Constraints on dark energy
A major challenge in cosmology is to determine the amount of dark energy, expressed as a fraction of the critical density by Ωdark = ρdark/ρcritical, and its degree of gravitational repulsion, which is characterized by the quantity w (see text). In a spatially flat universe Ωdark = 1 - Ωm, where Ωm is the amount of matter as a fraction of the critical density. Supernova data (red region) suggest that matter - both ordinary and dark matter - accounts for less than 50% of the total or critical density of the universe, and possibly as little as 20%. Since the relationship between distance and redshift depends on the repulsive strength of the dark energy (see figure 1), the supernova data also suggest that w is less than about -0.8. Measurements of the cosmic microwave background also place limits on these two numbers (blue region): the apparent angular sizes of features in the temperature-anisotropy pattern likewise depend on the distance-redshift relation, although at a much higher redshift of z ~ 1100. The overlap of the two sets of results suggest that dark energy accounts for between 62% and 76% of the total critical energy density, and that the value of w lies between -1.3 and -0.9.