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Solving Two Mysteries at Once

The portions of universe that we can see appear to consist almost entirely matter, rather than an equal balance of matter and antimatter. At the same time, most of the universe seems to be made of something we can't see at all - dark matter. These two facts are among the outstanding puzzles in physics. A new paper proposes that they may actually be two aspects of the same mystery.

Ever since Einstein penned his famous equation, E=mc^2, physicists have known that it's easy to make matter out of nothing but pure energy, provided that you end up with an equal balance of the normal matter and antimatter. Matter, of course, is the stuff you, I, everything around us, and everything we can see in the universe appears to be made of. Antimatter is like matter's Bizarro World twin. An antimatter proton (antiproton), for example, is the opposite of a proton in every way except mass, which means that if one ever runs into a proton, the two will instantly annihilate in a burst of energy.

Because the universe was created out an energetic blast known as the Big Bang, you might expect that half the universe would be matter and the other half antimatter. If that were true, however, everything in the universe should eventually be destroyed as the two types of matter mixed. Instead, we're left with a surplus of normal matter. That's mystery number one.

In recent years, mystery number two reared it's puzzling head. Much of the universe, it seems, doesn't appear to be made of normal matter at all. Instead, the universe is filled with a material so completely alien to us that we call it simply dark matter (for lack of a better name). We know it exists because it affects the orbit of stars and galaxies, and even the structure of the universe. We have yet to make or capture a bit of dark matter, so beyond realizing it's here, we're pretty much in the dark on dark matter.

One solution, which is being proposed in a paper coming out soon in the journal Physical Review Letters, is that the amount of dark matter we can't see balances the matter we do see.

That's not to say dark matter is antimatter. We don't know much about it, but we can tell it's not simply the opposite of regular matter. Otherwise there would be detectable dark matter stars and planets, as well as occasional dark matter meteorites blasting holes in the Earth.

Instead, it may be that dark matter is only the opposite of matter in one way. Specifically, a particle of dark matter may have a negative baryon number. In case you've never heard of baryon numbers, don't worry; it's just a number that we think should be zero for the universe as a whole, but instead appears to be very large. Regular matter particles like protons have positive baryon numbers. If dark matter had the opposite sign of baryon number, everything would add up to give universe the zero overall baryon number that we would expect from the Big Bang. That would be a very tidy answer that ties up two loose ends at the same time, and makes the existence of the universe slightly less mysterious.

The paper is only theory at this point, but its authors suggest at least one experimental test of their proposal. Very rarely, a bit of matter may run across a bit of dark matter and annihilate to produce a signal that looks a lot like a proton spontaneously self-destructing (i.e. proton decay). The problem is, people have been looking for protons actually self-destructing for a long time now, and no one has seen any sign of it happening yet. So it may turn out, right or wrong, that the theory may never be tested. And theory without experimental confirmation is little more than a just-so story, like the tale of how the leopard got its spots, why giraffe has such a long neck, or anything explained by string theory.

To tell you the truth, though, I enjoy a good just-so story every now and then. But it'll be a lot more enjoyable if we can find a way to turn this particular bit of mythology into science.


  1. Check out the electric universe hypothesis. It answers the questions, it predicts the consequences; actual science in action. Pity astronomers have no understanding of plasma physics, it would save them from having to keep saying: wow, we didn't expect this at all..."


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