Physicists have become keenly interested in the first billion years of the universe—the stretch between the Big Bang and the formation of the first stars during which galaxies began to form. During the last 600 million years or so of this period, the neutral interstellar galactic medium–and even pre-galactic medium–became ionized with ultraviolet radiation emitted by the first stars glowing in the earliest, growing galaxies. An understanding of the physics of this stretch, called the "Epoch of Reionization," or EoR, would connect the physics of the modern universe to the Big Bang.