Jeremy87 said:
So basically, mathematically it's correct that space is indeed expanding?
Would it still be "OK" to think everything ...
I'd say GR's correct in the provisional sense that nobody has yet come up with a math model that gives a better fit to the observed changing geometry of the world. All human theories eventually get improved or replaced. No final ultimate "correct".
GR says more than mere uniform expansion. You can have expansion at different rates in different directions (which could never by imitated by mere shrinkage of objects).
GR says you can have expansion along one axis and contraction on another, briefly, and then it can reverse. There can be vibrations in geometry, in effect. Not the kind of thing you could make a "shrink" copy of.
If someone hasn't been paying attention they may think that GR says that the whole universe is undergoing absolutely uniform expansion by the same amount at the same time-rate everywhere, so
the appearanceof that could be mimicked by uniform shrinkage.
But that is not what is really happening. That is just a nice on-average
approximation that you get from GR by making the simplifying assumption that matter is evenly distributed thru space and everything is happening uniformly.
That is a wonderfully useful approximation, because on large enough scale matter does indeed seem to be roughly uniform. The number and sizes of galaxies are more or less on average the same from place to place. But it is not true down at the level of local detail.
So there is a lot going on with the geometry of space and time that we OBSERVE and which would not be mimicked by some "shrink" scenario.
And time doesn't even run at the same rate everywhere. So if your scenario requires shrinking photons (as you say) and everything else except for large-scale distances, you would have a hard time saying on what
time-table and at what
rate to shrink things. It would differ all over the map and it would change from time to time even in a given location.
So to answer your "is it OK?" question: sure it is OK to think about the cosmos any way you please. We are free to fantasize the world as we like best. And science does not provide absolute truths, only what might be the simplest most reliable model at any given time. We grope our way towards a better understanding of still-mysterious nature.
So if you really enjoy picturing some kind of pattern of shrinkage, it is surely OK!
But I would say it is not
advisable. You'd miss a lot of the richness that way and put obstacles to your further understanding. You're more apt to get led down a blind alley, I think, and be poorly equipped to appreciate how dynamic geometry is.
What do you find that's so hard to understand about the basic expansion model of the cosmos?
It is a simplified approximation derived from GR (our welltested law of gravity). It is far from being the whole GR theory! Just a simplified picture derived from the full theory that fits the largescale date remarkably well.
It says basically that around year 400,000 space was filled with hot gas, uniform to within a tiny fraction of a percent, so we approximate by saying uniform. It was just then cooling enough to become transparent---it was about 3000 degrees kelvin then.
What has cooled that gas is the expansion--that is what has allowed it to gather into denser patches and condense into stars. The expansion of distances since then has been roughly 1000 fold, so the expansion of volumes has been roughly a billion fold.
We still see the ancient light that was released by the glowing hot gas (3000 degrees) at that time, because it had just cooled enough to become transparent. Since distances have increased 1000 fold since then, the temperature of the light has gone DOWN by 1000 fold. (short wavelengths = high temperature, longer wavelengths = cooler light) So the temperature is now about 3 degrees kelvin, and we measure that. We are still swimming in that soup of ancient light that comes more or less uniformly from all directions.
Everything since year 400,000 pretty much follows from that. Cooling by expansion, condensing into stars, gathering into larger clouds and clusters. Slowing of motions relative to the background soup of ancient light. Some lesser but interesting details about the evolving RATES of expansion. That's the broad outline.
That's the basic story of expansion cosmology. I'm curious to know what, if anything, you find hard to understand about it---so hard you flip over to wanting to picture stuff shrinking.
Of course there is EARLY universe history, what happened before about year 400,000 of expansion, especially around the very start. That is not so well understood.
But the story from the moment of clearing, when the hot gas cooled enough to turn transparent and the ancient light was released, that story is well understood and comparatively simple.
If you had to say what the hardest thing to understand about it is, what would that be? I'd like very much to know because there may be others who get stuck at the same point.