Horizons of Cosmology: Exploring Worlds Seen and Unseen is the fourth title published in the Templeton Science and Religion Series, in which scientists from a wide range of fields distill their experience and knowledge into brief tours of their respective specialties. In this volume, highly esteemed astrophysicist Joseph Silk explores the vast mysteries and speculations of the field of cosmology in a way that balances an accessible style for the general reader and enough technical detail for advanced students and professionals. Indeed, while the physical laws and origins of the universe can be endlessly complex, even Einstein once mused that they could be explained simply enough to be grasped by nonspecialists. To that end Silk begins by introducing the basic story of the major discoveries in cosmology over the past century—wherein we learned that we live in an expanding universe populated with galaxies and stars. The middle chapters examine a number of contemporary puzzles such as dark matter and dark energy. The last third of the book looks at the human side of cosmology and moves to the more philosophical frontiers of the field, such as concepts of multiverses and time travel—areas of exploration where some crossover into speculative territory becomes unavoidable. In the past century alone, our understanding of the universe has expanded exponentially, and it will be fascinating to see what discoveries the next hundred years hold. Few books will provide such a thorough understanding of where we have been and what might lie ahead as Horizons of Cosmology.