Planets in Other Universes: Habitability constraints on density
  fluctuations and galactic structure
  Motivated by the possibility that different versions of the laws of physics
could be realized within other universes, this paper delineates the galactic
parameters that allow for habitable planets and revisits constraints on the
amplitude $Q$ of the primordial density fluctuations. Previous work indicates
that large values of $Q$ lead to galaxies so dense that planetary orbits cannot
survive long enough for life to develop. Small values of $Q$ lead to delayed
star formation, loosely bound galaxies, and compromised heavy element
retention. This work generalizes previous treatments: [A] We consider models
for the internal structure of galaxies and find the fraction of galactic real
estate that allows stable, long-lived planetary orbits. [B] We perform a large
ensemble of numerical simulations to estimate cross sections for the disruption
of planetary orbits due to interactions with passing stars. [C] We consider
disruption due to the background radiation fields produced by the galaxies. [D]
One consequence of intense galactic background radiation fields is that some
portion of the galaxy, denoted as the Galactic Habitable Zone, will provide the
right flux levels to support habitable planets for essentially any planetary
orbit. As $Q$ increases, the fraction of stars in a galaxy that allow for
habitable planets decreases due to both orbital disruption and the intense
background radiation. However, the outer parts of the galaxy always allow for
habitable planets, so that the value of $Q$ does not have a well-defined upper
limit. Moreover, some Galactic Habitable Zones are large enough to support more
potentially habitable planets than the galaxies found in our universe. These
results suggest that the possibilities for habitability in other universes are
somewhat more favorable and far more diverse than previously imagined.
