Astronomy notes is an educational resource for introductory astronomy classes for undergraduates. This section discusses the Hubble constant, Olbers' Paradox, Hubble's Law, the cosmological principle and the perfect cosmological principle.  In the next section evidence for the Big Bang are presented such as the abundance of helium and hydrogen, the Cosmic Microwave Background, and the expansion of the universe.  The next section discusses the shape of the universe as either being flat, open, or closed, and why inflation, the cosmological constant, and dark matter play an important determining role.