-TERM
Arguments favouring
> 0 at the
present epoch essentially stem from four sets of observations:
(i) The age issue: A high value of the Hubble constant
H0 ~ 80 km/sec/Mpc predicts a short age of the
universe which is incompatible with the ages of the oldest stars (12 -
16 Gyr) unless the universe is open
(
m <
0.1) or flat and
dominated m +
= 1.
The appeal of this argument has somewhat decreased following
recent Hipparcos parallax measurements indicating a lower value
H0 
67 km/sec/Mpc and also a lower age for globular clusters:
11.5 ± 1.5 Gyr. Still, recent observations of
old galaxies at high redshifts
are extremely difficult to accommodate
within the framework of a flat matter dominated cosmology unless
the Hubble parameter is very small (H0
45 km/sec/Mpc;
section 4.1).
(ii) Structure formation: The standard COBE normalized
cold dark matter model of structure formation with
m = 1
appears to be in serious conflict with observations.
The situation may be remedied if the universe is flat, with most of matter
smoothly distributed in the form of a cosmological constant and only
a small fraction
m
h 
0.2 in
clustered matter.
(Here h is the Hubble constant in units of 100 km/s/Mpc).
Studies of the abundance and evolution of clusters of galaxies and of
lensing by clusters also appear to favour a low density universe
(section 4.6).
(iii) Baryon excess in clusters: In a spatially flat universe with
m = 1 the
mass fraction in baryons in the Coma cluster is expected to greatly
exceed nucleosynthesis bounds
leading to what has been called
the `baryon catastrophe'. The mass fraction in baryons can be kept in
agreement with nucleosynthesis constraints only if
m
h 0.16
[210]
(m
includes contribution from baryons and clustered dark matter).
Agreement with the Inflationary scenario which strongly favours a
spatially flat universe then suggests that the remaining
mass might be in the form of a cosmological constant.
(iv) High redshift supernovae and the cosmic microwave background:
Preliminary results
from this rapidly advancing field of cosmology
suggest that the universe may be accelerating universe with a dominant
contribution to its energy density coming in the form of
cosmological -term.
These results, when combined with CMB anisotropy observations on
intermediate angular scales, strongly support a flat universe
m +
= 1 with
~ 0.6 - 0.7
(sections 4.3 &
4.4).
In the first half of this paper we shall briefly review the present observational status of the cosmological constant referring the reader to the original papers and earlier reviews [26, 37] for more details.