Open universe: density of matter is low > gravity is weak > universe expands forever. Closed universe: density of matter is high > gravity will halt the expansion > universe will re-contract. If this happens repeatedly, we will have an oscillating universe. Flat universe: universe contains the so-called critical density of matter > galaxies barely manage to avoid re-collapse. Hydrogen and helium are depleted > no new stars can form. 1 x 1014 years existing stars run out of fuel. 1 x 1017 years all stars will have experienced ~100 disruptive encounters, flinging their planets into space. Evaporation of stars and collapse of galaxies into the central black holes. Protons decay (???) p+ > e+ + v (a positron and neutrino) e+ + e- > y (a photon = a burst of radiative energy) The big fizzle = an ever-expanding gas of electrons, protons, neutrons, photons.