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

[marcus]

, I had thought by tying the speed of the escalator to Hubble would allow for the backward, steady and forward motions as the photon traveled to its destination ...

I thought that illustrative example was apt. Didn't intend to sound overly critical. To me it suggests the moving sidewalks you get at airports, going down long corridors. Like an escalator but on the level, with no steps. And you stay on them for a long time, if it is a large airport.

the only thing is, as with all our familiar experiences when used as analogies, the analogy is always missing something. It is never perfect. So I wanted to pick out what was missing.
Different size distances expand at different speeds. So there is not just one speed, as there is with a moving sidewalk or escalator. You clearly realize this, but I thought I should emphasize that. Still a good analogy because we have all been on one, and tried to walk against the flow at some time in our lives. whereas not all of us have tried to swim or row a boat up river.

The following, I though made a really important point:
===quote Ogr8===
One of the biggest problems I see is what Terry Pratchett called "Lies we tell children." Such as electrons have orbits (Bohr model) and then later show the cloud model of Erwin Schrödinger, or the universe 'exploded' in a Big Bang and then later try to explain expansion and receding objects. I can't help but feel it would be easier if simplistic but ultimately false explanations were avoided even if this meant a more complicated but at least close to accurate version is first. Preconceptions are so hard to unlearn sometimes. Socrates taught us to ask why, now we seem to teach versions just to avoid having to answer questions until later.
==endquote==

And we are told with firm conviction that nothing can move faster than c. But then we learn that the distances to most of the galaxies we see today are, today, increasing faster than c. And that the distances to most of them WERE increasing faster than c at the time they emitted the light that we are now getting from them.

So somehow "receding" or becoming farther away is different from "moving" which happens when you actually get somewhere, approach a destination etc. IOW change your position relative to other stuff.

I think one should try not to be embittered or resentful of being mentally jerked around by the popularizers.
they are like professional translators of poetry. Our common language English is not perfect or complete, it still has to grow or evolve (dynamic changing geometry is one area where it might).

If you give a beautiful rhymed rhythmic richly meaningful German or Spanish poem to 3 different professional translators, and they each find the somehow analogous poem in English, with the best of intentions and professional integrity, you will probably still be able to find something wrong with each translation.

I'm not criticizing you for complaining about the physics and cosmology "Lies we tell to children". I am in a sense agreeing with you. Maybe the television science special producers should set higher standards, or maybe there should be a National Academy or Royal Academy review board, to vet the scripts of programs before they are shot and aired. It is difficult to know where to begin.

But in the meantime we can just see if we can do just a tiny bit better (and allow for our own inevitable partial failure as well).

 

[marcus]

Naty, thanks for looking up what Roger Penrose and Brian Greene had to say.I thought your own three paragraph draft introduction was pretty well thought out and worded also.

Clearly, the beginner has to believe what the figure tells. But what are we doing? We trust in the LCDM model.

Yes! Probably an honest introduction would have to say "this is an introduction to the best-fit model cosmos we have so far--it's generally agreed on to use for now, and based on our most reliable laws of gravity, geometry and motion, but cosmologists are working to find things wrong and ways to modify it. This is only the best fit to the data so far collected about the cosmos, by the observations made so far."

Or something like that.

 

[marcus]

One of the biggest problems I see is what Terry Pratchett called "Lies we tell children." ...
... I can't help but feel it would be easier if simplistic but ultimately false explanations were avoided even if this meant a more complicated but at least close to accurate version is first. Preconceptions are so hard to unlearn sometimes...

You got me to look up the "Lie-to-children" article in Wikipedia:
http://en.wikipedia.org/wiki/Lie-to-children
This is definitely something to think about. Pratchett seemed to be saying that the right L2C might serve to prepare the learner's mind for a better understanding later.

The article quoted something from Discworld which seemed to be justifying temporary pedagogical simplification, if it's well done.

I tend to believe, as I think you do, that misconceptions are hard to unlearn and have a serious cost, so I'd favor making something as truthful as you possibly can as long as you can say it reasonably concisely in the learner's own language, in words the learner already understands.
And if you have to use an unfamiliar word define it---make sure the learner knows what you mean.

It's something to mull over---what Pratchett said about Lies-to-children.

If we had a "Cosmology lies-told-to-children" thread here in this forum, I think I might now be expressing dissatisfaction with most of those we come across in the media. But what do you say *instead*?

 

[Ogr8bearded1]

I thought that illustrative example was apt. Didn't intend to sound overly critical. To me it suggests the moving sidewalks you get at airports, going down long corridors. Like an escalator but on the level, with no steps. And you stay on them for a long time, if it is a large airport.

the only thing is, as with all our familiar experiences when used as analogies, the analogy is always missing something. It is never perfect. So I wanted to pick out what was missing.

I didn't think you were being critical but that I had missed something in my example as presented. In fact, I was pleased when you said it had you mulling it over as it meant that I was actually able to contribute to your effort. In simplifying my analogy I think I entered into my own L2C problem. I will admit that sometimes when trying to explain what my mind is thinking it feels as if my brain is being twisted in my skull.

One thing I know I'm missing is the Hubble Distance or Boundary aspect and how the photon and space act when it enters that area. My model doesn't indicate the recession of the object that emitted the photon either. Also, if used, it should be pointed out that the appearance of a step at the bottom isn't new space being added but an inflation/expansion of the space already there. Your moving sidewalk could have better benefits too by making it easier to add additional ones to the side at different speeds and/or directions and have the photon move across to the next one as it enters another stage in the journey. At that point telling what the new movement of the next sidewalk indicated and how or why it relates to what we have observed.

Another place I see a lot of confusion happens is when talking about the Universe. Sometimes it means everything that can and cannot be seen (such as objects whose light will never reach us) and other times it only means the Observable Universe. While those more advance may know which is being referred to when talking about the Universe, others could be confused. Like in "what is the shape of the Universe," or 'the Universe is expanding' immediately makes one think "Into what?" If it was clear that we meant the Observable Universe is expanding, and its expanding into the rest of the Universe. Then we could say we don't know if the rest of the Universe is finite or infinite and work our way into inflation and how it is different from expansion and then not use the words interchangeably.

I don't have a problem with the L2C aspect of some models IF it is made clear at the time presented of the shortcomings it may have, what had to be left out or is not accounted for at the time or other such qualifiers. From what you said in your follow-up posts I can see you understand those issues already.

A section that covers old theories and why they have been discarded or don't fit new information we have learned could also be very important. Its too easy with the internet to find papers less than 20 years old by respected scientists that new data disproves or at least seriously brings them into question. In the past you could avoid these by staying with current publications and texts, now with the internet old and new appear together. With the way search engines work, the older works could even be more likely to appear before the new works.

 

[timmdeeg]

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

BTW Timdeeg, if you would glance back to post #21 and take a look at the graph of the red curve (D_then) and the blue curve (Hubble radius R).

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.

 

[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.