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The evidence for dark energy has received strong
corroboration from cosmic microwave background
(CMB) results (Balbi et al. 2000; Lange et al. 2001b;
Spergel et al. 2003) which are sensitive to the total
energy density, combined with galaxy power spec-
trum and cluster abundance measurements (Bahcall
et al. 1999; Efstathiou et al. 2002; Percival et al. 2002;
Allen et al. 2003) which probe the matter density QM,
or with an H0 prior (see Figure 1). Two of these three
independent measurements would have to be in error
to make dark energy unnecessary in the cosmological
model.
The dark energy might be due to the cosmological
constant term in Einstein's equations, which implies
vacuum energy with w= -1. Alternatively, it could
be due to a dynamical scalar field with w : -1 and/or
time-varying w. The fundamental importance of a uni-
versal vacuum energy has sparked a flurry of activity
in theoretical physics with several classes of models
being proposed (e.g. quintessence (Ratra & Peebles
1988; Caldwell et al. 1998; Ferreira & Joyce 1998),
Pseudo-Nambu-Goldstone Boson (PNGB) models
(Frieman et al. 1995; Coble et al. 1997), cosmic de-
fects (Vilenkin 1984; Vilenkin & Shellard 1994), and
modified gravity (Dvali et al. 2000; Carroll et al.
2003).
All these models explaining the accelerating Uni-
verse have observational consequences for the super-
nova Hubble diagram (Weller & Albrecht 2002). The
luminosity distance as a function of redshift can for
convenience be parameterized to first order with effec-
tive dark-energy parameters, e.g. wconst, or wo, wa.
A constant wconst can represent dark-energy models
such as the cosmological constant while w wo +
wa(1 - a) is successful at describing a wide variety
of both scalar field and more general models (Lin-
der 2003a, 2004). Predictions of dark-energy models
can be either compared directly to supernova Hubble
diagrams or to the dark-energy parameters as fit by
the data. Placing some constraints on possible dark-
energy models, Perlmutter et al. (1999b); Garnavich
et al. (1998); Perlmutter et al. (1999a); Knop et al.
(2003) find that for a flat universe, the data are con-
sistent with a cosmological-constant equation of state
and 0.2 < QM < 0.4 (Figure 2), or generally wconst <
-0.6 at 95% confidence level. When combining su-
pernova with other results, such as from a variety
of CMB experiments and large-scale-structure mea-
surements(Hawkins et al. 2003; Spergel et al. 2003),
Knop et al. (2003) find wconst 1.05 0.15 ( 0.09
3 11111111111111111
No Big Bang
2
Supernovae
1 - SNAP SN
STarget
- CMB
0
-1
04
I , I I I, , , I I I
2
3
Fig. 1.- There is strong evidence for the existence
of a cosmological vacuum energy density. Plotted are
QM-QA 68% and 95% confidence regions for super-
novae (Knop et al. 2003), cluster measurements (based
on Allen et al. (2003)), and CMB data with Ho pri-
ors (outer counters from Lange et al. (2001a), inner
contours from Spergel et al. (2003)). These results
rule out a simple flat QM 1, QA 0 cosmol-
ogy. Their consistent overlap is a strong indicator
for dark energy. Also shown is the expected confi-
dence region from just the SNAP supernova program
for QM 0.28, QA 0.72.
3
1
