So far, we have only described the Big Bang model in general terms: on the largest scales we can observe, the universe appears nearly uniform, it is currently expanding, and there is strong evidence that it was hotter and denser in the past. Now we would like the answers to some more specific questions:
- What types of matter and energy fill the universe? How much of each?
- How rapidly is the universe expanding today?
- How old is the universe today?
- What is the overall shape of the universe? Open, flat, closed, or otherwise?
- How is the expansion changing with time?
- What is the ultimate fate of the universe?
In this section, we address each of these questions in turn by summarizing the observations that inform each of these questions. There are many useful probes of the nature of our universe, each of which constrains one or more particular aspects of the Big Bang model and our understanding of structure formation. Indeed, the coming decade is being dubbed the era of precision cosmology as observations of supernova, galaxies and clusters, the cosmic microwave background radiation and the abundance of light elements each becomes mature. Taken together, these data will strongly constrain the model of our universe and may even point to the need for a radical rethinking of our understanding of cosmology.