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The Question

(Submitted June 19, 2002)

I have been trying to understand the idea of an expanding Universe. As I understand what I have read, this concept is based on relativity. In this model, the universe should not be seen as simply a sphere with an expanding radius but rather it should be thought of as space itself expanding. If this is so, it would seem that even "local" space would be expanding and therefore the dimensions at the atomic level would be increasing (i.e., if the distance to a distant qalaxy doubles over a few billion years, then atomic dimensions also double, atomic spectra would shift, etc.) I assume that this is not so, but why? (A reference to a good book or paper would be helpful)

The Answer

This is a good question --- a very short answer is that local forces are dominant on the scale of atoms and they, rather than the large-scale structure of space-time, determine the structure of an atom. Even on the scale of galaxies, our own is being drawn toward Virgo cluster of galaxies. The expansion of the universe becomes the dominant factor only on the largest scales.

For a longer explanation, with references, see this page (part of the Usenet physics FAQ):

http://math.ucr.edu/home/baez/physics/Relativity/GR/expanding_universe.html

Hope this helps,

Koji & Georgia
for "Ask an Astrophysicist"

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