Acceleration and energy loss time scales as a function of the proton energy. The acceleration time scales are obtained for the values of the upstream magnetic field B₁ reported in figure and a downstream magnetic field B₂ = 4B₁. The thick lines correspond to a shock velocity of 2000 km s^-1, the thin lines to a velocity of 3000 km s^-1. The horizontal dotted line shows the estimated age of the Universe, for comparison.