Friday, April 20, 2012
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.