Chapter 1 Understanding the Foundations of Modern Cosmology Over the span of the last 100 years, theoretical physicists and observational astronomers have uncovered the birth-story of our Universe and have coaxed its key physical properties from observations of the sky. The first steps were taken by a handful of great scientists early in the twentieth century: Albert Einstein, Alexander Friedmann, Vesto Slipher, Abbe Georges Lemaître, Edwin Hubble, and George Gamow. These pioneers and others who followed in their footsteps were able to peek behind a curtain that has now been flung wide open. An evolving model of our expanding Universe has taken center stage, and its characteristics are nothing short of breathtaking. Its current and most popular form is called the LCDM model. It begins in a state where all regions of space are nearly uniformly filled with an unimaginably hot and high density of energy. The LCDM model is a sophisticated and refined hybrid of the Big Bang theory that was sketched in its most rudimentary form by Georges Lemaître around 1930. Lemaître was the first to hypothesize that the Universe began in a high-density state with a tiny "radius of curvature" only to evolve into our current state with a huge "radius of curvature". The mathematical basis for his model-as well as the basis for the LCDM model-derives from Einstein's theory of general relativity in a form suggested by Alexander Friedmann"--