xMonty said:
i am guessing that the closer stuff's photons have passed us and the further stuff's photons haven't gotten to us yet, is this correct?
Absolutely right.
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There is another conceptual tool which I anticipate you might eventually find a need for. That is the idea of an observer being at rest with respect to the CMB.
An observer is at rest wrt Background if the temperature of the microwave sky is almost exactly the same in all directions.
No doppler "dipole" in the temperature map.
An observer moving at a significant speed wrt Background will see a doppler hotspot ahead and a corresponding coldspot to the rear. The forward temperature increase will be the same fraction of the average sky temperature as the speed of motion expressed as a fraction of the speed of light.
The solar system is moving (as a result of combined motions of the galaxy and the sun's orbit within the galaxy) in the direction of constellation Leo at a speed which amounts to 1/8 of one percent of the speed of light.
So the microwave background is 1/8 of one percent hotter in the Leo direction. And actually the galaxies in that direction appear to be receding from us at rates which are 380 km/s less than Hubble Law would have us expect. This has to be allowed for when data is interpreted.
"Morally speaking" to be at rest wrt Background is to be at rest with respect to the ancient matter of the universe, or at least the light that emanated from it. Or equivalently to be at rest relative to the expansion process itself (which has been called the "Hubble flow"). Before the CMB was observed they had this same idea of being at rest, but the accepted term for it was "comoving with the Hubble flow". The Hubble law only works, is the same proportion in all directions, for an observer at universe-rest. So even before CMB was observed, and its dipole measured, astronomers could detect the motion of the solar system wrt the expansion process, and adjust the data to eliminate it.
This idea of an observer at rest wrt Background is useful in several contexts. Kind of basic.
I appreciated the lucidity of your questions and your response. Thanks for trying out Ned Wright's calculator. He has a fun computer animation to watch which shows photons struggling to get from one galaxy to another while the distances between galaxies are increasing. If you want, google "wright balloon model".
In the balloon model, points which stay at the same latitude/longitude are at rest wrt the expansion process, equivalently at rest relative to Background. In the animation, the galaxies (being approximately at rest) stay at the same location on the balloon surface, while the photons move at some fixed speed like 1 millimeter per second. Of course it is merely a 2D analogy, a 3D hypersphere is hard to imagine, so one resorts to the 2D toy version.