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

[vorlon13]

From the 1987 supernova, we could measure (via spectroscopy) how fast the side of the explosion facing us is coming towards us.

We can also see how fast the thing is expanding side to side. Pretty easy to work out how big it is, very precisely, at any given time, and then by seeing how big it looks from here, it's a simple matter to calculate very precisely how far away it is.

And the more precisely we know how far away stuff is that is relatively close, we can work out other associations and calibrate them to see how far away more and more distant objects are.

[Anomalocaris]

From the 1987 supernova, we could measure (via spectroscopy) how fast the side of the explosion facing us is coming towards us.

We can also see how fast the thing is expanding side to side.  Pretty easy to work out how big it is, very precisely, at any given time, and then by seeing how big it looks from here, it's a simple matter to calculate very precisely how far away it is.

And the more precisely we know how far away stuff is that is relatively close, we can work out other associations and calibrate them to see how far away more and more distant objects are.

The problem is I am not sure we've seen too many supernovas close enough for us to resolve the shell. So we can't use the apparent shell size to calibrate the distance and brightness of many types of supernova?

That gets back to the less well calibrated standard candle of presumably known intrinsic brightness.

[vorlon13]

160,000 light years is next door.

[Anomalocaris]

Yeah, but that's just one supernova. I am not sure that is a standard candle type of supernova. It pins down distance to the megellanic cloud, but doesn't tell much of anything else.

[vorlon13]

It tells you how far away ALL the weird little yon beasties are in the LMC.

And then when you see one like one of them in something (presumably) much further away, you can noodle out that distance too.

Think of it as a series of ladder rungs going away from us. The more accurately we can calibrate the lower rungs, the more accurately we can judge items on rungs further and further away.

And there are parallel 'rungs' if you will, it isn't just a single chain of measurements that gets you distances further and further out.

For instance, radio telescopes can resolve supernova shells much further away than visible light telescopes can. We've already seen how by just using geometry we can get a very accurate distance to the 1987 LMC supernova. We can 'see' ones much further away with radiotelescopes, and we can therefore know how far away they all are.

[Alex K]

So... logical deductions...

...BASED ON experience.

Sure, physical cosmology is science, not maths or philosophy, so logical deductions based on observations are the standard. In this case, if an expanding universe gives you a perfect prediction of the shape of the cosmic microwave background through the Sachs Wolfe-Effects, and the alternate hypothesis of mere galactic movement has no way of predicting this or to even accomodate it without completely fudging it, it's basic scientific reasoning that hypothesis 2 is basically dead for the time being.

[Rhondazvous]

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.

[Gawdzilla Sama]

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.

That's the introductory level stuff?

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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.

Use the fire engine siren passing you example. The sound "builds up" in front of the truck and "fades away" behind it. Blue shift is the "build up", red shift is the "fades away".

[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.

[Rhondazvous]

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.

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.