Even as the evidence for the Standard Model became stronger and stronger in the 1970s and beyond, so the evidence for general relativity grew in the latter half of the twentieth century. Any discussion of the Standard Model and physics beyond must confront Einstein's theory at two levels. First, general relativity and the Standard Model are very successful at describing the history of the universe and its present behavior on large scales. General relativity gives rise to the big bang theory of cosmology, which, coupled with our understanding of atomic and nuclear physics, explains – indeed predicted – features of the observed universe. But there are features of the observed universe which cannot be accounted for within the Standard Model and general relativity. These include the dark matter and the dark energy, the origin of the asymmetry between matter and antimatter, the origin of the seeds of cosmic structure (inflation), and more. Apart from these observational difficulties, there are also serious questions of principle. We cannot simply add Einstein's theory onto the Standard Model. The resulting structure is not renormalizable, and cannot represent in any sense a complete theory. In this book we will encounter both of these aspects of Einstein's theory. Within extensions of the Standard Model, in the next few chapters, we will attempt to explain some of the features of the observed universe. The second, more theoretical, level, is addressed in the third part of this book. String theory, our most promising framework for a comprehensive theory of all interactions, encompasses general relativity in an essential way; some would even argue that what we mean by string theory is the quantum theory of general relativity.