About 400,000 years after the Big Bang, the dark universe cooled enough that particles were able to form the first atoms (hydrogen, helium), and then light. This is known as the “reionization” era. It is the most important period to astrophysicists because features and conditions in the reionization, including dark matter, determined the evolution of the universe. But this early epoch is inaccessible to telescopes. Astrophysicists devise very complex models to test their theories of the reionization and formation of the earliest galaxies. Dr. Vogelsberger, known for some of the most accurate simulations of the earliest galaxy formations, describes the Thesan project he developed with other scientists at MIT, Harvard University, and the Max Planck Institute for Astrophysics. This is of particular interest now that the James Webb Space Telescope (JWST) will eventually be able to site these earliest formations. Dr. Vogelsberger will also discuss other related simulation projects.