  Brane-world models where observers are trapped within the thickness of a
3-brane offer novel perspectives on gravitation and cosmology. In this essay, I
would argue that the problem of a late epoch acceleration of the universe is
well explained in the framework of a 4-dimensional de Sitter universe embedded
in a 5-dimensional de Sitter spacetime. While a 5D anti de Sitter space
background is important for studying conformal field theories -- for its role
in the AdS/CFT correspondence -- the existence of a 5-dimensional de Sitter
space is crucial for finding an effective 4D Newton constant that remains
finite and a normalizable zero-mode graviton wave function.
