Brane-world solutions, standard cosmology, and dark radiation

New exact solutions of brane-world cosmology are given. These solutions inc lude an arbitrary constant C, which is determined by the geometry Outside t he brane and which affects the cosmological evolution in the brane-world. I f C is zero, then the standard cosmology governs the brane-world as a low-e nergy effective cosmological theory. However, if C is not zero, then even i n low-energy the brane-world cosmology gives predictions different from the standard one. The difference can be understood as "dark radiation'', which is not real radiation but alters cosmological evolutions. (C) 2000 Publish ed by Elsevier Science B.V. All rights reserved.