2005A&A...434...45M

What is the influence of cosmology (the expansion law and its acceleration, the cosmological constant...) on the dynamics and optics of a local system like the solar system, a galaxy, a cluster or a supercluster? The answer requires the solution of the Einstein equation with the local source, which tends towards the cosmological model at large distances. There is, in general, no analytic expression for the corresponding metric, but we calculate here an expansion in a small parameter, which allows us to answer this question. First, we derive a static expression for the pure cosmological (Friedmann-Lema itre) metric, whose validity, although local, extends in a very large region around the observer. This expression appears as the metric of an osculating de Sitter model. We then propose an expansion of the cosmological metric with a local source, which is valid in the very wide neighborhood of the local system. This allows us to calculate exactly the (tiny) influence of cosmology on the dynamics of the solar system: it results that, contrary to some claims, cosmological effects fail to account for the unexplained acceleration of the Pioneer probe by several orders of magnitudes. Our expression provides estimations of the cosmological influence in the calculations of rotation or dispersion velocity curves in galaxies, clusters, and any type of cosmic structure, necessary for precise evaluation of dark matter and/or cosmic flows. The same metric can also be used to estimate the influence of cosmology on gravitational optics in the vicinity of such systems.