Résumé
The characteristic formalism uses outgoing null cones to define coordinates, and is well-developed in numerical relativity for gravitational wave extraction. The formalism is well-suited to cosmology, since cosmological data are observed on past null cones. In the case of spherical symmetry, density and radial velocity data on an initial null cone define a well-posed evolution problem leading to the past behaviour of the universe. Observationally, the data can be obtained from galactic number counts or age data, and the distance red-shift relation. When red-shift drift data becomes available, constraints on the theory of gravity, such as the cosmological constant, will become feasible.