hep-ph/9906513 Cosmology of One Extra Dimension with Localized Gravity

We examine the cosmology of the two recently proposed scenarios for a five dimen-sional universe with localized gravity. We find that the scenario with a non-compact fifth dimension is potentially viable, while the scenario which might solve the hierarchy problem predicts a contracting universe, leading to a variety of cosmological problems. ∗Research fellow, Miller Institute for Basic Research in Science. The main theme of the first 20 years of the hierarchy problem has been to modify parti-cle physics around the TeV scale. In the past two years it has become evident that another successful route to solving the hierarchy problem is to modify the nature of gravitational interactions at distances shorter than a millimeter [1, 2, 3]. This modification can be most simply achieved by introducing compact extra dimensions. One recent proposal which at-tracted enormous attention is to lower the fundamental Planck scale M ∗ all the way to the TeV scale, by introducing large extra dimensions [2]. The observed Planck scale is then just an effective scale valid for energies below the mass scale of the Kaluza-Klein (KK) excita-tions. The consequence of this proposal is that the necessary size of the extra dimensions is determined by the KK reduction formul