The realization that UHECRs probably come from active galactic nuclei (AGNs) is, says Cronin, "a fundamental discovery."
"When I first proposed that UHECRs are produced by nearby AGNs, people were rather skeptical," he told Sky & Telescope.
Biermann is confident that the link between UHECRs and AGNs will stand the test of time.
The unresolved disparity between HiRes and AGASA has added a new urgency to the UHECR problem, and the tools to resolve it are now under construction.
These so-called ultrahigh-energy cosmic rays (UHECRs) have energies exceeding [10.sup.20] eV--enough to accelerate a single proton to 99.999999999999999999999 percent of the speed of light.
Calculations show that supernovae cannot generate UHECRs, so it makes sense to look for sources among the most conspicuous energy factories in the universe: quasars and gamma-ray bursts billions of light-years away.
Thanks to the GZK cutoff, UHECRs cannot be the result of quasars, gamma-ray bursts, or anything else at cosmological distances.
It's a situation that scientists working on UHECRs want to change.
In the decade since evidence for UHECRs was first announced, theorists have raced ahead of their experimentalist colleagues, hoping to predict where and how such powerful particles are made.
In contrast, UHECRs are moving too fast to be deflected by the Milky Way's magnetic field.
With no obvious connection between UHECRs and particular astronomical objects, researchers have begun to look for more subtle clues to the particles' origins.
"Bottom-up" theories postulate that UHECRs start off as low-energy protons and then are powered up by a natural accelerator.