 the Universe is flat, rather than closed. The transition from expansion to contraction is caused by introducing negative potential energy, rather than spatial curvature. Furthermore, the cyclic behavior depends in an essential way on having a period of accelerated expansion after the radiation and matter-dominated phases. During the accelerated expansion phase, the Universe approaches a nearly vacuous state, restoring very nearly identical local conditions as existed in the previous cycle prior to the contraction phase. (Steinhardt and Turok 2002: 2) 