ASTRONOMICAL TESTS OF THE COLD DARK MATTER SCENARIO

An analysis of theories for the origin of large-scale cosmic structure by Peebles & Silk ( 1 988) opened with the question, "Will our feeble minds ever comprehend the evolution of the Universe?" A good question. It would be easy to be pessimistic about this matter. When we understand a phenomenon well, we can make testable predictions. We are confident that we understand the solar system, in good part because we can predict eclipses and cometary apparitions. But our record of predicting what new studies of large-scale structure will find has been abysmal. In addition, the theories have seemed, to "outsiders" not working actively in the field, fantastic in the extreme, highly dependent on physics for which there was no laboratory evidence and often seemingly at odds with existing astronomical knowledge. But I believe that there are strong grounds for optimism as well. First, increasingly, the theories proposed are falsifiable in the classic Popperian sense: They make definite predictions which, if tested and found wanting, provide evidence against the theory. Second, the range and depth of obser­ vational constraints are growing rapidly. The standards by which theories may, in principle, be measured grow stricter by the day. Three examples of new observational constraints come to mind: First, fluctuations in the CBR (Cosmic Background Radiation) have now been reliably detected on the 1 0° angular scale by the COBE satellite, thus normalizing all theories on that scale (Smoot et al 1 992); second, from spectral information, CO BE has set strict limits on the mean Sunyaev-Zeldovich y parameter of the