The Axidental Universe
  Theories with several hundred axion fields have enormous numbers of distinct
meta-stable minima. A small fraction of these local minima have vacuum energy
compatible with current measurements of dark energy. The potential also
contains regions suitable for inflation, and gives rise to a natural type of
dark matter. First-order phase transitions from one minimum to the vicinity of
another play the role of big bangs and produce many bubbles containing evolving
Friedmann-Lemaitre-Robertson-Walker universes. The great majority either
collapse in a tiny fraction of a second, or expand exponentially forever as
empty, structureless universes. However, restricting to those bubble universes
that form non-linear structure at some time in their history we find
cosmologies that look remarkably similar to ours. They undergo about 60 efolds
of inflation, making them flat, homogeneous and isotropic, and endowing them
with a nearly scale-invariant spectrum of primordial density perturbations with
roughly the observed magnitude and tilt. They reheat after inflation to a
period of radiation domination, followed by matter domination with roughly the
observed abundance, followed by vacuum energy domination at roughly the
observed density. None of these features require any model building or small
parameters. Instead, all dimensionful parameters in the theory can be set equal
to the grand unified scale 0.01 M_p, and the dimensionless parameters are order
one and can be chosen randomly. The small value of dark energy ultimately comes
from non-perturbative gravitational effects, giving an exponentially small
vacuum energy density. Therefore, random axion landscapes can account for many
of the apparently tuned features of our universe, including its current
enormous size, age, and tiny energy densities compared to the scales of
fundamental physics.
