d_c = 4 Is the Upper Critical Dimension for the Bak-Sneppen Model

Numerical results are presented indicating d_c = 4 as the upper critical dimension for the Bak-Sneppen evolution model. This finding agrees with previous theoretical arguments, but contradicts a recent Letter [Phys. Rev. Lett. 80, 5746 (1998)] that placed d_c as high as d = 8. In particular, we find that avalanches are compact for all dimensions d≤4 and are fractal for d>4. Under those conditions, scaling arguments predict a d_c = 4, where hyperscaling relations hold for d≤4. Other properties of avalanches, studied for 1≤d≤6, corroborate this result. To this end, an improved numerical algorithm is presented that is based on the equivalent branching process.