Antimatter's role in the architecture of the universe

[Carid]

An airport. A plane to catch. A magazine with an eye-catching cover. A quick flip through the pages then off to the plane. All the elements were there for a deep and critical understanding of the role of antimatter in the formation of the Universe. Or not.

The facts as told by the journalist.

Matter and antimatter formed in equal quantities.
Some of it was annihilated pretty swiftly, but by no means all.
The matter, clinging together by gravity, formed the long tendrils of galaxies, dust clouds, and stars.
The antimatter, shunning gravity (the anti-apple falls upwards), spread out to create the immense voids, pushing the matter into large scale web-like structures.
This was supposed to explain inflation, the large scale structures in the Universe, and the difficulty in locating the anti-matter.
It was also supposed to remove any need for dark matter and dark energy.
Nice shopping list.
Is this yet another journalist on science-inducing steroids or is there any physical basis for what I read in 30 seconds?

 

[Wallace]

The matter, clinging together by gravity, formed the long tendrils of galaxies, dust clouds, and stars.
The antimatter, shunning gravity (the anti-apple falls upwards), spread out to create the immense voids, pushing the matter into large scale web-like structures.

No. Anti-matter behaves exactly the same way as matter in relation to gravity (the apple still falls down). We understand the origin of the web-like structure of the Universe, and it is not caused by anti-matter.

 

[DaveC426913]

- the anti-apple falls upwards

False, as Wallace points out.

 

[noblegas]

Matter and antimatter formed in equal quantities.

There is more matter than antimatter. If there were equal amounts of matter and anti-matter that make up the universe, the universe would be destroyed. (http://www.sciencedaily.com/releases/2009/03/090325132855.htm)

 

[Vanadium 50]

There is more matter than antimatter.

This is not known. It's not excluded that some galactic clusters/superclusters are matter and others are antimatter.

If there were equal amounts of matter and anti-matter that make up the universe, the universe would be destroyed.

This is not correct.

 

[Dmitry67]

This is not known. It's not excluded that some galactic clusters/superclusters are matter and others are antimatter.

There is more matter (p+e-) then antimatter because of CP violation (Sakharov mechanism if I remember correctly). So you suggest that parameters of the Standard Model, like CP violating phase, vary from place to place??

 

[DaveC426913]

There is more matter than antimatter.

This is not known. It's not excluded that some galactic clusters/superclusters are matter and others are antimatter.

Elsewhere on PF it has been argued that there would be some pretty stong evidence if there were other systems made of antimatter, notably:
- a lot of hard radiation at the matter-antimatter interfaces, and
- Earth-based detection of anti-matter cosmic particles.

While it's not ruled out, I think we agreed that it would be pretty difficult to swallow.

If there were equal amounts of matter and anti-matter that make up the universe, the universe would be destroyed.

This is not correct.

OK, maybe not completely destroyed. Any and all contact would result in annihilation and release of gamma radiation. On a grand enough scale with enough mixing, we'd have a universe where there's be lot of gamma radiation and little matter.

 

[Vanadium 50]

There is more matter (p+e-) then antimatter because of CP violation (Sakharov mechanism if I remember correctly). So you suggest that parameters of the Standard Model, like CP violating phase, vary from place to place??

The CP phase in the CKM matrix is too small to generate an asymmetry as large as a 100% matter universe would suggest.

 

[Vanadium 50]

Elsewhere on PF it has been argued that there would be some pretty stong evidence if there were other systems made of antimatter, notably:
- a lot of hard radiation at the matter-antimatter interfaces, and
- Earth-based detection of anti-matter cosmic particles.

The annihilation radiation at the interface gets lost in the diffuse x-ray background if it's sufficiently distant. ~100 Mpc sticks in my mind as a ballpark estimate.

Earth-based detection of anti-matter cosmic ray particles is worthless. You get antiprotons, but these can and are produced in hadronic showers in the upper atmosphere. You need to see antinuclei, and there is presently only one measurement I am aware of that went into space to do this: AMS.

 

[DaveC426913]

The annihilation radiation at the interface gets lost in the diffuse x-ray background if it's sufficiently distant. ~100 Mpc sticks in my mind as a ballpark estimate.

Earth-based detection of anti-matter cosmic ray particles is worthless. You get antiprotons, but these can and are produced in hadronic showers in the upper atmosphere. You need to see antinuclei, and there is presently only one measurement I am aware of that went into space to do this: AMS.

Whether or not it is easy or hard to do, it is my understanding that, generally, scientists are agreed that there are not large quanitites of anti-matter out there, that there are not whole galaxies made of antimatter.

Are you claiming this is not the general understanding?

 

[Dmitry67]

The CP phase in the CKM matrix is too small to generate an asymmetry as large as a 100% matter universe would suggest.

it is too small so it creates only a small disbalance of matter over antimatter (about 10^-8). So it is small but big enough to create the matter we see.

 

[Vanadium 50]

Whether or not it is easy or hard to do, it is my understanding that, generally, scientists are agreed that there are not large quanitites of anti-matter out there, that there are not whole galaxies made of antimatter.

Are you claiming this is not the general understanding?

Yes. I am claiming, in fact, that the whole motivation of the AMS experiment is that we simply don't know if there are distant antimatter galaxies.

Now, I would expect people to be surprised if it was discovered that there were antimatter galaxies. But not shocked.

 

[Astronuc]

I believe the idea behind AMS was to get above Earth's atmosphere in order to remove the antimatter which is generated in the Earth's atmosphere by virtue of high energy proton-proton collisions. This way, the detected antimatter in theory has extra-galactic (outside the Milky Way) origins.

 

[Wallace]

Vanadium, sorry but your hypothesis would violate any kind of Big Bang scenario. The early universe (say the first few hundred thousand years, before and including re-combination) was extremely homogeneous, so it would not have been possible to have isolated, separate matter and anti-matter regions. If some region was made of only antimatter, the boundary between it and the bordering matter part (bearing in mind that the Universe is quite dense at this stage) would have annihilated, leaving a gaping hole and leaving a tell tale sign in the CMB (as observed by WMAP and other probes). The pre-recombination universe was opaque, so the extra inhomogeneous heating that would be produced in such a scenario would have resulted in a markedly different anisotropy power spectrum. That is before you try and explain why CP violation varies with space...

As Astronuc pointed out, the anti-matter thaT various missions have been looking at recently is that made in high energy processes in stars and AGN. This is not 'primordial' anti-matter, but trace amounts produced by astrophysical processes.

You're going to want to provide some references if you want to keep speculating on your personal theory ;)

 

[noblegas]

This is not correct.

If one matter particle were to come into contact with one anti-matter particle , they would annihilate each other instantly. I probably should have said our universe as we know it now would not exist , for radiation rather than matter would make up all or most of the universe just like it did at the early stages of the big bang , when there was equal amounts of matter and anti-matter.

 

[Vanadium 50]

it is too small so it creates only a small disbalance of matter over antimatter (about 10^-8). So it is small but big enough to create the matter we see.

Actually, it's not. I've been trying to trace back to the first papers that point this out, but it seems like it was so obvious, nobody bothers to cite it. As far back as Cohen, Kaplan and Nelson in 1990 it's treated as intuitively obvious. If you'd like to see the statement in print, see e.g. slide 10 of http://www.fnal.gov/orgs/utev/talk-slides/B_movie_fermilab.ps" .

I believe the idea behind AMS was to get above Earth's atmosphere in order to remove the antimatter which is generated in the Earth's atmosphere by virtue of high energy proton-proton collisions. This way, the detected antimatter in theory has extra-galactic (outside the Milky Way) origins.

Yes, AMS can look at extra-galactic antiprotons. But the thing they are really shooting for is to find anti-nuclei. There is no known process in matter galaxies that produce antinuclei, so even one anti-helium or anti-iron nucleus would settle the issue.

If one matter particle were to come into contact with one anti-matter particle , they would annihilate each other instantly.

The annihilation rate is not as large as you think. Annihilation is only one of many ways an electron and positron can interact - for example, an electron and positron can scatter off each other without annihilation. This is, in fact, the way that this evades cosmological problems (and the motivation for Sakharov to write his famous paper). What I don't know is whether this is consistent with baryon acoustic oscillations.

Note that I am not arguing that there are antimatter galaxies: I'm arguing there is a limit at which we are unable to tell.

 

[Astronuc]

Yes, AMS can look at extra-galactic antiprotons. But the thing they are really shooting for is to find anti-nuclei. There is no known process in matter galaxies that produce antinuclei, so even one anti-helium or anti-iron nucleus would settle the issue.

Yes - I should have clarified anti-matter in general, but I believe the orginial goal with AMS-01 was to distinguish between cosmic anti-protons as opposed to local anti-protons, and the bonus would be anti-matter nuclei. It flew briefly on the Shuttle - so didn't have a lot of collection time.

I attended a presentation by Samuel C. C. Ting back in 2005 where he described AMS-02. It was supposed to be installed in ISS, but the last I heard, it's been postponed until 2010 (and that's probably tentative). :grumpy:

 

[Vanadium 50]

AMS-01 didn't have the sensitivity to push much past the limit where we already know from other measurements that everything is matter. Flight time is a big reason for this - the shuttle doesn't stay at orbit for more than a couple of days.

AMS-02 is in big trouble. It's shuttle mission was canceled, and Prof. Ting has been relentlessly lobbying Congress to get them to require NASA to reinstate it. I don't know how successful he has been: the signals seemed mixed when I was paying attention to it. If it manages to fly, though, it will be ~1000x more sensitive than AMS-01. This is about what it takes to definitely settle the issue one way or the other - at least for the visible universe.

 

[Wallace]

Vandium, it's got nothing to do with the level of CP violation, it is about spatial homogeneity. The production of matter and anti-matter in the homogeneous primordial soup, followed by annihilation, happens the same everywhere, unless you postulate that the laws of physics vary from place the place. It doesn't matter what the details of the asymmetry is, what matter is that there is any asymmetry (which leaves matter left over) and that whatever matter is left over is at uniform density everywhere.

We do know that there cannot be anti-matter galaxies because of the evidence from WMAP that the early Universe was almost perfectly homogeneous. What you are saying violates the entire standard model of cosmology.

 

[Vanadium 50]

Remember, I am not arguing that there are antimatter galaxies. I am arguing that there is relatively little direct observational evidence against them at large distances. This was, in fact, the motivation of AMS.

I brought up CP violation to address the suggestion that CKM CP violation was large enough to cause the baryon asymmetry. It's been known for >= 20 years that it's not. One can wiggle out of this in many ways, including distant antimatter galaxies. This isn't my favorite option (I would be interested in seeing if the PMNS matrix could do it, once those parameters are measured) or probably anyone's favorite option.

I am certainly willing to believe that cosmological observations made since the AMS proposal disfavor or even exclude the possibility of distant antimatter galaxies. I brought up BAO in my last message. I would like to see a more quantitative argument, though. In particular, there is a perception on PF that matter and antimatter immediately annihilate, when in fact this is not true. They can interact, thermalize, and not destroy each other, which will increase homogeneity. That means the argument becomes quantitative. I'd like to see that.

 

[George Jones]

Remember, I am not arguing that there are antimatter galaxies. I am arguing that there is relatively little direct observational evidence against them at large distances.

And this is the line taken by a standard undergrad astrophysics text by Carroll and Ostlie, which states

"All but 0.01% of cosmic rays which sample our Galaxy are matter rather than antimatter. The few anitparticles that are detected can be explained as the result of high-energy interactions, such as the creation of proton-antiproton pairs when two energetic protons collide. Rare ultra-high-energy cosmic rays (UHECR) ... Observations of UHECRs reveal no evidence of antimatter. ... This indicates that within the region of space extending at least as far as the Galactic halo, and perhaps as far as the Virgo cluster, some 16 Mpc away, the universe is overwhelmingly composed of matter."

 

[Chalnoth]

Remember, I am not arguing that there are antimatter galaxies. I am arguing that there is relatively little direct observational evidence against them at large distances. This was, in fact, the motivation of AMS.

I brought up CP violation to address the suggestion that CKM CP violation was large enough to cause the baryon asymmetry. It's been known for >= 20 years that it's not. One can wiggle out of this in many ways, including distant antimatter galaxies. This isn't my favorite option (I would be interested in seeing if the PMNS matrix could do it, once those parameters are measured) or probably anyone's favorite option.

I am certainly willing to believe that cosmological observations made since the AMS proposal disfavor or even exclude the possibility of distant antimatter galaxies. I brought up BAO in my last message. I would like to see a more quantitative argument, though. In particular, there is a perception on PF that matter and antimatter immediately annihilate, when in fact this is not true. They can interact, thermalize, and not destroy each other, which will increase homogeneity. That means the argument becomes quantitative. I'd like to see that.

You haven't addressed Wallace's point, and it's a perfectly valid point. I'll try to make it again: during the epoch when the CMB was emitted, our universe was so uniform and smooth that there simply couldn't have been anywhere for the anti-matter to "hide" and not annihilate with normal matter. Because of this observed homogeneity, basically all of the primordial anti-matter must have annihilated significantly before the emission of the CMB.

 

[icarus2]

In particular, there is a perception on PF that matter and antimatter immediately annihilate, when in fact this is not true. They can interact, thermalize, and not destroy each other, which will increase homogeneity.

Hm...
really?

 

[Vanadium 50]

Really. If you look at, for example, a particle accelerator, a beam of positrons and electrons can pass through each other, disturbing the other beam, but with only a few or zero particle pairs annihilating. Of course, eventually there will be annihilation, but it's not instantaneous.

Hence my point that one needs to consider this quantitatively and to calculate what the time scale is for this annihilation.

 

[George Jones]

You haven't addressed Wallace's point, and it's a perfectly valid point. I'll try to make it again: during the epoch when the CMB was emitted, our universe was so uniform and smooth that there simply couldn't have been anywhere for the anti-matter to "hide" and not annihilate with normal matter. Because of this observed homogeneity, basically all of the primordial anti-matter must have annihilated significantly before the emission of the CMB.

What does one mean by "direct observational evidence for the lack of anti-matter?"

The worldlines of galaxies that we observe don't intersect the surface of last scattering in the same region as the worldlines of photons that we see from the surface of last scattering. So, observations of CMB together with the Cosmological Principal is needed.

 

[Chalnoth]

Hm...
really?

Well, only if the temperatures are high enough compared to the rest masses of the particles in question. When the temperature is wasn't too much smaller than the rest mass of the electron, for example, there were lots of electrons and positrons floating around together. Once the temperature dropped below a certain point, however (where the production rate of positrons/electron pairs dropped below the annihilation rate), there was basically a phase change and all of the positrons annihilated with electrons almost instantly.

Of course, this won't happen if your interaction rates are much slower, as is the case, for instance, with dark matter or neutrinos. We expect nearly half of each to be anti-matter.

 

[Chalnoth]

What does one mean by "direct observational evidence for the lack of anti-matter?"

The worldlines of galaxies that we observe don't intersect the surface of last scattering in the same region as the worldlines of photons that we see from the surface of last scattering. So, observations of CMB together with the Cosmological Principal is needed.

Given that the predictions of the behavior of nearby galaxies taken from the properties of the CMB match extremely well, throwing out the cosmological principle is crazy.

 

[Astronuc]

From the AMS website
http://ams.cern.ch/AMS/Description/overview.html#BriefPhys

Please refer to the second paragraph. It would seem to support Vanadium's comments.

The physics goals of the first AMS mission concentrate on the measurement of extraterrestrial anti-protons. In 10 days AMS will record more than 10 times the total number of anti-protons observed to date. The spectrum of these antiprotons will indicate or set limits on the nature of the missing, or dark, matter which is postulated to compose 90% of our universe.

The second mission will improve on these measurements and will also search for complex antimatter nuclei (anti-Helium, anti-Carbon, etc). The observation of such would indicate large regions of our cosmos are made of antimatter. From current observations, there are no indications that such regions should exist and the general feeling is they "ought" not to exist. However, no theory of the origin of the universe during the big bang can satisfactorily explain their nonexistence. Essentially, during the big bang there were equal amounts of matter and antimatter. Now we can't see any significant amounts of antimatter. The AMS experiment will help to answer where it might have gone.

. . . .

Now could it be that there is a difference in understanding between cosmologists/astrophysicists and HEP/particle physicists?

 

[Chalnoth]

Please refer to the second paragraph. It would seem to support Vanadium's comments.
Now could it be that there is a difference in understanding between cosmologists/astrophysicists and HEP/particle physicists?

Their argument is only talking about theories regarding baryogensis. It doesn't discuss any experimental results in cosmology, which pretty effectively rule out the possibility of any such thing already. Though I suppose it's always satisfying to push the error bars even further out, I think the main advantage to detecting heavier nuclei will be for detecting anti-deuterium, which will be produced in very small amounts in dark matter annihilations for most dark matter models.

 

[Astronuc]

Their argument is only talking about theories regarding baryogensis. It doesn't discuss any experimental results in cosmology, which pretty effectively rule out the possibility of any such thing already. Though I suppose it's always satisfying to push the error bars even further out, I think the main advantage to detecting heavier nuclei will be for detecting anti-deuterium, which will be produced in very small amounts in dark matter annihilations for most dark matter models.

OK, but they state, "The second mission will improve on these measurements and will also search for complex antimatter nuclei (anti-Helium, anti-Carbon, etc)." It seems there IS some expectation of finding anti-matter nuclei. In addition, there is also the statement "Essentially, during the big bang there were equal amounts of matter and antimatter," althought they also state, "Now we can't see any significant amounts of antimatter." It seems there is some expectation of finding some amount, not necessarily signficant, of antimatter. Or is it just a hope or fanciful wish?

 

[Chalnoth]

OK, but they state, "The second mission will improve on these measurements and will also search for complex antimatter nuclei (anti-Helium, anti-Carbon, etc)." It seems there IS some expectation of finding anti-matter nuclei. In addition, there is also the statement "Essentially, during the big bang there were equal amounts of matter and antimatter," althought they also state, "Now we can't see any significant amounts of antimatter." It seems there is some expectation of finding some amount, not necessarily signficant, of antimatter. Or is it just a hope or fanciful wish?

I don't think they have any realistic expectation of finding these things. If they do, they don't know much about cosmology (which wouldn't be a terrible surprise, if they're all high-energy people). But it's always good to test these things to death, as we never know when something unexpected is going to crop up.

 

[Astronuc]

I don't think they have any realistic expectation of finding these things. If they do, they don't know much about cosmology (which wouldn't be a terrible surprise, if they're all high-energy people). But it's always good to test these things to death, as we never know when something unexpected is going to crop up.

Even so - from "Shuttle experiment tries to detect antimatter"
http://web.mit.edu/newsoffice/1998/ams-0603.html

Professor Ting hopes to detect heavy antimatter nuclei of anticarbon or antihelium. An antimatter nucleus could signal the presence of a far-off antimatter galaxy because, unlike positrons and antiprotons, these heavy antiparticles are too massive to have resulted from interstellar particle collisions. An antihelium nucleus would prove that some antimatter survived the Big Bang. An anticarbon nucleus could mean that antistars exist, because carbon and heavier elements are created only in stars.

Now it maybe that particle physicists don't know much about cosmology - but some are certainly looking for evidence of the existence of antimatter in galactic quantities (I expect a star of anti-matter doesn't exist in isolation in a matter galaxy).

I expect that many of the AMS team are HEP and not necessarily astrophysicists.

 

[Chalnoth]

Even so - from "Shuttle experiment tries to detect antimatter"
http://web.mit.edu/newsoffice/1998/ams-0603.html

Now it maybe that particle physicists don't know much about cosmology - but some are certainly looking for evidence of the existence of antimatter in galactic quantities (I expect a star of anti-matter doesn't exist in isolation in a matter galaxy).

One thing you've got to realize is that physicists look for things they don't expect to find all the time.

 

[Carid]

Three pages of bright and vigorous debate about antimatter and the fact that cosmologists, astrophysicists and high energy physicists may not be singing from the same hymnbook. And all because I couldn't finish reading a popular science rag at the airport. Sweet!

 

[Chalnoth]

Three pages of bright and vigorous debate about antimatter and the fact that cosmologists, astrophysicists and high energy physicists may not be singing from the same hymnbook. And all because I couldn't finish reading a popular science rag at the airport. Sweet!

Er, well, scientists in general tend to disagree on all sorts of things. But if you press most any physicist who knows anything about the relevant fields (there are always a few fringe exceptions), I can guarantee that you'll find nearly all of them agreeing here that actual detection of a significant number of helium and heavier anti-matter particles is extremely unlikely.