The KSVZ Axion Model with Quasi-Degenerate Minima: A Unified Model for
  Dark Matter and Dark Energy
  We consider the possibility that dark matter and dark energy can be explained
by the minimal KSVZ axion model. This is possible if the lowest energy
metastable minimum of the scalar potential has zero energy density, which is
possible in theoretical models of vacuum energy cancellation based on spacetime
averaging and in models based on energy parity. Dark energy is then understood
as being due to the energy density of the metastable electroweak vacuum
relative to a second quasi-degenerate metastable minimum. The requirement of
quasi-degenerate minima is a non-trivial condition which completely determines
the form of the potential for a given value of the axion decay constant,
$f_{a}$, and the PQ scalar self-coupling, $\lambda_{\phi}$. The existence of
the second quasi-degenerate minimum imposes a new lower bound on the axion
decay constant, $f_{a} \geq 2.39 \times 10^{10} \, \lambda_{\phi}^{-1/4}$ GeV.
If the PQ symmetry is broken after inflation then the lower bound on $f_{a}$
implies a lower bound on the amount of axion dark matter,
$\Omega_{a}/\Omega_{dm} \geq (0.28-0.46)\,\lambda_{\phi}^{-0.291}$, where the
range is due to the uncertainty in the amount of axion dark matter produced by
vacuum realignment, cosmic strings and domain walls, therefore at least 30$\%$
of dark matter must be due to axions if $\lambda_{\phi} \lesssim 1$. When
axions constitute all of the dark matter and the PQ symmetry is broken after
inflation, $f_{a}$, and so the form of the scalar potential, is completely
fixed for a given value of $\lambda_{\phi}$, with only a weak dependence on
$\lambda_{\phi}$. This will allow the inflation and post-inflation evolution of
the model to be quantitatively studied for a given inflation model and
dimensionally natural values of $\lambda_{\phi}$.
