Towards Cosmography of the Local Universe

Abstract

Anisotropies in the distance-redshift relation of cosmological sources are expected due to large-scale inhomogeneities in the local Universe. When the observed sources are tracing a large-scale matter flow in a general spacetime geometry, the distance-redshift relation with its anisotropies can be described with a geometrical prediction that generalises the well-known Friedmann-Lemaître-Robertson-Walker result. Furthermore, it turns out that a finite set of multipole coefficients contain the full information about a finite-order truncation of the distance-redshift relation of a given observer. The multipoles of the distance-redshift relation are interesting new cosmological observables that have a direct physical interpretation in terms of kinematical quantities of the underlying matter flow. Using light cones extracted from N-body simulations we quantify the anisotropies expected in a Λ cold dark matter cosmology by running a Markov chain Monte Carlo analysis on the observed data. In this observational approach the survey selection implements an implicit smoothing scale over which the effective rest frame of matter is fitted. The perceived anisotropy therefore depends significantly on the redshift range and distribution of sources. We find that the multipoles of the expansion rate, as well as the observer's velocity with respect to the large-scale matter flow, can be determined robustly with our approach.