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 ...