We are all in the gutter,
but some of us are looking at the stars…Oscar Wilde, Lady Windermere’s Fan.
As we begin the study of the universe, we have to confront a fundamental question: Are the laws of physics that hold on or near the Earth also valid in distant regions of the universe? And are these laws of physics also valid at all times? When cosmologists look at distant regions of the universe, they see them as they were a long time ago – they see a galaxy at a distance of, say, 1010 light-years as it was 1010 year ago. To interpret the data collected in such observations, we need to know the laws of physics that govern these regions, far away in space and in time.
Newton set a precedent for the universal validity of physical laws in his cosmological speculations. He conjectured that the (apparently) static distribution of the “fixed” stars was the result of an equilibrium of their mutual gravitational forces. He assumed that his inverse-square law for the gravitational force was also valid for distant stars, and that “the fixed stars, being equally spread out in all points of the heavens, cancel out their mutual pulls by opposite attractions” (Newton, 1713).
Einstein intended to follow Newton’s precedent by applying to the entire universe the field equation he had posited for the Solar System. But when he found that with these field equations he could not obtain a static solution for the mass distribution of the universe, he introduced new physics, in the form of a cosmological term added to the field equations. After the discovery of the expansion of the universe, Einstein promptly retracted his adoption of the cosmological term, and he thereafter strictly followed the example of Newton in treating the universe by the same laws as apply within the Solar System.