Primordial Black Holes: Do They Exist and Are They Useful?

Recent developments in the study of primordial black holes (PBHs) are reviewed, with particular emphasis on their formation and evaporation. It is still not clear whether PBHs formed but, if they did, they could provide a unique probe of the early Universe, gravitational collapse, high energy physics and quantum gravity. Indeed their study may place interesting constraints on the physics relevant to these areas even if they never existed. Preface It is a great honour to speak at this meeting because Katsuhiko Sato has been a friend and colleague for 25 years. I first met him when he visited Cambridge in 1980. At the time he was a guest professor at NORDITA and I was a research fellow at the Institute of Astronomy. It was a particularly important period for Katsu because he had just written his famous paper on inflation (arguably the first paper on the topic, although its publication was delayed). I remember our first meeting very vividly: he came to my office and asked to take my photograph because he said I was famous. Nobody had ever told me I was famous before, so I was quite chuffed! Of course, Katsu is now much more famous than me, so I intend to repay the compliment by taking a photograph of him at this meeting! Although we have not written any papers together, Katsu was the first Japanese physicist with whom I interacted and he has had an important influence on my career. This is because meeting him was one of the factors which prompted me to spend a year at Kyoto University in 1982 and I have made many subsequent visits to Japan. On several of these visits I have enjoyed the hospitality of Katsu and his wife, so I would like to express my gratitude for this. Indeed I would say that Katsu's scientific contribution lies not only in his own work but also in the role he has played in encouraging collaboration with Western scientists (many of whom are attending this conference). In my case, this collaboration is particularly strong, since two years ago I married a Japanese lady. It is strange to reflect that this might not have happened without the chain of events triggered by Katsu coming to my office in 1980! I've chosen to talk about primordial black holes, partly because this is one of my own lifelong interests, but also because …