We know that the universe in expanding because of the fact that galaxies are observed to be rushing away from us in all directions. How do we know this to be true? This is the fourth of the videos in my new series of Cosmology. I'm going through Dr. Barbara Ryden's textbook "Introduction to Cosmology". I used this textbook at William Paterson University. This course will cover the current state of the science of Cosmology. Here are the topics of this video: Introductory Cosmology Chapter 02: Fundamental Observations Section 03: Redshift is Proportional to Distance Some things covered: Nearly all Galaxies Show a Redshift... Does cosmic redshift happen by chance? Does the redshift violate the cosmological principle? How can we quantify the expansion of spacetime? What are the Hubble Time and Distance? How does measuring the rate of expansion give us the age of the universe? What is the Steady State Model? Why does the Big Bang win over the Steady State? What's beyond the cosmic horizon? NGC 1068: 2MASS J18082002−5104378: The brightest (V = 11.9) ultra metal-poor star: The First Stars: A Low-mass Formation Mode: GOODS:.