Cosmology deals with the nature of the universe. It can be broadly divided into three great ages. The first began in the 6th century BC with the Pythagorean concept of a spherical Earth that is part of a universe in which the motions of the planets are governed by the harmonious relations of natural laws. The second began in the 16th century with the Copernican revolution. This in turn led into Newton's infinite universe. The third began in the early 20th century with Albert Einstein's theory of general relativity and developed into the expanding universe we know today. Einstein's general theory of relativity extended the new space and time concepts of the special theory of relativity from the domain of electric and magnetic phenomena to all of physics and, particularly, to the theory of gravitation. By building on Einstein's previous work on special relativity, general relativity sought to deal with accelerating frames of reference. This in turn led to the principle of equivalence. By dealing with accelerating frames of reference, general relativity provides astronomers with the best theory to predict the effects of gravity. The book examines in detail new and important work in this field.