Despite the dreams of science fiction fans everywhere, no antimatter galaxies lurk in the far corners of the universe. So concludes a trio of theorists who calculated the energy that would be emitted when huge domains of matter and antimatter meet and annihilate, then compared the result to the gamma-ray glow that pervades the sky. Their finding, to be reported in the Astrophysical Journal in February, may sound like a victory for conventional wisdom, but it underscores a long-standing mystery: why the big bang displayed such blatant favoritism toward matter.
The universe that sprang from the big bang should have contained equal parts of matter and antimatter. But cosmologists have long known that our cosmic neighborhood is all matter. The favored explanation is that soon after the big bang, a slight asymmetry somehow developed between matter and antimatter, which enabled matter to win out. Another possibility, however, is that the universe started off with equal amounts of matter and antimatter, in separate clumps. When the newborn universe went through a spurt of exponential growth, called inflation, these clumps grew so quickly that they never had time to annihilate completely. If so, the universe today would have huge, separate domains of matter and antimatter.
If these domains are big enough, astronomers could easily have overlooked the gamma rays from matter-antimatter annihilation at their boundaries, says Boston University physicist Andy Cohen. The annihilation would have begun in the early universe, so that the gamma rays would be smeared out and red-shifted to lower energies. Conceivably, this annihilation signal could explain the observed gamma ray background.
So Cohen, Alvaro de Rújula of CERN, and Sheldon Glashow of Harvard University tested the idea by computing the spectrum of diffuse photons from matter-antimatter annihilation in the early universe. The three physicists conclude that even in the most conservative analysis, matter-antimatter annihilation should produce a signal five times as large as the observable diffuse gamma ray background. "It's an awfully big effect," says Glashow.
The University of Chicago's David Schramm says the analysis definitely reinforces the "prior prejudices" of theorists that the antimatter isn't there. It also lengthens the odds for a planned search for antimatter cosmic rays, such as nuclei of anticarbon, coming from distant antigalaxies (Science, 12 January 1996, p. 142). The experiment, led by physicist Sam Ting of the Massachusetts Institute of Technology and CERN, is scheduled to be tested on the space shuttle this May. "We're not exactly saying it's impossible for [Ting] to find antimatter," says Glashow. But "if he finds it, he upsets the whole apple cart."