**2.8. Baryon mass fraction evolution**

This application of SZ effects has been dealt with earlier, for
individual clusters (Sec. 2.2). The
additional difficulty
for examining the evolution is only that of knowing the cosmological
parameters sufficiently well that the investigation of clusters at
different redshifts can work to a constant metric radius, or
alternatively to some other well-defined radius (such as
*r*_{200}).