Mrs. Pinetar made fun of me tonight because I was trying to explain this to our five year old.
He soaked it all up and seemed to enjoy it. How hard can it be? It went like this:
“Buddy, a guy in the 1920’s looked through his telescope and found that everything was flying away from us. So the whole universe must be expanding. But since Gravity is an attractive force between all the “stuff” in the universe, we must be slowing down some, but they look really really far away, they see that the stuff is flying apart faster and faster and they don’t know why!”
Then he wanted to know about relativity and I said, “oooh, that’s hard, daddy barely understands that. Relativity says that when you go really fast, time slows down for you, and you get smaller and heavier. I told you it was hard to understand, but you understood that Dark Energy thing, right?”
And he shook his head, “not really”.
Ok, but we tried. We had just watched an hour documentary on “space stuff”. He was all fired up.