For many people “The Big Bang Theory” is a CBS sitcom following the tangled lives of four geeky Caltech students. But what is the real Big Bang Theory? Most people know it concerns the beginning of the Universe, but exactly what does the theory say and how firm is the evidence for it? In this class, we’ll journey out into the galaxies and back to the primordial fireball, always paying attention to how we know what we claim to know. Ultimately, we’re pursuing an idea that has been present in all cultures at all times: what is the origin of our world, with its land and sky, sun and stars, and even ourselves? During this class, we’ll be exploring some wonderful themes. We’ll see how cosmic expansion helps us understand the unusual nature of the universe’s explosive origin about 14 billion years ago. We’ll use high-caliber data from large telescopes to unpack the evidence for two major, but invisible, components of today’s universe: dark matter and dark energy. Astronomers are extremely lucky because they can actually see the past simply by looking far away. Our largest telescopes reveal a billion year old universe filled with chaotic “infant” galaxies quite unlike today’s majestic “adult” spirals and ellipticals. We can even see the primordial fireball using microwave telescopes that reveal in exquisite detail the incandescent glow from hot gas, laced with deep harmonic tones of primordial sound. Pushing even earlier we arrive at the first hour and even the first minute, where we find conditions similar to the sun’s center or the inside a hydrogen bomb. Remarkably, the “heavy” hydrogen in the water you drink every day was made in that first minute! Finally, stepping back to frame everything, we’ll calculate the total energy of the Universe and find that it is zero – it sums to nothing! This in turn suggests a truly remarkable creation mechanism, called inflation, that creates everything from nothing and launches the expansion. While the evidence for inflation is not yet robust, there are important observations in the next decade or two that may help confirm its reality. Throughout, we will honor the overall intent of the “empirical engagement” by using the Big Bang Theory as a test case to explore how science works – how observations are used to test and refine a theory that is built using the known laws of physics. After a century of effort, this theory – with the exception of the creation mechanism itself – is about as detailed and robust as the theories of evolution and atomic structure. While these theories reached maturity some time ago, modern cosmology has only recently gelled and is therefore, arguably, the greatest scientific narrative of our current time.