**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 [26]. Recent work [27] 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.