Originally posted by rdt2
...If space itself is expanding, aren't all our measuring sticks also expanding? ...- answers in words of one syllable would be appreciated.
this is the most frequently asked question, or failure to ask it is the most frequent source of misunderstanding, in all of PF
At least AFAIK (as far as I know, such a useful abbr.!)
The answer SHOULD be available in words of one syllable. That is a very good idea. The answer is quite easy, but, since it is never given in words of one syllable, the people who ought to get it never do quite get it and so keep on asking and asking.
Suppose you have a steel measuring tape made mostly of iron atoms.
there is a preferred distance apart which is most comfortable for them, at room temp, to sit.
So iron atoms have a preferred spacing, and some number of spacings N will constitute a meter, or a kilometer.
Suppose you then stretch the tape. It will still want to go back to the earlier spacing. A meter will stay a meters----N iron atom spacings.
So you have two "galaxies" or other objects, say a kilometer apart. And distances between galaxies increase over time (this is what space expanding means). So you run the kilometer-long metal tape between them and wait for a billion years and come back and find they are say 1 percent farther apart. This has stretched the tape, and it does not like this. It has been stretched to where it is 1.01 kilometers long.
So you release one end and it contracts back to where it is 1.00 kilometers long. The "galaxies" are 1.01 km apart and the tape no longer reaches. After all the tape has only so and so many iron atoms in it (nobody came along and put more iron in the tape) and they have a preferred spacing that depends on their electric charge and their electrons.
So the tape does not quite reach any more.
This is what it means for galaxies to get farther apart. They get more METERS apart. And meters stay the same thing.