Antimatter Astronomy

In Cosmology by Brian Koberlein16 Comments

In astronomy we study distant galaxies by the light they emit. Just as the stars of a galaxy glow bright from the heat of their fusing cores, so too does much of the gas and dust at different wavelengths. The pattern of wavelengths we observe tells us much about a galaxy, because atoms and molecules emit specific patterns of light. Their optical fingerprint tells us the chemical composition of stars and galaxies, among other things. It’s generally thought that distant galaxies are made of matter, just like our own solar system, but recently it’s been demonstrated that anti-hydrogen emits the same type of light as regular hydrogen. In principle, a galaxy of antimatter would emit the same type of light as a similar galaxy of matter, so how do we know that a distant galaxy really is made of matter? 

The basic difference between matter and antimatter is charge. Atoms of matter are made of positively charged nuclei surrounded by negatively charged electrons, while antimatter consists of negatively charged nuclei surrounded by positively charged positrons (anti-electrons). In all of our interactions, both in the lab and when we’ve sent probes to other planets, things are made of matter. So we can assume that most of the things we see in the Universe are also made of matter.

However, when we create matter from energy in the lab, it is always produced in pairs. We can, for example, create protons in a particle accelerator, but we also create an equal amount of anti-protons. This is due to a symmetry between matter and antimatter, and it leads to a problem in cosmology. In the early Universe, when the intense energy of the big bang produced matter, did it also produce an equal amount of antimatter? If so, why do we see a Universe that’s dominated by matter? The most common explanation is that there is a subtle difference between matter and antimatter. This difference wouldn’t normally be noticed, but on a cosmic scale it means the big bang produced more matter than antimatter.

But suppose the Universe does have an equal amount of matter and antimatter, but early on the two were clumped into different regions. While our corner of the Universe is dominated by matter, perhaps there are distant galaxies or clusters of galaxies that are dominated by antimatter. Since the spectrum of light from matter and antimatter is the same, a distant antimatter galaxy would look the same to us as if it were made of matter. Since we can’t travel to distant galaxies directly to prove their made of matter, how can we be sure antimatter galaxies don’t exist?

One clue comes from the way matter and antimatter interact. Although both behave much the same on their own, when matter and antimatter collide they can annihilate each other to produce intense gamma rays. Although the vast regions between galaxies are mostly empty, they aren’t complete vacuums. Small amounts of gas and dust drift between galaxies, creating an intergalactic wind. If a galaxy were made of antimatter, any small amounts of matter from the intergalactic wind would annihilate with antimatter on the outer edges of the galaxy and produce gamma rays. If some galaxies were matter and some antimatter, we would expect to see gamma ray emissions in the regions between them. We don’t see that. Not between our Milky Way and other nearby galaxies, and not between more distant galaxies. Since our region of space is dominated by matter, we can reasonably assume that other galaxies are matter as well.

It’s still possible that our visible universe just happens to be matter dominated. There may be other regions beyond the visible universe that are dominated by antimatter, and its simply too far away for us to see. That’s one possible solution to the matter-antimatter cosmology problem. But that would be an odd coincidence given the scale of the visible universe.

So there might be distant antimatter galaxies in the Universe, but we can be confident that the galaxies we do see are made of matter just like us.

Comments

  1. If the only difference is charge, then what is the difference between a neutron and an anti-neutron (or any other particles with zero charge)?

    1. Author

      It’s not just electric charge, but other things like strong force charge. Neutrons are made of three quarks, and anti-neutrons are made of three antiquarks.

  2. Neutrons (anti neutrons) have a neutral charge, not a zero charge which is a very important distinction.

  3. OK. That seems to make it clearer.
    Does the old ‘An anti-particle may be regarded as a normal particle with the time sign reversed.’ apply to quarks.
    (my adviser at university was writing software for CERN when the top quark was found – so, a long time since I studied any of this.)

  4. I don’t understand this statement:


    In all of our interactions, both in the lab and when we’ve sent probes to other planets, things are made of matter. So we can assume that most of the things we see in the Universe are also made of matter.

    As far as I can tell, that’s a logical fallacy.. a big one in fact, in the same vein as the original flat earth paradigm.. how can tests here on earth and in our immediate local vicinity justify the presumption that the entire vastness of the universe is identical?

    It seems to me like the best we can do right now is shrug – we need at least some info from other star systems and galaxies before making any assertion at all

    1. Author

      You have to take that quote in context. It seems reasonable to assume, however, we should look at things more closely (which is done in the rest of the post). To use your flat earth analogy. It’s reasonable to assume the Earth is flat, however … and then point out that detailed evidence shows the Earth is round.

  5. “any small amounts of matter from the intergalactic wind would annihilate with antimatter on the outer edges of the galaxy and produce gamma rays”

    what about the Gamma ray halo around our galaxy, or the extragalactic cosmic rays we know so little about, or even those mysterious gamma ray bursts?

  6. Have really no matter/anti-matter annihilations been observed in the universe?
    Anti-matter form, in tiny quantities, by cosmic radiation in Earth’s upper atmosphere or magnetosphere, I’ve heard. I suppose that it is known because it annihilates and emits characteristic gamma rays. But beyond that.

    Is the mass really annihilated, or could it remain as a shadow in the shape of dark matter, undetectable in labs?

  7. What happens it you consider CP-violating processes in distant galaxies? Are they observable? I could imagine that they would be able to tell the difference between matter and antimatter.

  8. Is antimatter a matter of definition? I mean can we define matter to be made of positive and negative matter particles? That way we can drop the antimatter term. This would make some radioactive reactions make sense e.g. positron decay. Surely, the radioactive isotope isn’t really creating antimatter; or is it?

    1. Author

      The names we use aren’t really that important. It’s the process and relation between the two that matters, and that wouldn’t change.

    2. Antimatter is distinctly different from matter when examined at a quantum mechanical level.
      Reactions between particles and antiparticles not only require properties such as energy or charge to be conserved, but other properties which are quantum mechanical in nature and differentiate matter from anti matter, such as baryon and lepton number.
      Gamma radiation produced through matter/antimatter annihilation is a signature for the conservation of baryon and lepton number.

      Then there are the specific physical properties.
      For example neutrons and anti neutrons have the same neutral charge and mass but have opposite magnetic moments of the same magnitude.

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