As an analogy think about telling someone the story of your childhood and growing up and organizing it in episodes that depend either on how old you were or how big you were, when whatever it was happened---your family moved to a different town, you made some new friends, you started doing things your parents didn't know about with the neighbor girl out in the barn...whatever. You could either show with your hand, "when I was just this high, a toddler, my dad took us for a ride on the rollercoaster..." Or label the story by a fraction of your grown height, "when I was half-grown we used to get into awful fights in the schoolyard..." Or alternatively you could label each episode by your AGE: when you were 3, when you were 6, when you were 11 years old,...etc. ================= What I'm wondering is which seems preferable to you for running a model of the cosmos on: size or time? Which seems more convenient, informative, or natural to you---if you had to choose what the main variable input would be: to input a SCALE (when distances had grown to 1/5 or 1/4 of their present size...etc) or to input elapsed TIME since the start of expansion (when expansion had been going on for 2 billion years, for 5 billion years...etc)---which would you like the main variable to be? There may be no "right" answer---one might have to have a pair of calculators in tandem (not quite but somewhat as Ned Wright does) one where you key in a time and the other where you put in the scalefactor or its reciprocal, 1/a, the expansion which has occurred since the light was emitted. But if a tandem pair or a dual input option were NOT available, and you could only access the model in one mode, which would you feel more comfortable with or find more useful? Which makes the most intuitive sense to you?