Physical cosmology up to the time of Einstein

Gravity is the one long-range force which acts upon all matter. Soon after Isaac Newton had completed the unification of the laws of gravity and celestial physics through his discovery of the inverse square law of gravity, he appreciated that the unique form of this law has important consequences for the large-scale distribution of matter in the Universe. In 1692–1693, the cosmological problem was addressed in a remarkable exchange of letters between Newton and the young clergyman Richard Bentley (1662–1742), later to become master of Trinity College, Cambridge. The correspondence concerned the stability of a Universe uniformly filled with stars under Newton's law of gravity. The attractive nature of the force of gravity meant that matter tends to fall together, and Newton was well aware of this problem. His first solution was to suppose that the distribution of stars extends to infinity in all directions so that the net gravitational attraction on any star in the uniform distribution is zero. As he wrote,

The fixt Stars, everywhere promiscuously dispers'd in the heavens, by their contrary attractions destroy their mutual actions.

Newton made star counts to test the hypothesis that the stars are uniformly distributed in space and found that the numbers increased more or less as expected with increasing apparent magnitude. The problem, which was fully understood by Newton and Bentley, was that a uniform distribution of stars is dynamically unstable.