Intrinsic uncertainty on the nature of dark energy

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We argue that there is an intrinsic noise on measurements of the equation of state parameter w=p/ρ from large-scale structure around us. The presence of the large-scale structure leads to an ambiguity in the definition of the background universe and thus there is a maximal precision with which we can determine the equation of state of dark energy. To study the uncertainty due to local structure, we model density perturbations stemming from a standard inflationary power spectrum by means of the exact Lemaître-Tolman-Bondi solution of Einstein's equation, and show that the usual distribution of matter inhomogeneities in a ΛCDM cosmology causes a variation of w -- as inferred from distance measures -- of several percent. As we observe only one universe, or equivalently because of the cosmic variance, this uncertainty is systematic in nature.