Negative vacuum energy densities and the causal diamond measure
  Arguably a major success of the landscape picture is the prediction of a
small, non-zero vacuum energy density. The details of this prediction depends
in part on how the diverging spacetime volume of the multiverse is regulated, a
question that remains unresolved. One proposal, the causal diamond measure, has
demonstrated many phenomenological successes, including predicting a
distribution of positive vacuum energy densities in good agreement with
observation. In the string landscape, however, the vacuum energy density is
expected to take positive and negative values. We find the causal diamond
measure gives a poor fit to observation in such a landscape -- in particular,
99.6% of observers in galaxies seemingly just like ours measure a vacuum energy
density smaller than we do, most of them measuring it to be negative.
