If you placed a giant ruler between the sun and our nearest star for example, wouldn't the divisions on the ruler expand at the same rate as the universe - therefore maintaining a constant distance? If you draw 5 divisions between 2 points on a balloon, there will always be only 5 no matter how much you inflate the balloon.
How can this be reconciled with observed red-shift?
By expansion, we don't mean only the surface of the sphere is expanding; it is a volume expansion rather than surface expansion. We believe that in a short period of time (compared to the age of Universe,) the rate of expansion is fixed. However, this doesn't mean that the relative velocity between any two objects is the same. Depending on the "distance" between two objects, the relative velocity would be different. This is why the "distance" will never be fixed in such a volume expansion. And, if you do some research, you will be surprized that we have different definitions of distance in large scales and that's why I used quotes when referred to it.
Thanks,
The raisin bread analogy of the expansion of the universe is useful to get an intuition on what is the process.
Why are the raisins not expanding at the rate of expansion of the bread? Because the dough is a soft elastic medium while the raisins are solid.
In exactly the same way, the stability of clusters of galaxies , galaxies and planetary systems is retained because the attractive force of gravity is much stronger than the effective dispersive force exerted by the expansion of the universe . This is more so for systems bound by electromagnetic and strong forces, as your imaginary ruler.
