(Mis-)Interpreting supernovae observations in a lumpy universe

Details (Mis-)Interpreting supernovae observations in a lumpy universe (Mis-)Interpreting supernovae observations in a lumpy universe

2011-09-12

arxiv.org/abs/1109.2484v2

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

18 pages and 3 figures. v2 has extended discussion

Scientific paper

Light from 'point sources' such as supernovae is observed with a beam width of order of the sources' size -typically less than 1 AU. Such a beam probes matter and curvature distributions that are very different from coarse-grained representations in N-body simulations or perturbation theory, which are smoothed on scales much larger than 1 pc. The beam typically travels through unclustered dark matter and hydrogen with a mean density much less than the cosmic mean, and through dark matter mini-halos and hydrogen clouds. Large dark matter halos are rarely encountered directly and so are mainly experienced through their Weyl (tidal) curvature. How observations of many such beams averages this Weyl curvature into the Ricci curvature of the background is not understood. Standard analyses predict a huge variance for such tiny beam sizes, and nonlinear corrections appear to be non-trivial. It is not even clear whether under-dense regions lead to dimming or brightening of sources, owing to the uncertainty in modelling the expansion rate. By considering different reasonable approximations which yield very different cosmologies we argue that modelling ultra-narrow beams accurately is a critical problem for precision cosmology. This could appear as a discordance between angular diameter and luminosity distances when comparing SN observations to BAO or CMB distances.

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