Catching Cosmic Clues

by Joanne Baker

THE AUTHOR HENRY JAMES WROTE THAT "EXPERIENCE IS … A KIND OF HUGE spider-web of the finest silken threads suspended in the chamber of consciousness, and catching every airborne particle in its tissue." Particle astrophysicists are trying to weave their own webs by building vast detectors on Earth and in space that will ensnare cosmic particles and so teach us about the building blocks of the universe.

Thanks to enormous progress in cosmology in recent years, astrophysicists are both pleased and perplexed. On the one hand, they have succeeded in nailing down the universe's mass, geometry, and expansion rate. But on the other, they have discovered that 95% of the stuff of the universe is in two unknown forms that they have named "dark matter" and "dark energy." Only 5% is normal matter: electrons, protons, and neutrons. Pinning down the nature of this missing mass and energy is difficult, because dark matter does not absorb light or interact with normal atoms; the dark energy driving accelerated cosmic expansion is even more intangible. Particle physicists may, however, have the tools to test some ideas. In this special issue devoted to particle astrophysics, a rapidly developing interdisciplinary area, six Perspectives cover not only candidates for dark matter but also the physics of the Big Bang fireball, neutrinos, cosmic rays, and sources of extreme-energy gamma rays such as black holes.

Neutrino physics has leapt ahead in recent years, with measurements of neutrino mass and oscillations between different types, or flavors. The next frontier is neutrino astronomy, capturing neutrinos from sources more distant than the Sun, and vast arrays of detectors are being built under the ice in Antarctica and under the Mediterranean Sea to do this. Neutrinos hardly interact with normal matter at all, but occasionally they do and produce ghostly flashes of light that detectors can catch. If the universe's hidden mass takes the form of other particles, then axions and WIMPs (weakly interacting massive particles) are the prime suspects. Experiments, many hidden below ground to isolate the detectors from other stray particles, have been designed and are being implemented to spot these exotic particles via their recoil off other nuclei. Currently, these detectors are modest in size, but detectors now on the drawing board could weigh as much as a ton.

High-energy particles can also be used for astronomy. Cosmic-ray observatories are nearing the sensitivities required to detect individual sources in the sky, thus testing acceleration physics. Cosmic rays are created by extreme astrophysical sources such as supernova shock waves, gamma-ray bursts, and near black holes. Very-high-energy gamma-ray emission from these sources is already detectable with new telescope arrays and has constrained the physics of particle jets emanating from compact stars and black holes.

Particle astrophysics is an exciting area brimming with promise. As scientists come together to combine their know-how, maybe in the next decade we will find the missing matter, and crown the already remarkable achievements of cosmology.

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