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Matter-antimatter imbalance - how do we know?

  1. Nov 5, 2007 #1
    In my thread regarding the telescopic detection of antimatter, it was concluded that we cannot discern anti-matter from matter when observing it via telescope. As a follow-up, I have another question - or rather a few tightly connected questions:

    If we cannot tell the difference when looking at it, how do we know that some of the other galaxies, or maybe even some areas in our own galaxy, are not predominantly made of antimatter (I'm ignoring other possible ingredients such as dark matter)? Has any research been done to check this, at least on the level of a mathematical model? What sort of phenomena would we expect to observe if this was the case, if any?

    And for that matter (pun not intentional), how do we know that there is in fact an imbalance on the universal scale? What evidence do we have of this? Could it not be only a local imbalance, in principle? [Note of clarification: I am not asking how we know that the imbalance exists. Clearly, we are here, so it must exist. What I'm asking is how do we know it's on a universal scale, rather than local. Or rather - how do we know it's the same everywhere?]

    [Please forgive the somewhat speculative nature of this post. I am not trying to propose any new "theories". I merely wish to discuss the current mainstream understanding of this issue.]
    Last edited: Nov 5, 2007
  2. jcsd
  3. Nov 5, 2007 #2
    As far as i know this result is basically an extrapolation of data from our local area. Given that we're pretty sure our local environment is only matter and not its evil twin we know that there will be no M-AM annihilation processes occuring. Thus if we saw vast quantities of energy being released from galaxies as they destroyed themselves via these processes we would know they probably did contain a mix of both. Moreover Its generally concluded that if there was any antimatter (with an infinite lifespan ie not created from virtual pair production) it would have, over the last 13b years or so, found its way to normal matter and destroyed it. So youre right in the sense that there could be isolated pockets of antimatter which, according to that site, would look no different to normal matter, but its very unlikely.

    Hope that answers your question =]
  4. Nov 5, 2007 #3
    Yes, if the matter and antimatter were close together, they would sooner or later annihilate, and we would expect to see the associated energy release. But would it not be reasonable to assume that where such events could occur, they have already occurred, for the most part, a long time ago? Then in the nearer regions of space such events should be very rare today (if they occur at all), whereas in the farther (read: older) regions they may be harder to detect (due to the vast distances involved).

    You say that it is possible that there are isolated pockets of antimatter, but that this is very unlikely. Why is it unlikely? How did we reach this conclusion?

    And I'd like to push this one step further:
    We know that in the early universe there was both matter and antimatter, and they were gradually annihilating each other. I am guessing that we see that energy release as part of the CMBR. And we know, since we exist, that in our region there was a little more matter than antimatter, so in our region the matter "won", and our region became matter-dominated (or M-dominated in short). But is it not possible that other regions had slightly more antimatter back then, and so they became AM-dominated instead, in just the same fashion? I would think that if there are whole clusters of AM galaxies out there, they would tend not to be bothered by neighboring clusters of matter galaxies, since the two are moving away from each other as the universe expands... What evidence do we have against this? Do we have any such evidence, even theoretical?
    Last edited: Nov 5, 2007
  5. Nov 5, 2007 #4
    You are right in saying that the inflationary period would have an effect on M/AM interactions and that is exactly what happened just not in the way you described. In the early opaque universe full of high energy photons and particles in a plasma things were annihilating and recombining in pair production the whole time. Generally the virtual particles would destroy each other soon after creation giving off the same photon they started with but once inflation (of space not in space) hit this changed as space was stretched photons cooled down and moved apart and the short range interactions that goverened these annihilation processes could not span that space. (i MIGHT be confusing this with what happens at an event horizon, its been a while since i did any of this stuff :P)

    Now as for the ratio in the early universe its generally accepted that given we live in a matter (or perhaps antimatter who would know?) universe (as far as we can tell/test) then there would have been a triumph of matter of AM in the early universe. The reason we can assume this is during the period around 300,000 years after the big bang everything was in reasonably close proximity (inflation hit shortly thereafter) and this, i beleive, was ample enough time for any leftover naturally occuring antimatter to have been annihilated, destined to become an object alternating between virtual photons and virtual matter antimatter pairs. Its a good question and its true that it might not all have been used up before the inflation period hit, but given the attraction of M to AM it seems strange that there would be pockets of AM in the early universe seperate from the rest of the matter that was in turn shot off to great distances by inflation. Remeber this stretched everything equally.

    The short answer as far as i know would be to say that it was all used up long before the distances became large enough to overcome the forces pulling them together.
  6. Apr 17, 2011 #5
    Is it not true there have been recent studies that may imply that M and AM are repulsive to one another, and if this is the case would we need a rethink of the whole BB theory?
  7. Apr 18, 2011 #6
    I'm not certain that the discussion is correct. I'll have to research it, but I think you could see a difference if you look at some of the hyperfine transitions. Matter and anti-matter behave in slightly different ways if you put them into a magnetic field, and I think you should be able to see some spectral line differences.



    At the boundary between matter and anti-matter we'd expect to see a specific time of radiation. Specifically a gamma ray line of 511 Kev

    It so happens that we do know that there is something in the center of the milky way that is generating anti-matter.


    Now if there were a galaxy that was anti-matter, we'd expect to see a "glow" around it.
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  8. Apr 18, 2011 #7
    Yes. We are pretty sure that all of the matter-antimatter annihilations happened before the hydrogen and helium formed. If you have large amounts of antimatter after the CMB was formed, then you'd see a "gamma ray glow." If you have large amounts of antimatter on at 3 minutes after the big bang, then all of the hydrogen/helium abundances go funny.



    1) we don't see 511 Kev gamma radiation
    2) see above for the nuclear arguments

    Actually, this would make a nice intro astronomy homework problem :-) :-)
  9. Apr 18, 2011 #8
    There are two different questions:

    1) Suppose the universe had repulsive antimatter. What would it look like?

    People have been thinking about this for the last several decades....


    2) Is antimatter repulsive (or does it behave gravitationally different from normal matter)?

    That's something that is in the process of being measured.....


    It turns out to be tricky to measure.....
    Last edited: Apr 18, 2011
  10. Apr 18, 2011 #9
    Also I like the quote from the antimatter paper

    "The terrible tragedies of science are the horrible murder of beautiful theories by ugly facts."
  11. Apr 18, 2011 #10

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    Atomic Parity Violation arises from the interference of the Z and photon parts of atomic transitions. It is difficult to see in the lab, even though the effect is maximal (the Z coupling to electrons is almost purely axial, and the photon is purely vectorial). It is impossible to see in space - the effect is too small.

    Atomic CP violation, what you would need to distinguish matter from antimatter spectra, is *at minimum* 1000x smaller. It's probably many orders of magnitude smaller - it's second-order weak, it's heavily GIM suppressed.

    Not as much as you might at first think, as the IGM is quite thin. Above a few 100 MPc's, any putative annihilation radiation gets lost in the diffuse x-ray background.

    Yes, but there are very strong indirect arguments. We know photons and matter fall at the same rate to within 2%, so if you had upward falling antimatter, it violates conservation of energy - you could build a perpetual motion machine. Also, you would see an effect in Eotvos-style experiments, because sea antiquarks in the proton would gravitate differently than electrons.
  12. Apr 18, 2011 #11
    You are giving up too easy.....

    Yes but the magnetic fields that you can get in astrophysics is a lot, lot larger than you can get in the lab.

    The reason this comes up is that if parity violation can kick a one solar mass to 200 km/s


    then I'm not convinced that you can't see something. Give me a 10^6 solar mass black hole, 10^16 gauss of magnetic fields, heat everything from anything from 10 K to 10^6K. I'm not convinced that there won't be *some* sign of antimatter.

    But then the density of the IGN increases as you go back in time. Also once you have substantial red-shift then annihilation radiation gets red shifted which means that it will eventually out of the x-ray background.

    Also you start having active galactic nuclei. If you have a quasar pumping jets of positrons into the IGN, you'll see something.

    Sure. But it's a nice thing to do the experiment.
  13. Apr 18, 2011 #12

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    Large agnetic fields don't help. In fact, they make things harder. The problem is that the only source of the weak force is the nucleus, and its very far away from where the electron spends most of its time. Having a large Zeeman shift of both parent and daughter levels doesn't make your signal larger. (Actually, it makes it worse for technical reasons)

    300-500 MPc is the limit, and that gets you to z = 0.1. The universe at z = 0.1 is not very different than it is today. Going further back and your putative source gets even weaker, and gets buried even further in the diffuse x-ray background - i.e. the same AGN's that you are counting on for your signal are generating an enormous background.

    Many experiments are "nice". This one is "nice". I wish we had the resources to do all the nice experiments out there. Often we're faced with the question of whether you use this particular facility to measure X, Y or Z.
  14. Apr 18, 2011 #13
    So if you have lines from highly ionized atoms that can't be observed on earth. That would help, right? The reason I'm wondering about quasar emission lines is that you see a lot of atoms with lots of electrons stripped off.

    I'm been wondering about signal. The thing about quasar forbidden lines is that they are produced by a steady state cascade of electrons. You can have an extremely strong signal from a line with a very weak transition if it turns out that you have a lot of electrons getting tossed in the source level of the line. Also forbidden lines are very sensitive indicators of environment because even tiny changes in transition probability can result in large spectra changes. It's often the case, that you can't see a given transition, but the transition rate changes the electron distribution so that the lines that you can see have very large changes.

    Also with Zeeman shifts. If you have a sufficiently high magnetic field then you start getting higher order quantum effects that shift the location of the line. Something that would be interesting to see if any of those higher order effects are influenced by the neutral current. The reason I'm interesting in this is that when you have 10^16 gauss, you end up with all sorts of weird nuclear processes like neutrinos and electrons coming out through bremsstralung. With huge magnetic fields, I'd be interested in thinking of parity non-conserving processes that are something other than the Z-electron interaction.

    Your point that none of the experiments that we do in the laboratory will give us a signal in space is well taken, but there is are so many physics processes that I find it very difficult to believe that there wouldn't be *any* sign of parity violation.

    I'm thinking about z=6 or z=3000.

    Not sure. It's an enormous background, but the 511 kev line is at specific frequency. Also there is the issue of resolution. The location of the annihilation may be shifted from the background.

    Also, what if the signal comes from behind the AGN background? If we have an anti-matter galaxy, then it's reasonable to assume that it didn't pop into existence suddenly, but that there were anti-matter proto-galaxies, at which point we should see something like the Lyman-alpha forest, only with gamma rays.

    There's also the issues of limits. If there was one galaxy or even one planet that space aliens changed into anti-matter, then it would be tough to spot. However, if we assume that the universe has anti-matter/matter zones that are randomly distributed then things are different. If the size of the zone was 50 kpcs (i.e. just larger than one galaxy), then we ought to see a ton of transitions going back to the CMB. If the size of the zone was half the observable universe then maybe we wouldn't see the glow.

    Something that would be interesting to calculate are the limits of the number of anti-matter zones.

    That's when it's nice to have a scientist that good at politics and writing proposals. A lot of science involves selling the idea that spending money to do X is more important than Y. It also helps to own the telescope. In every astronomy project that I've seen, the group that pays for the telescope gets a fraction of time to do whatever they want with it, and sometimes that group got their money from the estate of a rich millionaire that wanted to talk to God (which is what happened with UTexas).

    There is some similarity between the world of Hollywood and the world of big ticket science. Often you have a director or star that has this pet project that he wants to work on, so he gets a reputation making movies and finally gets the money and pull to work on the project he *really* is interested in. I've seen the same with big-ticket science. Figuring out what experiment gets done is a lot like figuring out what movie gets made.

    On the other hand, I'm more of the "indie film" kind of person. The thing about particle physics is that it's a lot like "Hollywood, big blockbusters." Tons of money, tons of politics, big giant teams. Astrophysics theory is relatively cheap, so you can go out with your digital camera, a few friends and make a movie without that bureaucracy.
  15. Apr 18, 2011 #14
    No it's not true :-)

    It is true that you often have theorists think "what if" so you'll find no shortage of papers thinking about "what if" antimatter is repulsive. Something that you have to be careful as a theorist is to not mistake "what if" with "what is."

    No we wouldn't. The big bang is based on a lot of observations. If it turned out that antimatter theory conflicted with the idea of the big bang then people would be more likely to rethink anti-matter theory than the big bang, because there is less data for how anti-matter behaved.

    If it turned out that anti-matter was repulsive, it would be *really* *really**weird* and so the first thing is to do a lot more experiments to see how anti-matter really behaves.
  16. Apr 19, 2011 #15

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    If you allow for higher z, two bad things happen: your line gets smeared (unless you can spot its parent galaxy; difficult at z = 6 and impossible at z = 3000) and it gets shifted to lower energy where the diffuse x-ray background is even brighter. Also, remember that less than 0.1% of an atom's annihilation energy goes into the 511 keV line.

    Let's look at a simpler problem. We know that our galaxy is emitting positrons by it's 511 keV line. We can look at Centaurus A, which should be emitting even more, and look for it's 511 keV line. We can't see it over the background.

    Now you want to find a smaller signal, farther away.

    Can't be done. There is too much going on in the sky at those wavelengths.
  17. Apr 19, 2011 #16
    I think it depends on what you are looking for. If there is a single galaxy in the universe that mysteriously gets turned into antimatter at some point in the "recent" past, then it might be hard to spot (although I'm not convinced yet that it will be impossible, see below).

    If you have a large fraction of the universe made up of antimatter galaxies then I'm pretty sure that you would see some sort of glow that gets redshifted to something that we can see. In particular, you should see a lot of annihilations in the proto-galaxies, which should stick out like a sore thumb because they happen before the stars are forming. You'd see something like the "Lyman-alpha forest" as the emission lines get shifted. Also, there would be a larger peak at the location of the proton-proton annihilations that would also get redshifted.

    For that matter, if there were substantial anti-matter, then the CMB would be very different, and you'd have impact on galaxy formation. I think part of the reason that people really don't think too much about this problem is that if there were large amounts of antimatter in the universe it would have to have formed near the big bang, at which point there are dozens of things that would go wrong.

    It's not clear to me that it's going to be a smaller signal. Yes, the IGM is thin, but your signal is going to get integrated over a sphere of tens of Kpc's. This should be a pretty simple calculation and I'll run it this evening.
    Last edited: Apr 19, 2011
  18. Apr 20, 2011 #17

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    You can "believe" what you like. I recommend, though, that you look at the literature. The proposal for AMS is a good place to start, because the whole premise of that experiment is that you get sensitivity far beyond where photons get you: the diffiuse x-ray background is fierce.
  19. Apr 20, 2011 #18
    Doing that. No use doing a calculation if someone has already done it.

    Basically, what the literature says is that if there is conventional antimatter in the universe, the pockets of antimatter needs to be either very big....


    In particular, the Bullet Cluster is either all matter or all-antimatter. If it was a mix of matter and anti-matter we would have seen something..... The data appears to rule out anti-matter galaxies. There is a calculation on page 2 that works out that the gamma ray flux from antimatter/matter is considerably higher than the observed X-ray flux from the cluster.

    There other possibility is that you have weird early universe stuff producing stellar size clumps of antimatter


    Curiously enough there are a *lot* of papers trying to figure out "what-if" dark matter had large parts of anti-matter, including some pretty non-conventional anti-matter (anti-supersymmetric particles)

    Last edited: Apr 20, 2011
  20. Apr 21, 2011 #19


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    If you ask me, the existence of the CMB is a slam-dunk for nearly all of the matter in our universe being made up of normal matter and not anti-matter. At the time of emission of the CMB, the entire observable universe was a smooth, nearly homogeneous plasma. It is fundamentally impossible for any anti-matter to survive such a smooth, cooling plasma. Anti-matter will, of course, be produced later through a variety of astrophysical processes, but only in very small amounts.
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