The Main Epoch of Metal Production in the Universe
  Clusters of galaxies allow a direct estimate of the metallicity and metal production yield on the largest scale so far. It is argued that cluster metallicity (about 1/3 solar) should be taken as representative of the low-z universe as a whole. There is now compelling evidence that the bulk of stars not only in cluster ellipticals but also in field ellipticals and bulges formed at high redshifts (z>~ 3). Since such stars account for at least 30% of the baryons now locked into stars, it is argued that at least 30% of stars and metals formed before z=~3, and correspondingly the metallicity of the universe at z=3 is predicted to be about 1/10 solar. 
Alvio Renzini (European Southern Observatory, Garching b. M\"unchen,   Germany)