  To derive loop quantum cosmology from loop quantum gravity, I apply the model
given in \cite{lin1} to a system with coupled gravitational and matter fields.
The matter sector consists of a scalar field $phi$ serving as a cosmological
clock, and other fields ${psi}$ providing physical spatial coordinates and
frames. The physical Hilbert space of the model is constructed from the
kinematical Hilbert space of loop quantum gravity, and the local observables in
the physical Hilbert space are constructed using the matter coordinates and
frames. A specific coherent physical state is then chosen, whose expectation
values of the local observables give rise to homogeneous, isotropic and
spatially flat gravitational and $phi$ fields at a late clock time. The
equations governing these fields may be derived using the symmetry of the
physical Hilbert space. When the matter back reactions from ${psi}$ are
negligible, the result gives a specific loop quantum cosmological model in the
$O(hbar^0)$ approximation, with calculable higher order corrections.
