  Cosmological observables are used to construct cosmological models. Since
cosmological observations are limited to the light cone, a fixed number of
observables (even measured to arbitrary accuracy) may not uniquely determine a
cosmological model without additional assumptions or considerations. A
prescription for constructing a spherically symmetric, inhomogeneous
cosmological model that exactly reproduces the luminosity-distance as a
function of redshift and the light-cone mass density as a function of redshift
of a $\Lambda$CDM model is employed to gain insight into how an inhomogeneous
cosmological model might mimic dark energy models.
