4.3. Equation of State of Dominant Energy
Remarkably, the above estimate for M brings the discussion of cosmological parameters full circle, with consistency obtained for a flat 12.5 billion year old universe , but not one dominated by matter. As noted previously, a cosmological constant dominated universe with M = 0.35 has an age which nicely fits in the best-fit range. However, based on the data discussed thus far, we have no direct evidence that the dark energy necessary to result in a flat universe actually has the equation of state appropriate for a vacuum energy. Direct motivation for the possibility that the dominant energy driving the expansion of the Universe violates the Strong Energy Condition came, in 1998, from two different sets of observations of distant Type 1a Supernovae. In measuring the distance-redshift relation [24, 25] these groups both came to the same, surprising conclusion: the expansion of the Universe seems to be accelerating! This is only possible if the dominant energy is "cosmological-constant-like", namely if < - 0.5 (recall that = - 1 for a cosmological constant).
In order to try and determine if the dominant dark energy does in fact differ significantly from a static vacuum energy - as for example may occur if some background field that is dynamically evolving is dominating the expansion energy at the moment - one can hope to search for deviations from the distance-redshift relation for a cosmological constant-dominated universe. To date, none have been observed. In fact, existing measurements already put an upper limit - 0.6 . Recent work  suggests that the best one might be able to do from the ground using SN measurements would be to improve this limit to - 0.7. Either other measurements, such as galaxy cluster evolution observations, or space-based SN observations would be required to further tighten the constraint.