Cosmology and Geometry (4)
The balloon analogy in two dimensions illustrates simply the idea of comoving
coordinates.
If instead of viewing the expansion of the balloon from the outside
I place myself on one of the dots on the balloon's surface, I appear to be
stationary and I see the other dots moving away from me (and in my immediate area
I see the apparent curvature of the balloon's surface decreasing).
Astronomers will also often speak (in three-dimensional space) of a comoving
volume. This means a volume of space that is moving uniformly with the
expansion.
Illustration in Two Dimensions
The above right animation illustrates the difference
between viewing the surface of a balloon from outside or from
comoving coordinates on its surface
(Ref).
Here are some still frames from this animation that will help clarify what you are watching:
Comoving coordinates in the real Universe correspond to extending this idea to three
spatial dimensions.
