•Formalism is built on notion
that the universe is homogeneous and isotropic, at least
on the largest scales. Is it???
•Locally the universe is not
homogeneous. Is
this an issue? Keep in mind.
•Characteristic gravitational
timescale: tc = 1/(Gρ)1/2 ~ 1010 years
•Also light speed gives us a natural distance scale: L ~ c/(Gρ)1/2 ~ 3000 Mpc