What Does Hubble's Law Mean?
You've probably heard it said that Hubble's Law tells us that the Universe is expanding, but how do we get from the plot of Hubble's Law to an expanding Universe?
(Hubble, Proceedings of the National Academy of Sciences, 1929, 15, 168)
One way to think about the expanding Universe is to imagine that the Universe is a loaf of raisin bread. The galaxies are the raisins, and the dough is the space between galaxies or large structures. When the loaf is left to rise, the raisins get further and further apart because there is more space between them.
(Credit: D. Ponzio and NASA's Imagine the Universe)
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 bread pan. 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.
Note that the raisins in our loaf remain the same size even as the bread itself gets larger. This is similar to the matter in our Universe – the matter is not expanding. Instead it's the space around the matter that's expanding.
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, therefore, 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.
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