Abstract
It has been claimed that the observed magnitude of the vacuum energy density is consistent with the distribution predicted in anthropic models, in which an ensemble of universes is assumed. This calculation is revisited, without making the assumption that the cosmic microwave background (CMB) temperature is known, and considering in detail the possibility of a recollapsing universe. New accurate approximations for the growth of perturbations and the mass function of dark haloes are presented. Structure forms readily in the recollapsing phase of a model with negative A, so collapse fraction alone cannot forbid A from being large and negative. A negative A is disfavoured only if we assume that formation of observers can be neglected once the recollapsing universe has heated to T ≥ 8 K. For the case of positive A, however, the current universe does occupy an extremely typical position compared to the predicted distribution on the A - T plane. Contrasting conclusions can be reached if anthropic arguments are applied to the curvature of the universe, and we discuss the falsifiability of this mode of anthropic reasoning.
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