 To take into consideration the impact of pressure, the other key component of Einstein's field equations, we need to consider the relationship between pressure and energy density. When we change the amount of energy in a box, the pressure on the walls changes. In a slowly changing volume, the constant of proportionality is called W and varies depending on the nature of the contents of the box. In physics, this is called the equation of state. For matter that is not actually moving inside the box, the pressure is zero, so W is zero. For radiation, it can be shown that W equals one-third, where the number three comes from the number of spatial dimensions. This gives us the same relationship between energy density and the scale factor that we had with Newton's version. With this and the Friedman equation, we can now calculate the history of the scale factor and thereby the history of the universe if we can determine its radiation energy density, matter energy density, and curvature.