The status of and
is summarised in
TABLE 9.
We have a reasonable convergence of the
parameter towards
a low value
= 0.15-0.4. The
convergence of
is
significantly better with the presence
of the cosmological constant that makes the universe flat.
Particularly encouraging is that the
parameters
derived with the aid of structure formation models agree with each other.
This is taken to be an important test for the
cosmological model, just as in particle physics when
many different phenomena are reduced to a few convergent
parameters to test the model.
There are yet a still highly discrepant results on
,
but it is not too difficult to speculate their origins.
On the other hand, the current `low
' means the values
that vary almost by a factor of three and effort is needed to
make these converge.
method | ![]() | ![]() | model used? |
H0 vs t0 | < 0.7 | ||
luminosity density +M/L | 0.1-0.4 | ||
cluster baryon fraction | 0.15-0.35 | ||
SNeIa Hubble diagram | ![]() | ![]() ![]() | |
small-scale velocity field (summary) | 0.2±0.15 | ||
(pairwise velocity) | 0.15±0.1 | ||
(Local Group kinematics) | 0.15±0.15 | ||
(Virgocentric flow) | 0.2±0.2 | ||
large-scale vel field | 0.2-1 | ||
cluster evolution (low ![]() | 0.2+0.3-0.1 | yes | |
(high
![]() | ~ 1 | yes | |
COBE-cluster matching | 0.35-0.45 (if
![]() | yes | |
0.20-0.40 (if
![]() ![]() | yes | ||
shape parameter ![]() | 0.2-0.4 | yes | |
CBR acoustic peak | free (if flat) | ![]() ![]() | yes |
> 0.5 (if open) | yes | ||
gravitational lensing | ![]() | ||
summary | 0.15-0.45 (if open) | ||
0.2-0.4 (if flat) | |||
0.6-0.7(?) | |||
The cosmological constant has been an anathema over many years because
of our ignorance of any mechanism that could
give rise to a very small vacuum energy
of (3 meV)4, and neither can we understand a zero
cosmological constant.
In mid-nineties the atmosphere was changing in favour for a non-zero
. The prime motivation was
the Hubble constant-age problem,
but the introduction of a non-zero
was helpful in many respects.
One theoretical motivation was
to satisfy flatness which is expected in inflationary scenarios
(Peebles 1984).
Ostriker &
Steinhardt (1995)
proclaimed a
`cosmic concordance' with a flat universe mildly dominated by
.
By 1997, only one observation contradicted with the presence
of a moderate value of
;
this was the SNeIa Hubble diagram presented by the
Supernova Cosmology Project
(Perlmutter et
al. 1997);
see Fukugita 1997.
In the next two years the situation changed. Two groups
analysing SNeIa Hubble diagram, including
the Supernova Cosmology Project,
now claim a low
and a positive
. On the other hand, the
Hubble constant-age problem became less severe due to our cognition
of larger uncertainties, especially in the age estimate.
The indications from SNeIa Hubble diagram are very interesting and
important, but the
conclusions are susceptible to small systematic effects.
They should be taken with caution.
We should perhaps wait for small-scale CBR anisotropy
observations to confirm a nearly flat universe before concluding
the presence of
.
In these lectures we have not considered classical tests, number counts,
angular-size redshift relations, and magnitude-redshift relations of
galaxies
(Sandage 1961;
1988),
in those testing for and
. Unlike clusters or large
scale structure,
where no physics other than gravity plays a role, the evolution of
galaxies is compounded by rich physics.
Unless we understand their astrophysics, these objects cannot be used as
testing candles. It has been known that galaxy number counts is
understood more naturally with a low matter density universe under the
assumption that the number of galaxies are conserved, but it is
possible to predict the correct counts with an
= 1 model where galaxies
form through hierarchical merging, by tuning parameters that control
physics
(Cole et al. 1994;
Kauffmann et
al. 1994).
It is important to work out whether the model works for any cosmological
parameters or it works only for a restricted parameter range. This does not
help much to extract the cosmological parameters, but it can falsify
the model itself.
We have seen impressive progress in the determination of the Hubble constant. The old discrepancy is basically solved. On the other hand, a new uncertainty emerged in more local distance scales. The most pressing issue is to settle the value of the distance to LMC. There are also a few issues to be worked out should one try to determine H0 to an accuracy of a 10% error or less. They include understanding of metallicity effects and interstellar extinction. The future effort will give more weight to geometric or semi-geometric methods. From the view point of observations the work will go to infra-red colour bands to minimise these problems.
In conclusion, I present in Figure 6 allowed
ranges of H0 and
(and
) for the case of (a) flat and
(b) open universes. With the flat case we cut the lower limit of
at 0.2 due to a strong constraint from lensing.
An ample amount of parameter space is allowed for a flat universe.
A high value of H0 > 82, which would be driven only by
a short
LMC distance, is excluded by consistency with the
age of globular
clusters as noted earlier. Therefore, we are led to the range
H0
60-82
from the consistency conditions.
For an open universe the coeval-formation
interpretation is compelling for globular clusters, or else no region
is allowed. The allowed H0 is limited to 60-70.
No solution is available if LMC takes a short distance.
![]() |
Figure 6. Consistent parameter ranges in
the H0 -
|
I would like to thank Rob Crittenden for his careful reading and many useful suggestions on this manuscript. This work is supported in part by Grant in Aid of the Ministry of Education in Tokyo and Raymond and Beverly Sackler Fellowship in Princeton.