'Returns dark energy equation of state at redshift ``z``.
Parameters
z : array-like
Input redshifts.
Returns
w : ndarray, or float if input scalar
The dark energy equation of state
Notes
The dark energy equation of state is defined as
:math:`w(z) = P(z)/\rho(z)`, where :math:`P(z)` is the
pressure at redshift z and :math:`\rho(z)` is the density
at redshift z, both in units where c=1.  Here this is
:math:`w(z) = w_0`.'

def w(self, z):

    if np.isscalar(z):
        return self._w0
    else:
        return (self._w0 * np.ones(np.asanyarray(z).shape, dtype=np.float))
