This book provides an overview of topics in high energy, particle and gravitational astrophysics, aimed mainly at interested undergraduates and other readers with only a modest science background. Mathematics and equations have been kept to a minimum, emphasizing instead the main concepts by means of everyday examples where possible. I have tried to cover and discuss in some detail all the major areas in these topics where significant advances are being made or are expected in the near future, with discussions of the main theoretical ideas and descriptions of the principal experimental techniques and their results.

Cosmology, particle physics, high energy astrophysics and gravitational physics have, in the last two decades, become increasingly closely meshed, and it has become clear that thinking and experimenting within the isolated confines of each of these disciplines is no longer possible. The multi-channel approach to investigating nature has long been practiced in high energy accelerators involving the strong, the weak and the electromagnetic interactions, whereas astrophysics has long been possible only using electromagnetic signals. This situation, however, is rapidly changing, with the advent of major cosmic-ray, neutrino and gravitational wave observatories for studying cosmic sources, and the building of particle physics experiments using beams and signals of cosmic origin. At the same time, theoretical physics has increasingly concentrated efforts in attempts to unify gravity with the other three forces into an ultimate theory involving all four. The intense activity in these fields is beginning to open new vistas onto the Universe and our understanding of Nature's working on the very small and very large scales. In this book I have sought to convey not only the facts but also the challenges and the excitement in this quest.