Holographic principle and string cosmology

String cosmology is revisited from a cosmological viewpoint of the holographic principle put forward by 't Hooft, and by Fischler and Susskind. It is shown that the holography principle requires the existence of a `graceful exit' mechanism, which renders the universe non-singular by connecting pre- and post-big-bang phases smoothly. It is proven that a flat universe is consistent with the holography principle only if it starts with an absolutely cold and vacuous state and particle entropy is produced during the `graceful exit' period. An open universe can always satisfy the holography principle no matter what the initial state of the universe is.