Anti matter question.
17 Dec 2005 20:13:11 UTC
Topic 190399
(moderation:
I posted a request in tyhe cafe for where I should ask questions like...
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For the curious, my current worry is the "prevalence of matter over anti-matter in the universe". My question being, how is it known that, for example, the Virgo cluster is made of matter and not anti-matter? I'd expect anti-Hydrogen to fuse to form anti-Helium and emit "photons" in the same way as matter Hydrogen? etc., etc., I can't see an obvious way of determining at huge distance, the charge on the subatomic particles comprising the observed objects?
Indeed, a slight prevalence of anti-matter, which may have anti mass and feel gravity as a repulsive force, may explain the cosmological constant.
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... that, and it was suggested I ask them here, so I did!
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Anti matter question.
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The best argument against that is that there would have to be a region of space in between us and the Virgo cluster where matter and anti-matter are present in roughtly equal amounts. This should result in a gamma rays from their mutual annilation. No source of gamma ray consistent with such a source has yet been observed.
As far as anyone knows there would be no differences between a matter or an anti-matter universe.
Anti-matter particles have positive energy and mass. Anti-matter does not provide any kind of replusive effect when interacting gravtationally with normal matter.
--- The best argument against
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The best argument against that is that there would have to be a region of space in between us and the Virgo cluster where matter and anti-matter are present in roughtly equal amounts. This should result in a gamma rays from their mutual annilation. No source of gamma ray consistent with such a source has yet been observed.
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But is that not simply an assumption? That assumes that the intergalactic medium is sufficiently populated with particles/anti-particles that their anihilation would produce sufficient photons of appropriate energy to be detected?
If that is, indeed, the current thinking, then my next question would be what evidence is there for particles in intergalactic space? Could it not be empty because the particles that may or may not have been there have long since anihilated?
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Anti-matter particles have positive energy and mass. Anti-matter does not provide any kind of replusive effect when interacting gravtationally with normal matter.
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My thoughts were not along the lines of matter/anti-matter. It was a simple thought that anti-matter may not be gravitationally attracted to other anti-matter, it may be repulsed. I don't know. I have never seen any evidence for the gravitational interactions for large amounts of anti-matter.
I do not suppose for a minute that this is, indeed, the case. I agree. Anti-matter appear to have real mass and thus should behave in the same way as ordinary matter, I just have not seen the evidence.
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RE: That assumes that the
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Good point but if take the discussion beyond just our galazy and the Virgo cluster then things change. If even a small portion of matter in the universe was in fact anti-matter then we should see regions where the separate domains colide. Galaxies collide all the time and large galaxies eat small ones regularly. So far as I am aware none of these collisions show any evidence of matter/anti-natter annilation.
--- Galaxies collide all the
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Galaxies collide all the time and large galaxies eat small ones regularly.
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Very true, but, and it is why I framed my question in the way I did, the observed galactic interactions take place within galactic clusters. You do not see a member of the Virgo cluster interacting with a member of the Fornax cluster for example.
I was wondering if a random distribution of matter/anti-matter in the early universe could have given rise to "patches" of matter and other "patches" of anti-matter in the universe of today. The region between them being empty, or emptied by anihilation.
It seemed a very obvious solution to the "prevelence of matter" issue, but I'd not seen it raised, or debunked.
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RE: I was wondering if a
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The distances between galaxies and galaxy cluster is small enough compared to their sizes that we should see gamma ray fireworks between such domains if they existed. Don't forget that clusters form super cluster and even larger structures which trade matter in much the same way colliding galaxies do. if you assume that the distribution of the separate domains is random then the size of the domains would have be as large or larger than the largest known structure not showing evidence annilation radiation.
We can also look at the cosmic microwave background for evidence. This suggests the the distribution of matter at the time of recombination was uniform to better than a part per thousand. If this is true then the boundries of matter/anti-matter domains should have butted right against each other. Again the annilation radiation from these regions should be observable (red shifted by factor of about 1000) as ultra-violet or soft x-rays. Some fairly through searches in these spectra ranges have turned up nothing so far.
Even if you accept the idea that there is just as much anti-matter as matter in the universe you have replace one problem with another. Towit: How did the matter and anti-matter get separated into isolated domains?
--- even larger structures
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even larger structures which trade matter in much the same way colliding galaxies do.
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Yes, but do they do it fast enough that our instrumentation/observation window, (hey - we've only been looking for a few decades), would detect it? We are not talking about collimated beams of matter/anti-matter being directed towards each other in a detector a few meters across, we are talking about a rareified medium where most particles pass each other like ships in the night and are millions of light years away. Of course, we will get a few direct hits, but then, we also get Gamma Ray bursts, (although this is probably a diversion).
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matter/anti-matter domains should have butted right against each other.
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But we are talking about an extremely high energy environment where everything was moving away from everything else at extreme energies. What mechanism is there that a particle/anti-particle pair that were moving apart at great energy would have anihilated?
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Towit: How did the matter and anti-matter get separated into isolated domains?
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You see, that is not a problem for me and my "model". In any random 3 dimensional distribution of 2 "things", there will be regions where one type of "thing" is more numerous than the other. If you have a situation whereby these things are mutually exclusive, the regions where they are evenly mixed will remove themselves from the final structure.
It is just the areas where there was already a preponderence of a or b that a or b will remain. Thus shortly after a big bang, there would have been a furious period of anihilation, generating vast amounts of energy leaving regions of space that were populated by a majority of a or a majority of b.
We do not, for example, see an even distribution of "stuff", it IS lumpy. Why is that?
The vast majority of the matter in the early universe would have disappeared, but what was left would already have been seperated. It is, thus, not necessary to produce a mechanism for creating these isolated domains, in "the Worley model", they arise naturally, and as a matter of course.
Spread that out, reduce the probabilities of interactions, and allow gravity to get going and the idea of "clumps" of matter and "clumps" of anti-matter would seem to be inevitable.
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RE: But we are talking
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Actually no I am not, I am refering to the time when the tempurature has dropped to about 3,000K. This is far below the temp need to create electron/positron pairs. This would be about 7,000,000 after the time nuclear process stopped. If there had been any spare matter/anti-matter annilation the going on during this period the smooth distribution of matter implied by the cosmic micro-wave raditon would have been distroyed.
It is possible to have regions of pure a and pure b to separate out of a binary mixture randomly. But according to the rules of probability I learned the frequency such regions is a function of Exp[-(linear size)^2/(some constant)]. So there should be many more small domains than large ones.
Sorry but we have definite evidence that the early universe was anything but lumpy. When average over large regions it not particularly lumpy today.
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What your suggesting may have occured before or during the inflationary phase. But that would make your anti-matter domain(s) effectively another universe.
--- the smooth distribution
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the smooth distribution of matter implied by the cosmic micro-wave raditon would have been distroyed.
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I find that an odd statement since the distribution "stuff" in the universe is anything but smooth.
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So there should be many more small domains than large ones.
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And over time, what would happen?
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Sorry but we have definite evidence that the early universe was anything but lumpy. When average over large regions it not particularly lumpy today.
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That surely is, at least, counter intuotive. If the distribution of "stuff", (I deliberately do not say matter), was even, why do we see "stuff" concentrated into very small regions of space seperated by vast expanses of "not stuff"? You/me, stars, galaxies, clusters, super clusters, filaments, whatever, they are tiny in relation to the gaps between them.
Wave upon wave of demented avengers march cheerfully out of obscurity into the dream.
RE: I find that an odd
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I suggest this link WMAP
The various region would spread out and overlap. In this case annilating each other.
The observable universe is more the 10,000,000,000 light years in radius. On a scale of even a 1000th of that the universe is almost perfectly smooth. Gravity has compressed the small differences in the early universe into much smaller regions. But this has not and can not change the large scale average.
Adrian, Antimatter feels
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Adrian,
Antimatter feels gravity just like ordinary matter. It's just got reverse electric charge and something called parity. The masses of antiparticles are the same. It's hard to measure those directly in the lab because antimatter doesn't last long, but you can ask "what if the masses were different?" and come up with all sorts of effects which would be observed as a consequence. Since they're not observed, gravity has to interact with antimatter the same as matter to ... um, it's been ages since I looked, but I think the experimental limits on the difference are on the order of one per billion. Or less. Small enough that no one bothers looking these days, anyway.
Mark has already pointed out the issues of the nonzero density of the intergalactic medium, not to mention galaxies colliding. And then there are cosmic rays.... Basically, we know pretty well that there aren't matter-dominated and antimatter-dominated domains anywhere we can see.
But what about where we can't see? After all, if the universe is only N billion years old, we can only see out to about N billion light years distance, the "Hubble horizon". There are various reasons to believe that the early universe went through a period of "inflation" which basically means there would be stuff outside the cosmological horizon we can't see. There could very well be antimatter-dominated regions out there, and indeed a lot of theorists suspect that for various reasons.
So how would we know? If this stuff is outside the Hubble horizon, we can't watch the pretty gamma rays. But some mechanisms might show up in the history of the early universe. They might even affect the spectrum of gravitational waves produced during the inflationary epoch. LIGO is looking for stuff like that (though not with Einstein@Home since the CPU cost is pretty small), though realistically it will be well into the next decade before we can constrain those particular mechanisms.
Hope this helps,
Ben