Here I talk about the foundations of why we think that the universe began in a dense, hot state, and cooled to the state we see it today. We run the clock backward in time, and we see how, using the fundamental principles of classical physics, especially thermodynamics, the universe was smaller and hotter ago. We see exactly what we mean by the Big Bang, what it covers, and what it does not cover. The Standard Hot Big Bang basically is a story from only about the first few billionths of a second until today. It’s just amazing what exactly we can do with classical physics and elementary general relativity. Supplement the videos with OpenStax Astronomy, 29: The Big Bang: 0:00 Introduction 1:32 Expansion of the Universe 3:41 Foundations of the Big Bang 4:59 The Big Bang is a Testable Model 6:24 Expansion & Hubble's Law 7:42 Cosmological Redshift 8:29 Cosmic Lookback Time 9:45 The Shape of the Universe 11:17 How Does Redshift Measure Expansion? 12:24 Redshift Measures the Change in Size 16:14 Gravity tries to pull everything together 18:33 Cosmic Dynamics: How the Universe Grows 20:48 How the Hubble Parameter depends on Density 28:37 The Definition of the Critical Density 30:37 The Cosmic Density Parameter: Density is Destiny! 31:30 The Geometry of the Universe 32:58 Cosmic Dynamics and the Geometry of Space 33:46 The Sum of all the Bits of Density of the Universe 35:58 The Big Bang's Hot Past 37:01 How Does the Big Bang Grow? 37:33 What Make the Universe's Expansion Change? 37:56 Normal Matter 38:50 Radiation Density 39:38 Curvature density 40:05 A Dark Energy Component 41:53 The Density Evolution of the Universe 47:00 Exceptions to the Hubble Time 49:12 The Age of the Universe.