Open Questions: Matter/Antimatter Asymmetry

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See also: The big bang -- Beyond the standard model -- CP symmetry violation.



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The term "baryogenesis" refers to the process in the very early universe through which baryons came to be. Baryons are heavy fermions (spin-1/2 particles) consisting of three quarks. The primary examples are protons and neutrons. Since the latter make up by far the largest part (by weight) of "ordinary" matter, they are rather important. There's quite a bit of observational evidence for the existence of baryons.

Baryogenesis turns out to be very closely connected to something else which, when you think about it, is rather surprising. This is the apparent fact of matter/antimatter asymmetry -- there seems to be far more matter in the universe than antimatter. I. e., most of the universe seems to be built out of protons and neutrons instead of antiprotons and antineutrons.

This is fairly evident as far as our own galaxy is concerned. Any time matter and antimatter come into contact, the consequence is instant mutual annihilation and the production of high-energy gamma radiation. We simply don't see anything which looks like this happening in our galaxy, even though the material of the galaxy -- stars and interstellar gas -- is in close enough contact that an effect would be quite obvious if any significant amount of antimatter were actually present.

It turns out that even between galaxy clusters and superclusters there could not be any difference in composition as far as matter vs. antimatter is concerned, without violent annihilation events being obvious from distinctive production of gamma radiation. The conclusion is pretty secure that -- at present -- almost all the universe is made of matter rather than antimatter.

But this is rather odd, when you stop to think about it. Exactly why should things be all matter, with no compensating amount of antimatter? This is rather asymmetrical. We tend to expect that it didn't just "happen" this way by chance. The universe "should" have started out very symmetrically between matter and antimatter. The expected preponderance of one over the other should be exceedingly small. It's like flipping a perfect coin trillions of trillions of trillions of times. Mathematically, you would not expect to get exactly the same number of heads as tails, but percentagewise it can be computed that the difference is neglible.

In this case, instead of "heads" or "tails" the coin comes up as a particle of matter or antimatter. Then immediately, in the early universe, all matter particles annihilate with a corresponding antimatter particle. Whichever type is slightly more numerous will be all that remains. It could be matter or it could be antimatter, but one or the other will necessarily disappear. (And the result came to be called "matter", while the loser was named "antimatter".)

So it's not so surprising that the universe appears to consist entirely of matter vs. antimatter. Some asymmetry is inevitable. The surprising thing is that there is far more matter (of whichever type) than should have been expected by chance alone. The preponderance of primordial matter over antimatter is too great to have happened by chance. In other words, the coin that nature tossed trillions of trillions of times was decidedly biased.

The existence of this bias is what ultimately needs to be explained.

This bias can be described in several different ways. One is the asymmetry between matter and antimatter which must have existed before annihilation to account for the remaining matter we observe now. Or you can talk of it in terms of the net baryon number of the universe -- the number of baryons minus the number of antibaryons. (The net baryon number was the same before and after annihilation.) Or you can simply talk about it in terms of "how many" baryons exist in the universe today (which are all in the form of matter rather than antimatter).

This last number is conveniently expressed in terms of the ratio of photons to protons in the present universe. (About 75% by weight of baryonic matter in the universe is hydrogen -- protons. Almost all of the remainder is helium, which is half protons. So for rough estimates, we can just regard all baryonic matter as protons.) The ratio of photons to protons can actually be measured. From the current "temperature" of the universe, i. e. the energy in the cosmic microwave background photons, we know there are about 400 million photons per cubic meter.

The density of baryonic matter (protons) is somewhat harder to estimate. But the extensive research which has been done to compare visible matter in the universe to "dark" matter indicates that the proton density is somewhere between .1 and .3 protons per cubic meter. This is roughly consistent with density estimates based on nucleosynthesis calculations that yield the observed abundances of helium, deuterium, and lithium. The net result is that the possible range of ratios between the numbers of photons and protons in the universe is between 1 billion to 1 and 4 billion to 1. (Another name sometimes used for this ratio is "specific entropy").

The bottom line is, we need to account for this particular quantity, even though it's still a little fuzzy. Is it just a purely arbitrary "initial condition"? If so, it is a very unlikely value. A much more credible initial condition would have the universe begin much more symmetrically, so that the proton/photon ratio would have been vanishingly small, instead of an improbably large one part per billion.

However you prefer to conceptualize this situation, around 1967 the Russian physicist Andrei Sakharov figured out a way to explain it theoretically, even starting from the assumption that the universe was highly symmetrical to begin with.

Specifically, he concluded that three conditions were necessary to account for the improbably large net baryon number of the universe:

  1. Baryon number is not a conserved quantity. It can start out zero but end up positive (as it apparently does).
  2. A type of fundamental symmetry known as CP symmetry must be violated -- even though in most processes it almost never is.
  3. The universe went through a stage of disequilibrium between matter and energy.
Note that none of these conditions by itself is sufficient. They all need to exist in order to explain baryogenesis.

The first of these conditions is what grand unified theories provide -- the nonconservation of baryon number. While this is consistent with the standard model, a mechanism that can actually change baryon number is needed. This is what grand unified theories do in supposing there are interactions which can convert quarks to leptons.

CP symmetry is short for "charge-parity" symmetry. This is composed of two other discrete symmetries. The first is charge conjugation symmetry (C) -- the exchange of particles with antiparticles. (The fact of baryon number not being conserved implies a violation of charge conjugation symmetry, which is basically the symmetry between matter and antimatter.) The second symmetry is parity (P) -- also known as handedness -- the exchange of right and left. This is manifested in the direction of motion of a particle compared with its direction of spin.

The fact that CP symmetry can be violated (though it very seldom is) is a well-established observational fact. The mechanism behind it, however, is still mysterious. Grand unified theories don't shed any light on this puzzle.

There is one more important type of discrete symmetry -- time reversal symmetry (T) -- change in the direction of time. And there is a very fundamental theorem -- the CPT theorem -- which says that when all three symmetries (C, P, and T) are applied to a process, the final result must be the same. Suppose then that any two symmetries are applied to a process and the result is not the same. Then the combination of the two symmetries is violated, and hence the third symmetry as applied to the process must also be violated so that the final result will not change. It follows from the theorem, then, that a violation of CP symmetry implies a violation of time reversal symmetry as well. So another way of stating Sakharov's second condition is that it must be possible to violate time reversal symmetry. In other words, the "arrow of time" must make a distinction between past and future. It has very recently been verified directly that there are processes where this occurs.

There are a number of other interesting things to say about CP symmetry and its violation, which we'll go into elsewhere.

The third of Sakharov's conditions -- disequilibrium -- is necessary so that reactions which proceed in one direction are unable to reverse themselves and become undone by proceeding with equal probability in the other direction. This is basically guaranteed by the conditions of rapid expansion and cooling early in the history of the big bang. What it amounts to is that reactions which took place at a certain stage cannot be undone, because the temperature decreases too rapidly (so that certain reactions become much less likely or impossible), and because the interacting particles move apart rapidly along with the expansion of the universe (so that they become much less likely to interact). In other words, the effects of certain particle reactions become "frozen in" as the universe expands and cools. The effects cannot be undone.

To make a rather long story short, grand unified theories play an important role in baryogenesis or (equivalently) matter/antimatter asymmetry. That role is to provide a mechanism for one of the necessary conditions -- the nonconservation of baryon number.

More detail on CP symmetry violation

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Site indexes

Sites with general resources

The PAMELA experiment - Firenze
Home page at Firenza (Italy) University of the Payload for Antimatter-Matter Exploration and Light-nuclei Astrophysics experiment.
Siegen University Web Page for PAMELA
Home page at the University of Siegen of the Payload for Antimatter-Matter Exploration and Light-nuclei Astrophysics experiment.
The Antimatter Factory at CERN
Description of the laboratory facilities and experiments.

Surveys, overviews, tutorials

Article from Wikipedia.
Antimatter: Mirror of the Universe
An elementary introduction to antimatter.
Where Does Matter Come From?
Overview of the asymmetry problem by Marcelo Gleiser.
The Baryon Asymmetry of the Universe
Good site by Alex Flournoy.
Baryogenesis - Why Are There More Protons Than Antiprotons?
Short explanation by David Brahm, part of the Physics FAQ.
Getting a Grip on Antimatter
August 2004 article from Explains theories and recent experiments related to assymetry of matter and antimatter.
The Reality of Antimatter
September 2003 article from Discusses information on antimatter deduced from astronomical observations.
The Symmetry, or Lack of It, between Matter and Antimatter
Slide presentation given by Helen Quinn at the 2001: A Spacetime Odyssey conference. Nice outline/survey of the history.
Baryon/anti-baryon inhomogeneity and big bang nucleosynthesis
Technical paper by Kevork Abazajian.
Probing the antiworld
October 2005 article from Physics World, by Mike Charlton and Jeffrey Hangst. "Half a century since the discovery of the antiproton, and more than 70 years since that of the positron, researchers at CERN can routinely produce millions of antihydrogen atoms. Mike Charlton and Jeffrey Hangst explain how these remarkable anti-atoms could be our best bet for understanding one of the most fundamental symmetries of nature."
Antimatter performs optical gymnastics
March 2005 article from Physics World, by John Eades. "Lasers have been used for the first time to create antihydrogen, which could allow precise spectroscopic measurements of anti-atoms."
Where did all the antimatter go?
August 2001 article from Physics World, about measurements of sin(2β) in B meson experiments. The results indicate a variance with predictions of the standard model. Article has some good links to related information.

Recommended references: Magazine/journal articles

Sizing up the Electron
Alexandra Witze
Science News, February 12, 2011
Measuring the inner shape of the famous particle could help solve a cosmic mystery.
Cosmic Antimatter
Gregory Tarlé, Simon P. Swordy
Scientific American, April 1998, pp. 36-41
The search for any evidence of antimatter in the universe is described.
The Cosmic Asymmetry between Matter and Antimatter
Frank Wilczek
Scientific American, December 1980, pp. 82-90
Observational evidence suggests that the universe consists almost exclusively of matter rather than antimatter. Grand unified theories of elementary particles, and the violation of CP symmetry may provide an explanation.

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Copyright © 2002 by Charles Daney, All Rights Reserved