Preposterous Universe

 
Auger North

Speaking of massive technological undertakings to explore high-energy particles, the Pierre Auger Observatory has announced that they've chosen the site of their northern-hemisphere location, which will be in Colorado. Auger uses a multi-technique approach to detect ultra-high-energy cosmic rays. There is a mystery about these UHECR's, namely that one experiment (AGASA) claims to observe them at higher energies (> 10²⁰ electron volts) than should be possible. That's because the distribution in the sky looks the same in all directions, which seems to indicate that they are coming from far outside our galaxy (since galactic events should be concentrated in the plane of the Milky Way). But the universe is somewhat opaque to very high-energy cosmic rays; they tend to bump into the low-energy photons of the Cosmic Microwave Background and lose energy themselves. This should put an upper limit on the energy, known as the Greisen-Zatsepin-Kuzmin cutoff. If AGASA is right that they have detected events above the GZK limit (and maybe they're simply mistaken), then something funny is going on, either in the origin of cosmic rays or in the way they interact with ordinary photons.

The southern site for Auger is well underway in Argentina (as shown in the picture). They are presently collecting data, but with the first science results yet to come. Perhaps the northern counterpart will also have cows.

~ posted by Sean Carroll at 4:52 PM ~ link ~ ~ ~