Originally Posted by batema
Thank you all but I have one last (at this time) question. The concept of it being uniform in every part of the sky when measured, is this uniform nature due to photons travelling from the same distance or is it uniformly distributed throughout the observable universe like the spread out between here and jupiter. What I am trying to ask is what we are measuring do the photons come from cool gas that is 380 000Ly less than 13.7 biil Ly away. The image in my head is of a balloon in which the edge is the source of the CMBR and it has travelled for this time to us and due to the expanding nature of space time has stretched by a factor of 1000 and therefore appears in the microwave section of the electromagnetic spectrum. I am a high school physics teacher and want to get this right before I blow my students mind.
There are times where the ballon model, as an explanation for our observations, just doesn't 'cut it'.
The present Standard Cosmological Model has been proposed to help us visualise what we observe. It may sound easy to invent a 'story', which fits everything we observe, but it isn't. Give it a try and you'll see what I mean. (Come to think of it, this would be an awesome experiment for your students to try to come up with a 'theory' which has a good fit for something which has many observable, (ie: repeatable), phenomena).
There are many, many, many empirical observations, mathematical certainties, and physical empirically demonstrable facts behind the Cosmological Model.
So with this model, the way to interpret the CMBR we observe, (and have it fit in nicely amongst other observable phenomena), would be to visualise it as being due to the extreme age of the photons. The photons would be scattered throughout the universe evenly. Now, with Hubble's Law and redshift being caused by expansion, the corollary to this would be that the things with the higher redshifts, are caused by the things moving away the fastest, and because the things moving the fastest are further away, we can infer that the CMBR is also the oldest thing in our observable universe and hence, the farthest away from us.
The ballon model fails when it comes to explaining what is really intended by 'uniform expansion' of the universe. As Steven said, all parts of the larger scale universe expand evenly, and it doesn't matter where you sit in that universe, the expansion in all directions seems the same.
My new catch cry which also applies to the Big Bang Theory, (or more precisely, the Standard Cosmological Model of the Universe) is:
'Try it on !' …. 'see if it fits (the facts) !' …. but whatever you do … 'don't believe
This 'catch-cry' certainly applies in the case of the present Standard Cosmological Model. For the time being, it provides an unprecedented best fit match for most of our observations.
Hope this helps.