Book excerpt: Just what is Einstein's Theory of Relativity? The Big Bang Theory? Curvature of Spacetime? What do astronomers mean when they talk of a 'flat universe'? This approachable and authoritative guide to the cosmos answers these questions, and more. Taking advantage of the distinctive Companion format, readers can use the extensive, cross-referenced background chapters as a fascinating and accessible introduction to the current state of cosmological knowledge - or, they can use the convenient A-Z body of entries as a quick reference to a wide range of terms and concepts. Entries include topics such as: Black Hole; Doppler Effect; Fermi, Enrico; Heat Death of the Universe; Life in the Universe; Olber's Paradox; Quantum Field Theory; Supernova; and much more.