Prediction and typicality in multiverse cosmology
  In the absence of a fundamental theory that precisely predicts values for
observable parameters, anthropic reasoning attempts to constrain probability
distributions over those parameters in order to facilitate the extraction of
testable predictions. The utility of this approach has been vigorously debated
of late, particularly in light of theories that claim we live in a multiverse,
where parameters may take differing values in regions lying outside our
observable horizon. Within this cosmological framework, we investigate the
efficacy of top-down anthropic reasoning based on the weak anthropic principle.
We argue contrary to recent claims that it is not clear one can either dispense
with notions of typicality altogether or presume typicality, in comparing
resulting probability distributions with observations. We show in a concrete,
top-down setting related to dark matter, that assumptions about typicality can
dramatically affect predictions, thereby providing a guide to how errors in
reasoning regarding typicality translate to errors in the assessment of
predictive power. We conjecture that this dependence on typicality is an
integral feature of anthropic reasoning in broader cosmological contexts, and
argue in favour of the explicit inclusion of measures of typicality in schemes
invoking anthropic reasoning, with a view to extracting predictions from
multiverse scenarios.
