The Raisin-Bread Universe

One of the most difficult ideas to convey in my classes on Cosmology is the notion that the universe can be expanding without there being a "center" that it is expanding away from.  This comes up especially in the context of interpreting the original data taken by Edwin Hubble used to make the first observational arguments that the universe is expanding, and originated in a "Big Bang".  Hubble (and his collaborator Humeson) discovered that all distant galaxies appear to be flying away from us, with the most distant ones flying away the fastest.  This would seem to imply that all of the galaxies used to be closer together, but it would also seem to imply that we're at a special central location (otherwise, why is everything rushing away from here?).

The correct interpretation of Hubble's observations is that space itself is expanding.  The galaxies aren't exactly moving, but the space in between them is growing over time.  No matter what galaxy you happen to live in, the expansion of space itself makes it look as though all the rest of the galaxies are moving away from you.

The traditional analogy that is used in this scenario is the "raisin bread universe".  Imagine that every galaxy is a raisin in a loaf of raisin bread.  As the bread bakes, the whole loaf expands.  The raisins all used to be closer together, but as the bread expands they all become more distant from each other.  No matter what raisin you happen to be, it looks as though all the other raisins are running away.  Same thing with the galaxies in the universe.

This is a pretty potent analogy for grasping what's meant by the expansion of the universe.  Yet, it's rarely given a good visual form.  Here are a few versions I could find:



The version above is from this website.


Here's an animated gif:

This one is from this webpage.

What do you think?  Could you do better?  Do these images work to convey the idea?