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Scientists in the late twentieth century are not the first to view galaxy formation as a phenomenon worthy of explanation in terms of the known laws of physics. Already in 1754 Kant regarded the problem as essentially solved. In his Univerlal Natural Hutory and Theory 0/ the H eaven$ he wrote;'If in the immesurable space in which all the suns of the Milky Way have formed themselves, we assume a point around which, through some cause or other, the first formation of nature out of chaoo began, there the largest mass and a body of extraordinary attraction will have arisen which has thereby become capable of compelling all the systems in the process of being formed within an enormous sphere around it, to fall towards itself as their centre, and to build up a system around it on the great scale.... Observation puts this conjecture almost beyond doubt.'More than 200 years later, a similar note of confidence was voiced by Zel'dovicb at an IAU symposium held in Tallin in 1911;'Extrapolating... to the next symposium somewhere in the early eighties one can be pretty sure that the question of the formation of galaxies and clusters will be solved in the next few years.'Perhaps few astronomers today would share Kant's near certainty or feel that Zel'dovich's prophecy has been fulfilled, Many, however, will sympathize with the optimistic olltlook of these two statements.

This book represents the proceedings from the NATO sponsored Advanced Research Workshop entitled'Observational Tests of Inflation'held at the University of Durham, England on the 10th-14th December, 1990. In recent years, the cosmological inflation model has drawn together the worlds of particle physics, theoretical cosmology and observational astronomy. The aim of the workshop was to bring together experts in all of these fields to discuss the current status of the inflation theory and its observational predictions. The simplest inflation model makes clear predictions which are testable by astronomical observation. Foremost is the prediction that the cosmological density parameter, no, should have a value negligibly different from the critical, Einstein-de Sitter value of 00=1. The other main prediction is that the spectrum of primordial density fluctuations should be Gaussian and take the Harrison-Zeldovich form. The prediction that n =l, in patticular, leads to several important consequences o for cosmology. Firstly, there is the apparent contradiction with the limits on baryon density from Big Bang nucleosynthesis which has led to the common conjecture that weakly interacting particles rather than baryons may form the dominant mass constituent of the Universe. Secondly, with n =l, the age of the Universe is uncomfortably short if o the Hubble constant and the ages of the oldest star clusters lie within their currently believed limits.