From the Register, talking about the recent CERN antimatter experiment. I'm not concerned with the experiment itself, but rather this claim

This, in turn, would help us understand how come our universe is asymmetrical, home to vastly more matter than anti-matter.

I'm very skeptical of this claim because it seems to be making a definitive statement about something that appears unmeasurable. Is there legitimate scientific backing to this statement?

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    If they had specified "... how come our observed/observable universe ..." would that be the end of your question? – ChrisW Jun 7 '11 at 13:16
  • @ChrisW -- I don't know. Even of what we can observe, I'm not clear on how one would go about measuring the dark matter content of areas of spaces light years away. We can observe a lot of the universe (from what little i know) but that doesn't automatically imply the ability to accurately measure. My hope is that someone with more knowledge of physics and astronomy can explain it. – Russell Steen Jun 7 '11 at 13:38
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    If you want someone to explain physics and astronomy then perhaps the physics.stackexchange.com site would be a good place to ask that. – ChrisW Jun 7 '11 at 13:41
  • @Russell: Dark matter is observable (albeit indirectly): Dark matter has mass, which means it affects gravitational fields, which is observable in deformations of the orbits of objects we can see. – ESultanik Jun 7 '11 at 14:16
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    Please note: The original claim was about the ratio between matter and antimatter. This should not be confused with dark matter. – Oddthinking Jun 7 '11 at 15:12

Matter/antimatter annihilation produces gamma rays at specific frequencies. That means we can detect regions of space where matter and antimatter are interacting. The logic showing matter antimatter asymmetry thus goes something like this:

  1. Obviously in our local area of the Universe (solar system, Milky Way) consists of matter.
  2. Can’t tell if distant galaxies consist of matter or antimatter– spectra etc. all the same. Universe could consist of domains of matter and antimatter, with net baryon asymmetry.
  3. If matter/antimatter domains are in contact, gamma rays produced at boundary from annihilation. Cosmic gamma ray background indicates domains must be at least ~Gpc in size.
  4. Voids between domains would show up in the CMB.

(list from The Origin of Matter-Antimatter Asymmetry - pdf)

The gamma ray background doesn't reveal domain borders. WMAP does not show voids between large domains.

-- So if there's a lot of antimatter out there, it's not in contact with matter anywhere that we can see, and it's not separated from normal matter by cosmic voids either. That doesn't leave much room for antimatter in the observable universe.



The answer to your questions is complicated. We know that in the Milky Way, and in the Local Group, matter must dominate antimatter by 10-15 orders of magnitude. We've also put limits on other galaxy clusters that indicate that the ratio is anywhere from about 5.5 to 8 orders of magnitude, depending on the cluster. We don't know whether the dominant component of these clusters is matter or antimatter; we just know that there's a very high asymmetry, and the consensus, applying the cosmological principle, is that it's more likely to be matter that dominates, rather than antimatter. If we could show that the dominant component in those clusters is matter, the answer to your question would be a firm yes.

In terms of techniques: we can measure the ratio of antimatter to matter on a variety of scales, from inside the galaxy (i.e. a few kiloparsecs) to entire galaxy clusters (on the order of tens of megaparsecs) by searching for the signature of matter-antimatter annihilation - namely, photons - or antiparticles themselves. Over the last half century, stringent limits have been placed on this ratio in different locations and regimes. Very low limits are known for our own galaxy, while for clusters far away, we have results that aren't as low but are still quite good - although we can't prove whether the predominant component in the cluster is matter or antimatter.

Galactic antimatter

In the interstellar medium (ISM) between stars, we can try to find pockets of antimatter (Steigman 1976). One simple direct detection method involves observing the polarization of light passing through the ISM and an effect called Faraday rotation, where light interacts with a large-scale magnetic field. Large quantities of antimatter (likely antiprotons and positrons) should leave a characteristic polarization signature on this rotation, but this has not been observed even since the first studies (Gardner & Whiteoak 1963 onwards). Indirect ISM measurements of the products of matter-antimatter annihilation (gamma rays, chiefly, as well as neutrinos) have very strongly constrained upper limits to galactic antimatter fractions:

  • In interstellar clouds: f ~ 10-15
  • In the interstellar medium: f ~ 10-15
  • In the galactic halo: f ~ 10-10

Steigman's review also mentions that only a few compact gamma ray sources have been found, implying a lack of stars made of antimatter, but this is quite out of date, as both Fermi and more modern gamma ray telescopes have found many more high-energy sources (see e.g. TeVCat for a list of high-energy galactic and extragalactic sources). That said, we've identified the nature of these sources (pulsar wind nebulae, high energy binaries, etc.), and antimatter stars can still likely be ruled out.

The upper bound on the fraction in the galaxy placed by Steigman is ~ 10-4, corresponding to about 10 million stars. This assumes that the stars were formed from condense pockets of antimatter, as the lifetime of an antiparticle in the ISM is only about 300 years. The calculation was done based on the expected luminosity from matter-antimatter annihilation as a star passes through gas, and compared to the Milky Way's total gamma ray luminosity.

Extragalactic antimatter

A key method of direct detection for extragalactic sources is to study cosmic rays. If a significant fraction of them are antimatter nuclei, this might be a strike against the idea of such strong matter-antimatter asymmetry. However, at a wide range of energies, we find fairly good limits, ranging from f ~ 10-1 to 10-4 (Steigman 1976, and presumably these have gotten better). Detecting antimatter cosmic rays (e.g. antihelium) would imply that an active galaxy containing predominantly antimatter could be the culprit.

We can study the fraction of antimatter on the scales of galaxy clusters by checking for matter-antimatter annihilation in the intracluster gas between galaxies; we should see gamma rays if there are significant amounts of both matter and antimatter. The ratio of x-ray flux (from hot gas) to gamma-ray flux (from annihilation) can be used to figure this out.

Data from EGRET (Reimer et al. 2003) constrains the ratio of the dominant component (either matter or antimatter) to the rarer component (either antimatter or matter) in a number of prominent clusters (Steigman 2008). For many, e.g. the Virgo Cluster and the Perseus Cluster, the fraction is around ~ 10-8 or lower. An outlier is the Bullet Cluster, which has a comparatively high ~ 10-5.5, but most clusters are well below this.

Now, we can't tell matter galaxies from antimatter galaxies at this scale, but we can tell that on cosmological scales, groups of matter and antimatter must be separated by tens of megaparsecs at minimum, certainly larger than the sizes of individual clusters.

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