaps infinite universe.
One of the lines of evidence against the existence of anti-matter is their absence in cosmic rays. A lot of cosmic rays are from other galaxies, and if a substantial number of them are made out of anti-matter, we would expect at least some antimatter particles. Not one has ever been detected.
It's true, of course, that observations like this do not completely rule out the possibility of antimatter galaxies and such. However, a lot of things are possible, like the existence of undetectable pink bunnies in my bedroom, or the existence of Santa Claus. That doesn't make them plausible; without evidence, things like anti-matter galaxies are wild guesses and nothing more.
We can only deal with the observable universe. The observable universe is our universe.
I dont completely understand. How did we actually prove that the universe is dominated by matter and not anti-matter?
Is it possible that our galaxy is one such distant galaxy made of matter and the rest of the universe is made of anti-matter such that the matter of our galaxy doesn't interact with the anti-matter?
Our galaxy is matter (by definition), where matter and anti-matter meet you get annihilation and a characteristic x-ray signal. At some point in the space between them the inter stellar medium from the matter galaxy would meet the ISM from the anti-matter galaxy and go bang. It's rather like having two continents with different religions - where they meet you would have a thin strip of land where people are constantly at war.
We don't see any such x-ray signal, however the ISM is very thin (very few particles/m^3) in the space between galaxies so it isn't completely ruled out.
The biggest argument against it is that we detect cosmic rays from other (nearby) galaxies and they are matter. We can also see the signal from the early universe when things were very much closer together and presumably any matter-antimatter interaction would be more common.
There are no identifiable x-rays in matter-antimatter annihilation of proton and anti-protons.
Most of the energy goes into pions. Electron positron annihilation is very common after proton beta decay of matter, so is not a good signature. Besides, the photons are gamma-boosted to MeV energies whenever the pair is moving. Pi-zero decay photons are meaningless. Nor is neutrino helicity a good signature. Antiprotons are created in our atmosphere by very high energy cosmic rays. The best signature might be the alpha to antialpha particle ratio.
Galaxy clusters are surrounded by hot ionized gas with a temp of tens of millions of degrees. so it seems reasonable to assume that if an antimatter cluster were next to a matter cluster that there would be a lot of annihilation occurring at the boundary between them.
I did some quick back of the envelope calculations and came up with this.
according to one website I found the escape velocity of a galaxy cluster is around 600 km/s. in addition to that, these clusters are an average of 20 Mparsecs apart so they are moving away from each other at 1500 km/s. (Hubble constant = 75 km/Mparsec). For the particles to reach the boundary so annihilation can occur the particles from each cluster must be moving at over 2000 km/s. the average velocity of room temp molecules is a few hundred m/s so hydrogen (each atom of which is 16 times less massive) at room temp should move at approximately (200 * √16) 1 km/s. To get a velocity 2000 times higher we need a temp 2000^2 times highter. room temp = 300 kelvin so we need 300*2000^2 kelvin=1.2 billion kelvin = 2.15999954 × 10^9 degrees Fahrenheit
I fine the concept of anti-matter interesting. If anti-matter were to exist, won't it be going in the opposite direction of the matter?
1] Uh, what direction is matter moving such that antimatter would move in the opposite direction?
2] Antimatter does exist.
3] Antimatter is exactly the same as matter except that, instead of -ive electrons, it has +ive positrons, and instead of +ive protons, it has -ive antiprotons.
The observable universe is YOUR universe Dave. Not everyone limits themselves like that.
Well, me and serious scientists.
Philosophers may not limit themselves...
It's pretty certain that there aren't any stars or planets made out of anti-matter in nearby galaxies. Anything close by would emit a specific type of radiation which we don't see in general.. But.....
There *do* seem to be objects near the center of the Milky Way that is generating anti-matter. (Also anti-matter isn't that hard to create. Particle accelerators do it all the time. The thing about anti-matter is that easy to make but it's hard for it to accumulate since the moment it touches matter then bzzzzttttt.....)
Now it's likely that there wasn't significant amounts of anti-matter in the early universe. That would cause lots of things to change. We think we more or less know the composition of the early universe from the abundance of helium.
There's another thread that talks about the possibility of distant galaxies being made of anti-matter. I'd be surprised if there is, and I'm pretty sure that we can think of ways of ruling that possibility out directly, but that requires more thinking.
Also the signature of anti-matter is this.....
E=mc^2. When an electron and an anti-electron hit each other and go bzzt..... Then it will produce exactly two packets of energy each with mc^2 of energy (it's 511-keV). So you look for that line and that's a signature for anti-electrons.
Also when things get hot enough it goes into the opposite direction, Two packets of energy hit each other and an electron and anti-electron pop out. It's very common.
What's *hard* is to have reactions that don't balance matter and anti-matter. In fact, we haven't directly observed any particle reactions that don't balance matter and anti-matter so the existence of matter is something of a mystery. Presumably there is some particle reaction at very, very high temperatures in which matter and anti-matter don't quite balance.
One of the theoretical insights on this was provided by Andrei Sakharov....
Basically, his argument is that given that matter exists, and assuming that you don't use the argument that "well the universe just started out with more matter than antimatter, so there" then you *must* have three conditions present at some point in the early universe.
Separate names with a comma.