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Question:
I have just read the Wikipedia article on the Metric expansion of space. In that article, the writer presents the raisin bread analogy and says "The dough between raisins in this model acts as the space between galaxies while the raisins as "bound objects" are not subject to the expansion." I have wondered for 30 years why scientists consider the locality of mass as being bound -- exempt from expansion. In other words, the ant on the balloon can measure the expansion of his two dimensional space using a yardstick -- which is exempt from the effects of the expansion of space. It would seem to me that the yardstick would expand, the raison would expand, as well as the ant. This results in our small observers not being able to detect the expansion of space in any way. However as light progresses toward us from a source nearly 12 billion years ago, it travels through space that is expanding and is consequently red shifted. Somehow the energy of each photon is spread out over a greater "distance", its energy reduced, and wavelength increased. I can nearly buy that explanation.



Replies:
It is not so much that galaxies, stars, and people are "exempt" from metric expansion of spacetime as that the forces holding them together are strong enough to hold them together despite the metric expansion of spacetime. If you pull one raisin to the right and another to the left, the raisins will move apart form each other. On the other hand, if you pull the right side of a raisin to the right and its left side to the left, you will not get a bigger raisin unless you pull really hard - a lot harder than you need to just move two raisins apart.

Your visualization of red-shift is pretty good, at least as far as I understand it.

Richard Barrans
University of Wyoming



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