Idea for cosmo Basics (minimum to understand meaning of expansion)

[marcus]

I am sorry, Marcus. I have just noticed that I somehow missed your post completely.


I think it's possible to make the diagram understandable, but would show the time axis vertically in order to be conform with the usual spacetime diagram showing the expansion of the universe. With a few comments it's in fact almost self explanatory. Otherwise the beginner searching the web could be confused.
As the expansion acts on photons and matter as well, I would show some worldlines of increasingly redshifted galaxies additionally.

More like the top row of this figure?
http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg

It has the pear-shaped or plum-shaped past lightcone but it does not have the trumpet-shaped future lightcone. It does have the worldlines of galaxies which we are seeing with various redshifts, which you mentioned.

I understand your thinking that is a very clear diagram, if that is the kind of thing you mean.

I have a slightly different point of view though. I would like to see beginners learn to use the table and diagram-making calculator. To actively manipulate the model, not just look at already-made diagrams. I don't see the horizontal time versus vertical time issue as making as big a difference.

 

[timmdeeg]

More like the top row of this figure?
http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg

Yes, Fig 1 in the paper "Expanding confusion", Davis&Lineweaver. It contains all what - at least to my opinion - belongs to the basics regarding the expansion of the universe, the worldline of the photon we currently receive (emitted then) , those of the galaxies, the Hubble sphere and the event - and particle horizon. The explanations regarding Fig 1 seem understandable even to the 'interested' layman.
Once recognising these interrelationships it might undoubtedly be worthwile to play with the calculator. And I agree it's good to show the future light cone too, with some care however, as nobody knows how an eventual time dependence of the dark energy would influence it. Well, it is anyhow interesting to simulate different scenarios .
After having clarified the CP (especially isotropic expansion) and 'at rest' the next step could involve dynamics. The Friedmann equations boiled down to energy density and pressure their meaning is quite easy to grasp. Almost no math is needed.