A Sober Assessment of Cosmology at the New Millennium

Cosmology is the dot com of the sciences. Boom or bust. It is about nothing less than the origin and evolution of the Universe, the all of everything. It is the boldest of enterprises and not for the fainthearted. Cosmologists are the flyboys of astrophysics, and they often live up to all that image conjures up. It is hardly surprising then that words written about cosmology are rarely balanced. They verve to one extreme or the other – from, What we can possibly know about the Universe is a set of measure zero (true); to, We are on the verge of knowing it all (no comment). Once or twice, I have probably been guilty of irrational exuberance when it comes to cosmology (see e.g., Turner 1999). (However, I will defend to my last breath the phrase I coined six years ago, Precision Cosmology.) Even the most serious scientists have been guilty of injecting philosophical arguments when discussing cosmology (cf., the papers of Bondi and Gold (1948) and of Hoyle (1948) on the aesthetic beauty of the steady state cosmology). Before I begin my sober assessment of cosmology, let me discuss the mission statement for the enterprise. As one might expect, this is where the divergence of views often begins. By my definition, cosmology is the scientific quest to understand the most salient features of the Universe in which we live. Since most of the history of the Universe is uninteresting thermal equilibrium, the interesting moments, those rare departures from thermal boringness which lead to notable features today, are a manageable set. Others take a much different view. As an extreme example consider Disney (2000), who defines cosmology as the quest to understand the entire space-time history of the Universe (in response to Disney, see Peebles 2000). Not surprisingly, he concludes that the achievements of cosmologists have been minimal and that cosmology may not be a science at all. While more than two decades ago the relativist Ellis (1975) educated us all on the impossibility of Disney’s goal – we are absolutely limited in our knowledge of the Universe by our past light cone – that has not prevented significant progress toward understanding how the basic features of our portion of the Universe came about as well as their implications for the Universe as a whole.