Bailyn, Charles. ASTR 160: Frontiers and Controversies in Astrophysics, Spring 2007. Yale OpenCourseWare: Astronomy, Accessed 22/9/14 http://oyc.yale.edu/astronomy/astr-160/lecture-16 License: Creative Commons Attribution-Noncommercial-Share Alike 3.0 license The third and final part of the course begins, consisting of a series of lectures on cosmology. A brief history of how cosmology developed into a scientific subject is offered. The discovery of dark energy, along with dark matter, played a crucial role in the development of cosmology. The lecture then discusses the discovery of spiral nebulae in 1920, as well as the "Great Debate" over what they were. Hubble's famous redshift diagram is presented as the basis for Hubble's Constant and Big Bang cosmology.