Cosmic mystery

3 November 1998

A recent paper by Glennys Farrar of Rutgers and Peter Biermann of the Max Planck Institute for Radio Astronomy in Bonn suggests interesting possibilities for the orbits of cosmic particles.

Cosmic rays, particles arriving from outer space, are generally believed to be the result of cosmic fireworks like supernovae. But this is not the whole story.

The energy spectrum of cosmic rays extends above 1020 eV, more than a million times the energy of CERN’s future LHC proton collider. Quite apart from the difficulty of imagining a mechanism which can produce such astronomical energies, the fact that such energies are seen at all is an enigma.

The universe is filled with cosmic background radiation, the faint echo of the Big Bang, discovered by Penzias and Wilson in 1965. Shortly after this discovery, it was pointed out that cosmic particles gradually lose energy by scattering off these photons. Theorists calculate that because of this continual attrition, no cosmic particle should be able to maintain an energy above about 5×1019 eV.

But some of them do. A handful of ultra-high-energy particles have been recorded which have somehow negotiated this brick wall. Perhaps some new kind of ultra-high-energy cosmic particle ­ “uhecron” ­ is able to shake off the interaction with the cosmic background radiation. Another possibility, put forward by Sidney Coleman and Sheldon Glashow, is that these extreme energies encounter relativistic effects which under ordinary conditions are too small to be noticeable.

The paper by Farrar and Biermann points out other interesting features of these extreme cosmic energies. Normally, galactic and intergalactic magnetic fields make charged particles loop around in tangled orbits so that it is impossible to tell from which direction they have come. However, the higher the energy, the “stiffer” these orbits become, so that the very high energy ones continue to move more or less in their original direction.

Farrar and Biermann point out that the handful of events above 1020 eV appear to come from the direction of quasars, stellar beacons from the dawn of time. If the particles are indeed quasar generated, some additional explanation is still needed for why these signals from the early universe are not eroded by the cosmic background radiation.

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