Solving a cosmic ray conundrum

Astronomers say they have solved a puzzle about the most energetic particles that smash into Earth. Known as ultrahigh-energy cosmic rays, these particles, mostly protons, each pack as much punch as a fast-pitched baseball.

According to a theory first proposed 42 years ago, the particles ought to rarely reach Earth with such high energies because many lose about 20 percent of their energy when they collide with photons from the cosmic microwave background—the Big Bang’s afterglow.

But several years ago, data from the Akeno Giant Air Shower Array near Tokyo suggested that the ultrahigh-energy cosmic rays reaching Earth were as abundant as lower-energy rays. Physicists concocted an assortment of exotic theories to explain why they didn’t see the predicted deficit, known as the GZK cutoff.

Now, another array, the High Resolution Fly’s Eye cosmic ray observatory at the U.S. Army Dugway Proving Ground in Utah, has weighed in on the problem. Nine years of observations with the now shuttered observatory reveal that the predicted deficit does exist, report Charlie Jui of the University of Utah in Salt Lake City and his colleagues in the March 14 Physical Review Letters. Data from the Pierre Auger Observatory in Argentina show a similar depletion. The new studies use a more direct method to measure the cosmic rays than did studies at the Japanese observatory, Jui says.

Jui’s team found ultrahigh-energy cosmic rays that had lost enough energy to suggest they’d come from sources at least 150 million light-years from Earth. The exact source of these cosmic rays remains a mystery, Jui says. Auger observations indicate that the rays come from supermassive black holes at the centers of galaxies (SN: 11/10/07, p. 291). But unpublished data from Fly’s Eye show no such correlation.

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