cosmology - How could we know that the universe is homogeneous and isotropic at a moment?

Cosmologists say that the universe is homogeneous and isotropic within our Hubble volume based upon the astronomical observations. But how can we argue that the universe is homogeneous and isotropic at any particular moment while we can't picture the entire universe simultaneously due to the finite speed of light? i.e. when we picture the universe we picture the past. The further we go the further in time we observe.