One of the major concerns of cosmology today is the
nature of dark energy (quintessence).
While a cosmological constant appears to be the simplest option, formidable
fine-tuning problems which confront the latter have led to theoretical
models being developed
in which both the dark energy as well as its equation of state are
functions of time.
Type Ia supernovae currently provide the strongest evidence for
dark energy. The very recent observations of a supernova at
z 
1.7
appear to rule out simple extinction and evolution effects as major
causes for the diminishing light flux from these high-z
objects. It therefore
appears that the dark energy is `real' and that the universe was
decelerating at z > 0.5
[60].
The observational situation is likely to improve as results from both
deep and extensive galaxy and galaxy cluster survey's come in. It is
well known that the in the near future
[63].
Indeed, recent results which measure the gravitational clustering of
over 100,000 galaxies in the 2dF survey, determine a matter
power spectrum which is consistent with
CDM and
inconsistent with SCDM
[64].
Combined results from CMB probes (MAP, PLANCK), galaxy surveys (2dF, SDSS, DEEP), weak lensing statistics and the possibility of a dedicated supernova telescope (SNAP) give rise to expectations that the nature of dark energy will be understood (at least at the phenomenological level) in the not too distant future.