  We identify 31 dimensionless physical constants required by particle physics
and cosmology, and emphasize that both microphysical constraints and selection
effects might help elucidate their origin. Axion cosmology provides an
instructive example, in which these two kinds of arguments must both be taken
into account, and work well together. If a Peccei-Quinn phase transition
occurred before or during inflation, then the axion dark matter density will
vary from place to place with a probability distribution. By calculating the
net dark matter halo formation rate as a function of all four relevant
cosmological parameters and assessing other constraints, we find that this
probability distribution, computed at stable solar systems, is arguably peaked
near the observed dark matter density. If cosmologically relevant WIMP dark
matter is discovered, then one naturally expects comparable densities of WIMPs
and axions, making it important to follow up with precision measurements to
determine whether WIMPs account for all of the dark matter or merely part of
it.
