This talk will briefly review the early evolution of the Universe, from the epoch when ionized hydrogen recombined - and the cosmic background radiation was released - to the epoch when hydrogen reionized. This is a very important period in cosmic history. It was when the first stars formed from the gas generated by the Big Bang. These early stars were formed by processes quite different from that of subsequent star formation, because the cosmic gas from the Big Bang was extremely poor of metals. In addition, the galaxies formed from the first few generations of these early stars had very low masses because, during their formation, the predominance of neutral hydrogen in the intergalactic medium shielded them from energetic ultraviolet radiation that otherwise would have ionized their gas content. Subsequently, ultraviolet emissions from these first stars and galaxies built up a cosmological ultraviolet background radiation that reionized hydrogen. As a result, the shield protecting low mass proto-galaxies from energetic radiation disappeared. Thereafter the formation of these ultra-low mass galaxies was no longer possible and eventually gave to the formation of the type of galaxies that predominate now. The talk will describe theoretical considerations underpinning the model of cosmic evolution during this period and the experimental results that support it.
Massimo Stiavelli earned his PhD at the Scuola Normale Superiore of Pisa. He did postdoctoral work at Rutgers University and was a fellow at the European Southern Observatory in Garching. After a stint at the Scuola Normale Superiore of Pisa he joined the Space Telescope Science Institute in Baltimore, where he is currently an Astronomer and Acting Mission Head for the James Webb Space Telescope. His main scientific interest is the formation and evolution of galaxies both from the point of view of theory and observations. He was the team lead for the Hubble Ultra Deep Field, and he has authored or coauthored 109 research papers in professional journals, 161 technical reports and other publications, and three books. He has served on and chaired numerous NASA committees, and he is Interdisciplinary Scientist on the Science Working Group of the James Webb Space Telescope. He is a member of the American Astronomical Society, American Association for the Advancement of Science, American Institute of Aeronautics and Astronautics and International Astronomical Union.
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