 Vacuum energy has a key implication for our flat cosmological model. We have seen that radiation with w equal to one-third dilutes by the cosmic scale factor raised to the fourth power. Relatively still matter that exerts no pressure has w equal to zero and dilutes by the scale factor cubed. But vacuum energy density with w equal to minus one is a constant. It does not dilute. Therefore the total amount of vacuum energy increases with the volume of the universe. In a small universe it would have a little impact, but today it is almost 70% of the energy density of the universe, filling the gap left by the matter radiation only number. Under this model the universe was matter dominated for most of its existence since the Big Bang. It was radiation dominated for a mere 47,000 years, matter dominated for 9.8 billion years, and currently in transition to complete vacuum energy domination. The name we use for this zero point quantum vacuum energy is dark energy. We use the symbol lambda to represent this component of the universe. The symbol was first used by Einstein as a cosmological constant to account for a static universe. It went by the wayside when Edwin Hubble discovered that the universe was not static, but it has now been repurposed to represent this vacuum energy.