The eminent Austrian-British philosopher Karl Raimund Popper (1902-94) is one of very few modern philosophers who is known by most and whose views have influenced course of science. to his famous demarcation criterion, first stated in 1933, a scientific theory must be falsifiable, that is, result in predictions of such a kind that they, should they disagree with observations, refute theory.1 Popper presented his falsificationist philosophy of science in Logik der Forschung from 1934, but it became generally known only with enlarged English translation of 1959, The logic of scientific discovery. The ideas in this book, and demarcation criterion in particular, played important role in cosmological controversy that raged from 1948 to about 1965 and in which new steady state theory confronted evolution cosmology based on Einstein's general theory of relativity.2 The leading steady state protagonist, Hermann Bondi, was fascinated by Popper's philosophy which he thought proved methodological superiority of steady state model over big bang models. 'The steady-state theory is far more testable than any other, he wrote in 1966, one year after discovery of cosmic microwave background. According to Popper and other philosophers of science, this makes it clearly preferable to alternative theories.3It may be less well known that Popper's views of science continue to play a role in modern astronomy and cosmology, and are widely accepted by practitioners of these sciences.4 Recently these views have become involved in another cosmological controversy, one concerned with multiverse proposal of numerous other universes in addition to one we live in.5 In so-called landscape version defended by Leonard Susskind and some other physicists, multiverse theory is incompatible with Popperian standards of testability. This has caused Susskind to dismiss falsifiability criterion and generally views of Popperazi. On other hand, Lee Smolin and other critics of multiverse cosmology emphasize necessity of Popperian methodology. Referring to Popper, Smolin suggests that scientists have ethical imperative to consider only falsifiable theories as possible explanations of natural phenomena.6In light of how influential Popper's philosophy has been in modern astronomy and cosmology, it is remarkable that he had very little to say about these sciences. Only on a few occasions did he refer to physical cosmology, a topic that does not appear in any of his major publications. While debate over steady state theory involved several philosophers (including Adolf Grunbaum, Milton Munitz, and Rom Harre), Popper remained silent. Yet he had interest in cosmology, which he called the most philosophically important of all sciences.7 From various scattered remarks in his books and papers it appears that he considered cosmology, although philosophically important, a somewhat immature science, and also that he had little confidence in victorious big bang model of universe.We learn more about Popper's attitude to physical cosmology from a letter he wrote me on 10 June 1994, just a few months before he passed away. The letter, covering seven handwritten pages, was a reply to some questions I had asked him in preparation of my book Cosmology and controversy. It is a source of considerable interest, both from perspective of history of cosmology and from perspective of history of philosophy. I can only quote from it now, after it has been deposited at Karl Popper Library in Klagenfurt, Austria,8 and copyright questions have been clarified.In his younger days, so Popper says in letter, he was an ardent admirer of Friedmann's suggestion, a reference to Russian physicist Alexander Friedmann who in 1922 had shown that there are expanding and other dynamical solutions to Einstein's cosmologica! …