Kolmogorov complexity in the Milky Way and its reduction with warm dark
  matter
  We discuss the Kolmogorov complexity of primordial patches that collapse to
form galaxies like the Milky Way; this complexity quantifies the amount of
initial data available to form the structure. We also speculate on how the
quantity changes with time. Because of dark-matter and baryonic collapse
processes, it likely decreases with time, i.e.\ information sinks dominate
sources. But sources of new random information do exist; e.g., a central black
hole with an accretion disk and jets could in principle broadcast small-scale
quantum fluctuations over a substantial portion of a galaxy.
  A speculative example of how this concept might be useful is in
differentiating between warm (WDM) and cold (CDM) dark matter. With WDM, the
initial patch that formed the Milky Way would have had few features, making the
present high degree of structure a curiosity. The primordial patch would have
had only several billion independent information-carrying `pixels' if the WDM
particle had a mass of 1 keV. This number of `pixels' is much less than even
the number of stars in the Milky Way. If the dark matter is proven to be warm,
the high degree of structure in the Milky Way could have arisen in two ways:
(1) from a high sensitivity to initial conditions, like an intricate fractal
arising from a relatively simple computer code; or (2) from random information
generated after the galaxy formed, i.e.\ not entirely deterministically from
the initial conditions.
