What Does Hubble's Law Actually Mean?
Why does Hubble's law work? Now its time to to imagine!
Don't get too hungry looking at these loaves of raisin bread. You are
not allowed to eat them! They will be used instead as a model for
you as you learn a little something about the structure of the universe.
First consider the small black dot at the far left of the image. Imagine
that just a few moments ago this loaf of raisin bread, raisins and all,
could be represented by this single dot which is a point we call a
singularity. It wasn't a point in space, however......... it was
space itself and inside that space were the raisins which we can imagine
make up the matter known in the universe. At this initial time, however, it
was very, very, small.
In real life as a loaf of raisin bread bakes, the yeast in the bread
makes the dough rise and expand. This expansion fills some of the "void"
in space around the breadpan. Our model of the bread as an expanding
universe takes on a different meaning (since the Universe as we know it is
not expanding into anything, such as another dimension.
There is just more space itself. The expansion of the dough in our
model represents the expansion of space itself and in the process the
raisins, which represent the matter we find in space, move away from each
other in all directions.
It's hard thinking about all this stuff!
Look carefully at the raisins in each loaf of bread. They are hard to see
but you can find them. Notice something subtle about the expanding loaf.
While the bread part of the loaf gets larger, the raisins remain the same
size! The matter in our universe on the grand scale is
not expanding. That would be a violation of the principle of conservation
of mass. However, the space around the matter is expanding. Does that
seem "raisinable" to you????
Think of two raisins, one of which is twice as close to you initially. If
the space between everything in the raisin bread is expanding uniformly, in
the time it takes the loaf to expand so that the closer raisin is now
twice as far away, the further raisin is now four times as
far away (the loaf is twice as big). In this time, therefor, the first
raisin has traveled a distance d (2d  d) while the second raisin has
traveled a distance 2d (4d  2d). The velocity you would observe for the
first raisin is d/t while that of the second raisin is 2d/t. Thus you see
that if the loaf is uniformly expanding, the velocity of distant objects
is directly proportional to their distance away from you, which is what
Hubble found!
