|Twentieth-century science completely overturned our view of cosmology. We now know that our solar system is one of many in our galaxy, and our galaxy is one of many in the universe. These galaxies are spread throughout space in a nearly uniform distribution, and distant galaxies are mutually moving apart from each other as the universe expands. Over ten billion years ago the entire collection emerged from an incredibly hot and dense state: the Big Bang.
In the twenty-first century, new discoveries are offering new challenges to our understanding. Ordinary matter comprises only about four percent of the stuff in the universe; the rest is dark matter and dark energy. Structures in the universe grew from a very smooth primeval state; the tiny deviations from perfect smoothness may have been caused by a period of inflationary expansion in the very early universe. New experiments are being designed to extend our understanding further into the unknown.
This primer provides a brief introduction to these ideas, the basic picture of modern cosmology. The intended audience includes anyone with curiosity about science; no technical background is assumed.