5.3. High Redshift SNIa Observations
The SCP
[74]
in 1997 presented their first results with 7
objects at a redshift around z = 0.4. These objects hinted at a
decelerating Universe with a measurement of
M =
0.88+0.69-0.60,
but were not definitive. Soon after, the SCP published a further
result, with a z ~ 0.84 SNIa observed with
the KECK I and HST added to the sample
[75], and
the HZSNS presented the results from their first four objects
[22,
87].
The results from both teams now ruled out a
M = 1
Universe with greater than 95% significance. These
findings were again superceded
dramatically when both teams announced results including more SNe (10
more HZSNS SNe, and 34 more SCP SNe) that showed not
only were the SN observations incompatible with a
M = 1
Universe, they were also incompatible
with a Universe containing only normal matter
[77,
83].
Fig. 4
shows the Hubble diagram for both teams. Both samples show that SNe
are, on average, fainter than would be expected, even for an empty
Universe, indicating that the Universe is accelerating. The agreement
between the experimental results of the two teams is spectacular,
especially considering the two programs have worked in almost
complete isolation from each other.
![]() |
Figure 4. Upper panel: The Hubble
diagram for high redshift SNIa from both the HZSNS
[83]
and the SCP
[77].
Lower panel: The residual of the distances relative to
a |
The easiest solution to explain the observed acceleration is to include
an additional
component of matter with an equation of state parameter more negative than
w < - 1/3; the most familiar being the cosmological
constant (w = - 1). Fig. 5
shows the joint confidence contours for values of
M and
from both
experiments. If we assume the Universe is composed only of normal matter
and a cosmological constant, then with
greater than 99.9% confidence the Universe has a non-zero cosmological
constant or some other form of dark energy.
![]() |
Figure 5. The confidence regions for both
HZSNS
[83] and SCP
[77] for
|
Of course, we do not know the form of dark
energy which is leading to the acceleration, and it is
worthwhile investigating what other forms of energy are
possible additional components.
Fig. 6 shows the joint
confidence contours for the HZSNS+SCP observations for
M and
wx (the equation of state
of the unknown component causing the acceleration). Because this
introduces an extra parameter, we apply the additional constraint that
M +
x = 1, as
indicated by the CMB experiments
[14].
The cosmological constant is preferred, but
anything with a w < - 0.5 is acceptable
[23,
77].
Additionally, we can add information about the value of
M,
as supplied by recent 2dF redshift survey results
[98], as
shown in the 2nd panel,
where the constraint strengthens to w < - 0.6 at 95% confidence
[69].