Even without dark energy, a negatively curved universe expands forever, with gravity negligibly slowing the rate of expansion. With dark energy, the expansion not only continues but accelerates. The ultimate fate of an open universe is either universal heat death, a "Big Freeze" (not to be confused with heat death, despite seemingly similar name interpretation ⁠— ⁠see "§Theories about the end of the universe" below), or a "Big Rip", where the acceleration caused by dark energy eventually becomes so strong that it completely overwhelms the effects of the gravitational, electromagnetic and strong binding forces.