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 .
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 . 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 .
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.