Observations on past null cones, numerical relativity, and cosmology

In the characteristic formalism, coordinates are based on null cones generated by radial null geodesics. The Einstein equations take hierarchical form, and so can easily be written as a system suitable for numerical evolution. In numerical relativity, the major interest in the characteristic formalism is for gravitational wave estimation by Cauchy characteristic extraction, but it is also useful in cosmology.

The characteristic formalism is applicable to cosmology, since nearly all data is a result of observations on our past null cone. It is in principle possible to measure the initial data required in order to calculate an evolution into the interior of the past null cone, and codes have been implemented to perform the evolution. In the case of spherical symmetry, it is now possible to apply this process using real data.