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

[AFTT47]

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.

My bolding of Brian's comments.

Brian, you are one of the strictest posters here regarding sticking to hard science. Where do you get off making an absolute statement regarding our longevity as a species with no justification whatsoever? Please tell us WHY you are so convinced we will not last that long as a species.

I happen to agree with you but probably for a different reason. I believe we will have so re-engineered ourselves by that point, we will not be the same species we are now. You make it sound like you are certain we will not survive in any form. Isn't that a bit presumptuous? Please correct me if I misinterpreted you.

[Alex K]

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.

Thanks Brian, your answer may be right, but I still think my question will help me understand what Alex said. He's a teacher, and a damn ggood one at that. In the couple years he's been working with me, he never once suggested that I couldn't understand something and I've always been able to zero in on the part of his answers that will hep me understand what he says. His patience is his gift.

I teach calculus to future child care workers with approx. 0 interest in maths. For that you either have infinite patience or you'll be very unhappy

[Succubus]

...Now the freaky thing about our universe's future, if we were to exist as a species, that long, which we will not,...

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My bolding of Brian's comments.

Brian, you are one of the strictest posters here regarding sticking to hard science. Where do you get off making an absolute statement regarding our longevity as a species with no justification whatsoever? Please tell us WHY you are so convinced we will not last that long as a species.

I happen to agree with you but probably for a different reason. I believe we will have so re-engineered ourselves by that point, we will not be the same species we are now. You make it sound like you are certain we will not survive in any form. Isn't that a bit presumptuous? Please correct me if I misinterpreted you.

My bolding I don't understand your objection.

[Gawdzilla Sama]

I am directly related to little mouse-types that lived in the Cretaceous. That does not mean I'm a late model Cretaceous mouse.

[Pat Mustard]

I understand about the rubber band/raisin bread dough analogies. My question is what made scientists look at the universe and say hey, you know what, the galaxies aren't moving. Space is expanding. What tipped them off to make this distinction?

Doppler effect. Pretty much everything we can measure which is outside the local group is showing red shift in its light spectrum over time, meaning it's getting further away.

[Brian37]

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.

---

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.

My bolding of Brian's comments.

Brian, you are one of the strictest posters here regarding sticking to hard science. Where do you get off making an absolute statement regarding our longevity as a species with no justification whatsoever? Please tell us WHY you are so convinced we will not last that long as a species.

I happen to agree with you but probably for a different reason. I believe we will have so re-engineered ourselves by that point, we will not be the same species we are now. You make it sound like you are certain we will not survive in any form. Isn't that a bit presumptuous? Please correct me if I misinterpreted you.

Because our sun and our solar system WILL DIE eventually. And no, sorry I do not see a day when humans will live forever, that is si fi crap. extend yes, forever no. I don't treat si fi woo any differently. If scientists are speculating at the future possibilities it is still up to them to get there not up to anyone to assume or blindly swallow.

[brewer]

I know that the space around my waist like is expanding galactically.

Or would that be contracting?

[bennyboy]

How do we know that rather than space expanding, matter isn't contracting, the idea being that light is exempt from shrinkage? What, for example, if one of the fundamental four forces is actually in flux?

This would also lead to a red shift, no?

[Alex K]

How do we know that rather than space expanding, matter isn't contracting, the idea being that light is exempt from shrinkage? What, for example, if one of the fundamental four forces is actually in flux?

This would also lead to a red shift, no?

Absolutely, because the receiving atoms would be smaller, the received light would look more long-waved to them.

If you take the equations of matter and spacetime in General Relativity, both interpretations are possible. One can interpret the change of the space metric as an increase in distance by changing the coordinates, or one can model it by rescaling all the masses and strengths of forces, which would lead you to a picture where the mass of the electron rises making the atoms smaller. This should be equally consistent, but one still has to change the definition of time (which comes natural though because all the physical clocks are ticking differently as well). The space expansion interpretation of the equations is much simpler though.

[Jehanne]

How do we know that rather than space expanding, matter isn't contracting, the idea being that light is exempt from shrinkage?  What, for example, if one of the fundamental four forces is actually in flux?

This would also lead to a red shift, no?

Absolutely, because the receiving atoms would be smaller, the received light would look more long-waved to them.

If you take the equations of matter and spacetime in General Relativity, both interpretations are possible. One can interpret the change of the space metric as an increase in distance by changing the coordinates, or one can model it by rescaling all the masses and strengths of forces, which would lead you to a picture where the mass of the electron rises making the atoms smaller. This should be equally consistent, but one still has to change the definition of time (which comes natural though because all the physical clocks are ticking differently as well).  The space expansion interpretation of the equations is much simpler though.

The nail in the coffin (in a positive sense) that proves the expansion of the Universe is gravitational lensing:

Distant quasars tend to change their brightness, causing them to flicker. As the light which creates the different images of the quasar follows paths with slightly different lengths, the images do not flicker simultaneously but are delayed with respect to each other by several days. This delay in flickering can be used to measure the Hubble constant which describes the speed of expansion of our Universe. While the relative time between two flickers is correctly represented in this animation, in reality the delays are in the range of days to two weeks.

https://phys.org/news/2017-01-cosmic-len...nsion.html