**10.2. The equation of state of the vacuum**

So far, we have assumed that the vacuum energy is exactly
a classical , or
at any rate indistinguishable from
one. This is a highly reasonable prior: there is no reason
for the asymptotic value of any potential to go exactly
to zero, so one always needs to solve the classical cosmological
constant problem - for which probably the only reasonable
explanation is an anthropic one (e.g.
Vilenkin 2001).
Therefore, dynamical provision of
*w*
*p*_{v} /
_{v}
- 1
is not needed. Nevertheless, one can readily take an
empirical approach to *w* (treated as a constant for
a first approach).

Figure 22 shows a simplified approach to this,
plotting the locus on (*w*,
_{m}) space
that is required for
a given value of *h* if the location of the main CMB acoustic peak
is known exactly. For
*h* 0.7, this is
very similar to the locus derived from the SN Hubble diagram
(Garnavich et al. 1998).
The solid circles show the updated 2dFGRS constraint of
_{m}*h*
= 0.18. In order to match the data with *w* closer to zero,
_{m} must
increase and *h* must
decrease. The latter trend means that the HST Hubble constant
sets an upper limit to *w* of about -0.54
(Percival et al. 2002).
This is very similar to the SNe constraint of
Garnavich et al. (1998),
so the combined limit is already close to *w* < - 0.8. The vacuum
energy is indeed looking rather similar to
.