Although the universe seems to be much the same at each position in space, it is definitely changing in time. This was not realised until the early years of the last century. Up to then, it was thought the universe was essentially constant in time. It might have existed for an infinite time, but that seemed to lead to absurd conclusions. If stars had been radiating for an infinite time, they would have heated up the universe until it reached their own temperature. Even at night, the whole sky would be as bright as the Sun, because every line of sight would have ended either on a star or on a
...more
