Observational constraints on the averaged universe
2012
Averaging in general relativity is a complicated operation, due to the
general covarianceof the theory and the non-linearity of Einstein's equations. The latter of these ensures that smoothing
spacetimeover cosmological scales does not yield the same result as solving Einstein's equations with a smooth matter distribution, and that the smooth models we fit to observations need not be simply related to the actual geometry of
spacetime. One specific consequence of this is a decoupling of the geometrical spatial curvature term in the metric from the dynamical spatial curvature in the
Friedmann equation. Here we investigate the consequences of this decoupling by fitting to a combination of HST, CMB, SNIa and BAO data sets. We find that only the geometrical spatial curvature is tightly constrained, and that our ability to constrain
dark energydynamics will be severely impaired until we gain a thorough understanding of the averaging problem in cosmology.
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