Bailyn, Charles. ASTR 160: Frontiers and Controversies in Astrophysics, Spring 2007. Yale OpenCourseWare: Astronomy, Accessed 23/9/14 http://oyc.yale.edu/astronomy/astr-160/lecture-23 License: Creative Commons Attribution-Noncommercial-Share Alike 3.0 license Reasons for the expansion of the universe are addressed at the start of this lecture, focusing especially on the acceleration of dark energy. Supernovae were the first evidence for the existence of dark energy. Two other proofs are presented. The first is the Cosmic Microwave Background, which is a form of electromagnetic radiation that is perfectly smooth and equal in all directions. It firmly supports the Big Bang theory. Projects attempting to measure it, such as COBE and WMAP, are discussed.