Negative vacuum energy densities and the causal diamond measure

Understand

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.