The Hot Big Bang

In an expanding universe, matter is diluted as the volume of the universe gets larger. Conversely, if we follow the expansion backwards in time, we find that the universe was denser in its past than it is today. In fact, the density of the universe grows without bound as we wind the clock back to the big bangBig bang. Furthermore, the temperatureTemperature of the universe also soars to extremely high values. How do we know this? And what does this imply about the conditions of the early universe?