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.