Cosmology was once a field of bold speculation and baffling unknowns, but new instrumentation and insights are turning it into a precision science. Where does our knowledge end and uncertainty stretch away?Martin Rees has spent more than sixty years at the centre of this transformation, as a global community of astronomers traced cosmic history back towards the Big Bang, discovered quasars, black holes and neutron stars, and begun to ask whether our observable universe is only one island in a vast multiverse. In Secrets of the Cosmos, Rees tells this story from the inside. Combining lucid explanation with personal recollection, he shows how great debates were settled – and how accidental discoveries, new technologies and dogged human ingenuity opened questions earlier generations could not have imagined.This is also a book about the limits of knowledge. Rees explores the very large, the very small and the very complex: dark matter, dark energy, gravitational waves, alien life, consciousness, artificial intelligence and the future of intelligence itself. For Rees, the vast cosmic perspective does not make life on Earth insignificant. Our planet has existed for 45 million centuries, but this is the first in which one species – ours – holds Earth’s future in its hands, and could jeopardise life’s immense potential.Authoritative, personal and full of wonder, Secrets of the Cosmos asks what we know, how we know it and what we may never know.