We have come a long way in four lectures, from Einstein's misbegotten introduction of the cosmological constant at the beginning of the last century to its triumphant return today. Einstein's blunder is now seen as the key to understanding the very beginning of the universe, as represented by the theory of inflation, as well as the universe today, dominated by the mysterious dark energy that makes up more than two thirds of the entire mass of the cosmos. I have tried to convince you of two things: first, that the study of the early universe is particle physics in a very real sense, and second that apparently exotic theories of the early universe such as inflation (and perhaps even elements of string theory or some other variant of quantum gravity) are predictive and testable. It is a difficult business, to be sure, compared to the clean physics at, say, an e ± collider, but what we learn about fundamental theory from cosmology is in many ways complementary to the lessons learned from more traditional particle physics.
Acknowledgments
I would like to give my warmest thanks to Harrison Prosper for organizing a wonderful summer school, and to the bright and motivated students who made lecturing there a rare treat. I thank Richard Easther and Brian Greene for helpful comments. ISCAP gratefully acknowledges the generous support of the Ohrstrom foundation.