How Do Scientists Know It's Space Expanding Not Galaxies Moving?

[Brian37]

When space expands, redshift is the sum of all expansion during the light travel. When Galaxies move, the redshift from the Doppler effect is the difference between movement speeds at the time of sending and receiving. Big difference.

Wow, I only had to read this three times to understand it. I think I'll have a clearer understanding of what you're saying if I understood redshifting mathematically. I'm thinking of it as a color. The color may fluctuate in intensity because of varying circumstances, but I still don't know enough about it to think of it as having sums and differences.

The math and formulas involved in cosmic distances can be confusing to laypersons. But really it is not a different principle than using a tape measure and say a cop's radar gun to detect speed of a car. I am over simplifying certainly. It is a slight bit deeper than that. 

But think of it this way, how many people can actually design and build a car from scratch, including the motor block itself. I don't mean plopping an old motor into a new chassy, but designing a mold, melting the metal, to make the block and the pistons. Then design the chassy itself, the interior the electronics.

But, you can certainly understand the idea of combustion, that is simple. Spark plug ignites the gas, the gas expands causing the pistons to move, in turn the arms of the pistons rotate the crank shaft, which turns the axle which turns the wheels.

Not many humans can understand say the deep math say at the level of a Hawking so if you don't get some of it that is ok, if you can, that is great, but anyone can certainly understand basic concepts.

As far as measuring cosmic distances and movement between galaxies it starts simply by measuring local stars then moving further outwards and they judge the age and color and size and position between other bodies along with speed. I don't know the formulas they do that with, but that is the basic idea, if I am not correct.

To the naked eye without a telescope everything looks like a star, but not every light in the sky is, some of those are entire galaxies in the deep backfield when looking at it with the naked eye with no aid. But, there was a time long ago where scientists originally thought our galaxy was all their was.

Now the freaky thing about our universe's future, if we were to exist as a species, that long, which we will not, but if we did exist long in the future, there will be a time when all those galaxies will be so far away it would seem the same way as it did before we knew there were other galaxies. If you think the universe is big now, you ain't seen nothing yet.

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You can also think of expansion this way. The galaxies are not moving but the space between them is?

Ok, say you are studying the ice caps melting. So you put a poll at one end say 100 yards, and a poll at the other end, and you see a crack at the 50 yard line. well the ice is floating on water, in the case of the universe, expansion, which causes each of those chunks to move further away from each other. By knowing the color and size of the poll, a sun in another galaxy you can judge the ice chunk galaxy's motion away from the other poll which you also know the color and size of.

Now anyone feel free to correct me if I am way off, but that is the way I am picturing it.