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What Does Hubble's Law Actually Mean?

Why does Hubble's law work? Now its time to to imagine!

expanding raisin bread

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.

girl thinking hard

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!

Quiz Click here for a quiz on what Hubble's law means.
Return Click here to return to the Doppler shift explanation.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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