Transcript: The fact that the early universe had a phase of superluminal or faster than light expansion means that it is not trivial to calculate the size of the observable universe. In other words the following simple idea does not work. The universe is eleven or twelve billion years old, so we might imagine that it must be eleven or twelve billion lightyears across. That’s not true because the early expansion was so rapid. Galaxies early in the expansion were being carried apart faster than the speed of light. Later on they were moving apart less rapidly, and eventually the light became able to reach us. So depending exactly on the cosmological model, the size of the observable universe is twenty-five to thirty billion lightyears across. The observable universe represents the horizon, the distance we can see or the distance within which light has had time to reach us in the history of the universe and the cosmic expansion. Given that we think that the universe is now accelerating, the future of the observable universe is very interesting. Acceleration, if it continues, will gradually take galaxies out of our horizon and away from being visible. So over long periods of time the accelerating universe will remove objects from our view. Over a time scale of many billions of years there will actually be less and less to study. Observational astronomy will not last forever. In the end, the acceleration will cause the only galaxies to be visible to be those that are bound to the Milky Way itself.