2The Emergence of Cosmic Structure
The Universe is thought to have begun with a great explosion-the Big Bang. At an early time, about 15 billion years ago, all the mass in the Universe was contained in a tiny region which was very dense and hot. Since then the Universe has been steadily expanding, cooling as it has done so and creating the conditions for the formation of stars, galaxies, planets and, eventually, life and people. This essay covers some of the most important events that have occurred during the Universe’s life history. In particular, it focuses on the physical phenomena through which the Universe evolved from its primaeval amorphous state to its present, highly structured complexity. Several fundamental aspects are fairly well understood, for others we have only some tentative ideas, and for many more we have had no more than a glimpse of what might have happened. Two specific issues are addressed:
What is the Universe made of? What physical processes gave birth to the galaxies and other structures?