Expansion of the Universe: Is There a Way to Tell?

[Darken-Sol]

everything I've read points to expansion of the universe. it seems to me that if everything were getting smaller the results would appear the same as expansion. is there a way to tell the difference? is there a difference, with nothing observable to compare size?

 

[russ_watters]

The two ideas really don't have much in common and considering that some of our particles are point particles already, I don't see how it could be possible for things to be shrinking. Remember, we can measure the expansion, but only on galactic scales. If "everything were getting smaller", it would have to include things on a small, local scale.

 

[HelloRd]

If it was a metric contraction or inversely proportional to the gravity field, how could we notice any local shrink? Isn't it the same as gravitationally bound structures wrt expansion of space?

 

[bcrowell]

http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html#Section8.2
See subsection 8.2.6.

 

[Darken-Sol]

some of our particles are point particles already

i am confused as to these "point particles". how do they differ from a singularity? not to get off subject.

 

[Chalnoth]

everything I've read points to expansion of the universe. it seems to me that if everything were getting smaller the results would appear the same as expansion. is there a way to tell the difference? is there a difference, with nothing observable to compare size?

Well, we usually consider atoms to be stable in size. With atoms being stable in size, there is no ambiguity: our universe is expanding. This point of view makes good sense to us, because we are made of atoms, and don't see ourselves as getting bigger or smaller (except through normal biological processes).

However, it is quite possible to write down equations for a universe that is static, but things within it (including atoms) are shrinking. This isn't really a very significant point, however, because it's just a statement that our definition of length is arbitrary, so that it's perfectly possible to define a different length scale that, according to our usual definition of length, changes in time. In fact, it turns out that this is a rather useful thing to do in cosmology, where we often use what are known as "comoving coordinates". Comoving coordinates are so-called because they move along with the expansion. So in these coordinates, there actually isn't any expansion, but things within the universe are getting smaller.

We usually don't think of it in that way, however, and translate back to coordinates where the matter, not the coordinate system, is expanding when measuring distances.