A theoretician's analysis of the supernova data and the limitations in determining the nature of dark energy

Current cosmological observations show a strong signature of the existence of a dark energy component with negative pressure. The most obvious candidate for this dark energy is the cosmological constant (with the equationof state w X = p/ρ = -1), which, however, raises several theoretical difficulties. This has led to models for a dark energy component that evolves with time. We discuss certain questions related to the determination of the nature of the dark energy component from observations of high-redshift supernovae. The main results of our analysis are as follows. (i) Even if the precise value of w X is known from observations, it is not possible to determine the nature of the unknown dark energy source using only kinematical and geometrical measurements. We have given explicit examples to show that different types of sources can give rise to a given w X . (ii) Although the full data set of supernova observations (which are currently available) strongly rule out models without dark energy, the high- (z > 0.25) and low- (z < 0.25) redshift data sets, individually, admit decelerating models with zero dark energy. Any possible evolution in the absolute magnitude of the supernovae, if detected, might allow the decelerating models to be consistent with the data. (iii) We have introduced two parameters, which can be obtained entirely from theory, to study the sensitivity of the luminosity distance on w X . Using these two parameters, we have argued that although one can determine the present value of w X accurately from the data, one cannot constrain the evolution of w X .