Re: [Phys-l] Cosmic background radiation

[John Denker <jsd@av8n.com>]

On 11/06/2006 12:23 PM, Aker, Leanna wrote in part:
> ....
> students calculate the age for the universe based on how much galaxies
> appear to have moved in a certain amount of time (the materials are two
> copies of many dots, the top one a transparency with the dots expanded
> on a copier).  

The problem with copiers is that when the universe is expanded
with a _copier_, the stuff in the universe (i.e. the dots)
gets expanded in proportion, which is not how the real universe
works.

I made a movie
  http://www.av8n.com/physics/img48/expansion.gif

This shows the dots moving away from each other (velocity
proportional to distance) ... but the dots themselves do
not get bigger!

The dots can represent atoms in the intergalactic gas, or
dust particles, or any other physics where the intra-dot
forces are strong enough to preserve the size of each dot,
while inter-dot forces are not strong enough to overcome
the "tidal" stress that tries to make everything expand.

The distinction I'm making is important, because if our rulers
etc. expanded along with the universe, the expansion would be
unmeasurable and indeed meaningless.