1 Cosmology today

Modern cosmology is based on the understanding that the Universe began in an event called the hot big bang, in which time and space were created, about 13.8 billion years ago. The Universe has cooled and space has expanded as time has progressed since then. The evidence for this is chiefly based on three sets of observations, which will be discussed in the following sections. A schematic timeline for the main events in the history of the Universe is shown in Figure 1.