Oh, hey, on the CMB front Peter Coles (http://telescoper.wordpress.com/) of Cardiff University says it better here https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html (1999). I strongly recommend the second paragraph, starting, "To visualise...", (with the tiny caveat that there is an unstated choice of gauge in the "about twice the speed of light"), and the fourth paragraph starting, "Imagine...".