The universe as we observe it today has a remarkable hierarchical structure: the stars are arranged into galaxies, the galaxies into clusters of galaxies, and the clusters of galaxies into "superclusters". Yet we know that about 10 billion years ago the universe was almost perfectly smooth and devoid of any structure. In fact, recent observations of the high degree of isotropy of the "cosmic microwave background radiation" imply that the density in the early universe varied from one point in space to another by only a thousandth of a percent. An intriguing question, then, is how the smooth and dense early universe evolved into its present-day form, containing a rich and regular hierarchical structure. In particular, when, and how did the very first star form?
The density of the early universe did have a slight initial spatial modulation. Consider one specific region in the early universe where the density was slightly above average. This region will exert a slight gravitational pull on the surrounding matter and decelerate it relative to the overall expansion of the universe. Since this makes the region even more dense, eventually the matter around this region stops expanding with the universe, turns around, and start collapsing to form an astrophysical object. Depending on the size of the initial seed region, this object could be a star, galaxy, or a galaxy cluster. Gravity alone does not select a scale for this process, but we know the inital conditions just after the big bang from the observations of the microwave backgound radiation. In particular, we know that the density fluctuations were stronger on smaller mass-scales, i.e. the smaller a region was, the more overdense it was expected to be. This, in turn, means that the observed hierarchical structure was built from the bottom up, i.e. smaller objects appeared first, which then later assembled to form the larger objects.
It turns out, however, that the pressure forces opposing the gravitational collapse become more important within smaller collapsing regions. Unlike gravity, pressure forces are complicated to calculate since they depend on every process that effects the temperature. This leads to the fundamental difficulty in deriving the sizes and properties of the very first objects that appear in the universe: the nature of these objects can not be established by simple gravitational considerations alone. I have addressed the following specific questions about the first objects and their observable consequences.
1. How small were the first objects in the universe?
2. How does radiation from the first objects effect the cosmic abundance of H2?
3. How does radiation effect the abundance of H2 molecules in dense clouds?
4. Are there observable signatures of the primordial stars?
5. Can we see the effects of dust from the first stars?