Illustration: Alan Stonebraker

Figure 1: The overall cosmic-ray spectrum showing the average slope of the spectrum over three important intervals: below the “knee” at $1015eV$, from the “knee” to the “ankle” just below $1019eV$, and around the predicted GZK cutoff near $1019$$1020eV$. (Lower inset) The high-energy data measured at the Auger telescope (the white triangles indicate upper limits only) [4]. Blue lines are guides to the eye, drawn using the spectral indices from Ref. [4] (Auger) above the ankle, and the spectral index from Ref. [3] (HiRes) below the ankle. The upper inset sketches the interaction between a cosmic-ray particle (proton or neutron) and a photon, $γ$, in the cosmic microwave background that results in the photoproduction of secondary $π$ mesons via a $Δ$ resonance.