Supernovae and the cosmic expansion
Observations of supernovae can be used to chart the history of the cosmic expansion. (a) The distance to a type 1a supernova is readily obtained from its luminosity, which is calibrated by its light curve and spectrum, and its observed intensity. (b) Meanwhile, the expansion of the universe shifts features in the supernova spectrum to longer wavelengths by a factor characterized by the redshift. (c) By plotting distance versus redshift for a large number of supernovae, we can chart how the universe has expanded over time. The orange circles are data points with the error bars omitted for clarity (see Riess et al. in further reading), along with the favoured theoretical prediction: a universe with 30% matter and 70% cosmological constant (blue). Also shown are predictions for a universe with 30% matter and spatial curvature (red dashed), and for 100% matter (purple dashed). The difference between acceleration and deceleration is revealed where the theoretical curves start to diverge. The transition from deceleration to acceleration is more subtle: the green line shows a coasting cosmos that is neither accelerating nor decelerating. The expansion speeds up close to where the data reach their maximum deviation from this curve (near z = 0.5). Hubble's view of the cosmic expansion was limited to objects at distances less than a few megaparsecs (i.e. a small region on the left of the figure).