The Universe Is Expanding

Abstract

Let us recall the situation in 1924. Hubble had just discovered the Delta Cephei stars in the Andromeda Nebula and had determined its distance. Curtis had triumphed over Shapley about the question of the island universes. From now on the universe would be full of galaxies, the nearest of which could be seen in the sky as spiral nebulae. Olbers’ paradox (see the Introduction) was still around in all its glory. According to this, if the universe has always been homogeneous and filled with stationary galaxies, then wherever we look, our line of sight must encounter a galaxy and the luminous surface of a star within it. However, if our line of sight misses all the stars in a galaxy and passes right through it, then we must still encounter another galaxy behind that one and the surface of one of its stars, or else a star in a yet more distant galaxy. Wherever we look we must finally encounter the surface of a star, and our night sky should be as bright as the surface of the Sun.

The unanimity with which the galaxies are running away looks almost as though they had a pointed aversion to us. We wonder why we should be shunned as though our system were a plague spot in the universe.

Sir Arthur Eddington (1882–1944), “The Expanding Universe”