This is a very basic question but what is the difference between isotropic and homogeneous? I mean, I can imagine a universe that would be isotropic but not homogeneous although this seems to select a prefered frame (am I worng on this?) But I don't understand how it would be possible to have a universe which would be homogeneous without being isotropic. It seems to me that homogeneous implies isotropic. And yet, I must be wrong because people would not insist on having both conditions satisfied.