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Re: Recession of Galaxies Metaphor



At 08:38 AM 2/21/01 -0800, Carl C. Gaither wrote:

"Imagine a giant jungle gym, with galaxies at every intersection.
Suppose you are invested with the power to make that jungle gym grow in
all directions. (Maybe it's made of living bamboo!) What would that
look like, from the point of view of any individual galaxy? If the
jungle gym doubled in size, then a galaxy's closest neighbor would be
twice as far away--at a distance we might call 2 JU (for Jungle
Units)--and it would be receding. The most distant neighbors, let's say
ten junctions away, will also double in distance, ending up 20 JU away.
So, as viewed by that particular galaxy, all the others are receding,
and the more distant ones are receding faster. In fact, an expanding
framework produces Hubble's Law, with all galaxies seen as moving away
at velocities or recession just proportional to their distances." (p.
48)

This is not bad, but it might benefit from a little refinement, to reduce
the potential for misunderstanding.

1) At an absolute minimum, we need to add a statement that the galaxies are
not expanding, only the distance between them.

2) One needs to ask whether they are lone-wolf galaxies, or bound in
clusters, in which case they wouldn't even move apart.

3) One needs to make it clear that the measuring unit is NOT defined by the
lattice of the jungle gym. This is troublesome, because other books use a
jungle-gym-like metaphor for the measuring system.

===========================

I appreciate the value of a D=3 metaphor over the usual D=2 balloon+pennies
metaphor. What I use is raisin-bread dough:

As the dough rises, the raisins move apart. The raisins don't get bigger,
just farther apart. A ruler embedded in the dough isn't significantly
affected. Any raisin sees all the others retreating, with velocity
proportional to distance. This is true for a raisin in the middle or
anywhere else. This is true whether the entire "universe" of dough is the
size of a bread-pan, or the size of a swimming pool, or even light-years on
a side.

I've never done it, but for students of a certain age it might be fun to
actually whomp up a batch of bread dough and let them watch it rise. Then
they get a biotechnology lesson along with their physics.