The standard model does not try to explain the cause of the Big Bang. It starts from the present conditions of average density and temperature in the Universe. If we go backwards in time, the Universe was smaller; its density and temperature can be computed for some typical epochs in the past.

1. (1) Average density. The present Universe is very unhomogeneous, so that its average density can be estimated only by using a very large volume, for instance, a cube of 500 million light-years on each side, for which the total mass of millions of galaxies can be assessed. The average density found by this method is a little less than 10-30 g/cm3.

2. (2) Average temperature. There are now still 3 billion photons at 2.7 K in the fossil radiation coming directly from the Big Bang, for each hotter photon arriving from the stars. These stellar photons are therefore the insignificant and negligible traces left by the fireworks from the primordial explosion. The average temperature of the Universe is 2.7 K.

3. (3) Expansion velocity. Its rate is given by the Hubble constant, H, which can be taken, for instance, as H = 25 km/second per million light-years.

In Table E.1, results have been rounded to the nearest factor of 10. The Universe cooled, as for example in an explosion gases cool as they fill a larger and larger volume, so that it is easy to compute the temperature and density at different times in the past.