Encyclopedia of Astrophysics: The Expanding Universe

Abstract: The expansion of the Universe is the basis of modern cosmology. This chapter outlines the theory behind the expansion of the universe, including the cosmological principle, distances, velocities, and accelerations. We provide basic derivations of the key equations and highlight some interesting features, such as superluminal expansion, how pressure increases gravitational attraction, the subtleties of conservation of energy in the expanding universe, and the existence of cosmological horizons.