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RussT
2006-Jan-01, 12:40 AM
I understand that the farther away the galaxies are the faster they are receding from us.
So, if I was in a galaxy 8 billion light years away, and looked back at earth...
I would see the galaxies 4 billion light years away (towards earth) getting smaller (because I am traveling faster than they are)...but, they should be traveling toward me and thus be blue shifted, correct???

Sam5
2006-Jan-01, 12:48 AM
You would see the same thing we see now, just different galaxies in different places. We would see your galaxy as being 8 bly away and you would see ours as being 8 bly away.

Kaptain K
2006-Jan-01, 01:59 AM
The situation is symetrical.

We would see you as being 8 billion light years away and moving away from us at some speed "X".
You would see us as being 8 billion light years away and moving away from you at some speed "X".

We would see a galaxy half way between us as being 4 billion light years away and moving away from us at a speed "X/2".
You would see a galaxy half way between us as being 4 billion light years away and moving away from you at a speed "X/2".

Tim Thompson
2006-Jan-01, 05:46 PM
As the Kaptain says, the situation is symmetric. This is one case where the good old balloon analogy serves to illustrate the point. Just imagine that you are on one of the dots, on the balloon, as it expands. It does not matter which dot you are on, you will see the same thing in every case, namely that all of the other dots on the balloon are moving away from you, whereever you are.

Likewise, in the expanding universe, you will see all of the other galaxies moving away with the cosmic expansion, except the "local" ones of course.

Sam5
2006-Jan-01, 08:54 PM
As the Kaptain says, the situation is symmetric. This is one case where the good old balloon analogy serves to illustrate the point. Just imagine that you are on one of the dots, on the balloon, as it expands.

We are not on the "surface" of a 2-D universe. We are inside a 3-D universe. In your model, if you go far enough away from a galaxy, you can see it being closer in the opposite viewing direction. But there is no evidence that our universe is like that.

Fortunate
2006-Jan-01, 09:48 PM
I'll borrow the example of an expanding chess board used by Ken G in a previous thread. He modified it in a particular way - I will modify it in a different way.

Consider a chess board with an infinite number of equal-sized squares extending infinitely far in every direction (or at least as far as the observable universe, if you prefer). Let us now suppose that the chess board is expanding so that each square is increasing in size at the same rate as each other square. Notice that this example does not make any square or any point special in any way. The expansion is not centered anywhere, and an observer at any point in one square will see points receding from him at the same rate as will an observer at any other point in that or any other square.

Each observer will see points that are farther away receding at greater speeds; the rate of recession will be proportional to the distance. In the time it takes for the scale to increase 10%, a point originally 1 light year away will have moved .1 light years farther away, but a point originally 2 light years away will have moved .2 light years farther away in that same amount of time.

What each observer actually sees is complicated slightly by the fact that it takes the light some time to travel from one point to the other, but the basic idea that objects further away seem to be receding more rapidly is preserved.

I hope that picture is clear. Now just consider a three-dimensional "chess board," and the analogous expansion.

The_Radiation_Specialist
2006-Jan-02, 12:26 AM
What if we travelled faster than speed of light to a nearby galaxy. Then we took a whopping large telescope and zoomed on earth. Then we could see earth in its early stages of forming. We could see history.

RussT
2006-Jan-02, 01:49 AM
The_Radiation_Specialist
Senior Member

It couldn't be that nearby, to see what you're suggesting. It would have to be about 5 billion light years away.

RussT
2006-Jan-02, 02:03 AM
Ladies and Gentlemen...since we "KNOW" the galaxies are receding from "US", we know that the galaxy 4 billion light years away, is moving toward the galaxy 8 billion light years away! So, the question is...if we were in the galaxy 8 billion light years away, would the galaxy 4 billion light years, that we know is moving toward it, be "BLUESHIFTED"???

grant hutchison
2006-Jan-02, 02:09 AM
Ladies and Gentlemen...since we "KNOW" the galaxies are receding from "US", we know that the galaxy 4 billion light years away, is moving toward the galaxy 8 billion light years away! So, the question is...if we were in the galaxy 8 billion light years away, would the galaxy 4 billion light years, that we know is moving toward it, be "BLUESHIFTED"???
No, the galaxy 8 billion light years away is receding twice as fast as the galaxy 4 billion light years away. So if we looked from the "4 billion" galaxy to the "8 billion" galaxy, we'd see it moving away from us; if we looked back from the "8 billion" galaxy, the "4 billion" galaxy would be lagging behind: from either galaxy, the other is receding, and therefore redshifted.

Grant Hutchison

RussT
2006-Jan-02, 02:33 AM
You can't have it both ways!!! If we are saying that the galaxies are moving away from us, then the galaxy 4 billion light years away must be moving toward the galaxy 8 billion light years away. So, if you were to say that it is red-shifted from the perspective of the galaxy 8 billion light years away (Looking toward us), then it would have to be moving "Toward" us!!!

I understand (as I said in the origanal post) that the galaxy 8 billion light years away, is moving away faster than the galaxy 4 billion light years away.

The other problem with this whole concept is...if the galaxies at the horizon (13+ billion light years away) are expanding away faster than the speed of light, that means that all the galaxies farther than that have to exponentially be going faster and faster to keep ahead of the ones just behind them, so what happens at 100,000 light years away and farther out to infinity?

Sam5
2006-Jan-02, 02:44 AM
Ladies and Gentlemen...since we "KNOW" the galaxies are receding from "US", we know that the galaxy 4 billion light years away, is moving toward the galaxy 8 billion light years away! So, the question is...if we were in the galaxy 8 billion light years away, would the galaxy 4 billion light years, that we know is moving toward it, be "BLUESHIFTED"???

No.

The galaxy that is 8 bly away is moving away from us much faster than the galaxy that is 4 bly away, so that means the 8 bly galaxy is moving AWAY FROM the 4 bly galaxy. The 4 bly and the 8 bly galaxies are moving in the same directions but at different speeds.

Roughly speaking, the 8 bly galaxy would be moving away from the 4 bly galaxy at about the same speed that the 4 bly galaxy is moving away from us. So, people in the 8 bly galaxy would see the 4 bly galaxy light as being redshifted.

Sam5
2006-Jan-02, 02:52 AM
You can't have it both ways!!! If we are saying that the galaxies are moving away from us, then the galaxy 4 billion light years away must be moving toward the galaxy 8 billion light years away. So, if you were to say that it is red-shifted from the perspective of the galaxy 8 billion light years away (Looking toward us), then it would have to be moving "Toward" us!!!




No.

If a train is moving away from us at 60 mph, we would hear a redshift in it’s whistle. If on the far side of it, away from us, there is another train going away from us at 120 mph, then that train would hear a redshift in the whistle of the first train, and the first train would hear a redshift in the whistle of the second train.

We would hear some redshift in the whistle from the first train (the one going 60 mph) but we would hear much more redshift in the whistle of the second train (the one going 120 mph relative to us).

The two trains would be separating (moving apart) at 60 mph, so they would both hear redshifts in each other’s whistles. From our point of view, train 1 would be moving away from us at 60 mph, and train 2 would be moving away from us at 120 mph. But trains 1 and 2 would be separating from each other at 60 mph. Among the three objects, us and the two trains, there would be no blueshift in the whistle signal.

phunk
2006-Jan-02, 03:47 AM
Something about using the term redshift to apply to sound just doesn't add up... :D

Should be calling it the doppler effect.

sorry for the nitpick.

Sam5
2006-Jan-02, 04:08 AM
Something about using the term redshift to apply to sound just doesn't add up... :D

Should be calling it the doppler effect.

sorry for the nitpick.

You are right. It's just easier to distinguish which Doppler Effect, the stretching or the shrinking waves of sound, by using the red/blue terms that are applied to light, especially in a short post.

Nereid
2006-Jan-02, 04:56 AM
You can't have it both ways!!! If we are saying that the galaxies are moving away from us, then the galaxy 4 billion light years away must be moving toward the galaxy 8 billion light years away. So, if you were to say that it is red-shifted from the perspective of the galaxy 8 billion light years away (Looking toward us), then it would have to be moving "Toward" us!!!

I understand (as I said in the origanal post) that the galaxy 8 billion light years away, is moving away faster than the galaxy 4 billion light years away.

The other problem with this whole concept is...if the galaxies at the horizon (13+ billion light years away) are expanding away faster than the speed of light, that means that all the galaxies farther than that have to exponentially be going faster and faster to keep ahead of the ones just behind them, so what happens at 100,000 light years away and farther out to infinity?Your misunderstanding here, RussT, has been addressed already.

However, behind it there may lurk a deeper misunderstanding (or simply a lack of understanding), one whose resolution gets you into some (mildly) counter-intuitive territory involving the application of GR to the whole universe.

There is an excellent article in a not-too-long-ago Scientific American, by Davis and Lineweaver (I think) on some popular misconceptions about cosmology; it dealt with your questions, and several others. They also wrote a more technical paper (http://bat.phys.unsw.edu.au/~charley/papers/0310808.pdf), published in ApJ?, where they provided numbers, math, equations and stuff.

Fortunate
2006-Jan-02, 08:17 AM
You can't have it both ways!!! If we are saying that the galaxies are moving away from us, then the galaxy 4 billion light years away must be moving toward the galaxy 8 billion light years away. So, if you were to say that it is red-shifted from the perspective of the galaxy 8 billion light years away (Looking toward us), then it would have to be moving "Toward" us!!!

Sometimes the terms "away from" and "toward" can be confusing. When used in a certain sense, they are relative to a particular persperctive (coordinate system). From our point of view, we are stationary, and the galaxy 4 bly away is moving "toward" the galaxy 8 bly away. From the point of view of the galaxy 4 bly away it itself is stationary, and we and the galaxy 8 bly away from us are moving away in opposite directions. From the point of view of the galaxy 8 bly from us, it itself is stationary, and the galaxy 4 bly from us is moving toward us.

But there is another way to use the terms "away from" and "toward," a sense in which they are absolute instead of relative to perspective. In this sense, two objects are moving "toward" each other if the distance between them is decreasing and are moving "away from" each other if the distance between them is increasing. In this sense of the terms, motion of a luminous object toward an observer causes the observer to see a blue shift and motion of a luminous object away from an observer causes the observer to see a red shift.

Thus, the object 4 bly from us is moving "toward" the object 8 bly from us in the relative sense (relative to our point of view) but is moving "away" from it in the absolute sense. So we have been trapped in a pun. The motion in the absolute sense is the one that determines the direction of the spectral shift.

RussT
2006-Jan-02, 08:45 AM
So, if what everyone is saying above is true, then we "CANNOT" tell if the whole universe is expanding away from us!

If we look in any one direction, it simply means that all the galaxies in that direction are simply moving in that direction faster than we are,and then when we look 180 degrees behind us, that we are simply going faster than the ones going in our same direction!

grant hutchison
2006-Jan-02, 01:53 PM
So, if what everyone is saying above is true, then we "CANNOT" tell if the whole universe is expanding away from us!

If we look in any one direction, it simply means that all the galaxies in that direction are simply moving in that direction faster than we are,and then when we look 180 degrees behind us, that we are simply going faster than the ones going in our same direction!But look to the "sides", and those galaxies are moving away, too: so we must be part of an expanding Universe. There's just no way we can say that we're at the centre of an expanding Universe, since everything is expanding away from everything else.

Grant Hutchison

RussT
2006-Jan-03, 12:27 AM
If the galaxy 8 billion light years away Can't tell that the galaxy 4 billion light years away is coming toward it (because the light is redshifted), then something is wrong!!!

For one thing, that would mean that the Great Attractor could be a singularity, and that we are simply traveling faster toward it than all the galaxies farther away from us, and we simply can't tell it, because they are all redshifted but coming toward us!

grant hutchison
2006-Jan-03, 12:37 AM
If the galaxy 8 billion light years away Can't tell that the galaxy 4 billion light years away is coming toward it (because the light is redshifted), then something is wrong!!!The galaxy 4 billion light years isn't moving towards the galaxy 8 billion light years away: it's moving away from it. It's moving away from us and moving away from the galaxy 8 billion light years away. Remember that all motion is relative, and these galaxies are caught up in the universal expansion of space.

Let's call them "galaxy four" (the 4-billion-ly galaxy) and "galaxy eight" (the 8-billion-ly galaxy).

The local view: Galaxy four is moving away from us, so we see it redshifted; galaxy eight is moving away from us faster, so we see it more redshifted.

The galaxy eight view: Looking back from galaxy eight, our galaxy appears to be receding, so they see us to be as redshifted as we see them. Galaxy four, which is moving away from us more slowly than galaxy eight, is lagging behind galaxy eight (and therefore receding from it), so galaxy eight inhabitants will see galaxy four redshifted, too, but less so than our galaxy.

The galaxy four view: From galaxy four, our galaxy appears to be receding in one direction, and galaxy eight appears to be receding in the other direction. Both appear redshifted.

Grant Hutchison

Tim Thompson
2006-Jan-03, 05:01 AM
We are not on the "surface" of a 2-D universe. We are inside a 3-D universe. In your model, if you go far enough away from a galaxy, you can see it being closer in the opposite viewing direction. But there is no evidence that our universe is like that.
We are inside a 4D universe, not a 3D universe, which I claim to be a valid statement of fact. In the model, as I analogized it, you would see the same object, in the opposite direction, if and only if, it were possible for light to follow such a trajectory in an expanding universe. Since the expansion is somewhat faster than the speed of light, in a general relativistic expanding universe, it is impossible and your criticism is invalid.

Fortunate
2006-Jan-03, 05:45 AM
We are not on the "surface" of a 2-D universe. We are inside a 3-D universe. In your model, if you go far enough away from a galaxy, you can see it being closer in the opposite viewing direction. But there is no evidence that our universe is like that.

Analogies are used to explain features of one object in terms of the corresponding features of another (usually simpler) object. Naturally, the objects are not identical in every way, or else the analogy would lose its value of simplifying the discussion. The balloon analogy is simply being used to illustrate the possibilty of an expansion in which every point is moving away from every other point.

bigbluestar
2006-Jan-03, 05:53 AM
I thinkt the major problem here is that russt is beliving that the universe is expanding in one direction. This is not he case. If you anlayse the redshift data for a universe expanding in one direction. The galaxies infront have a higher redshift than the ones at back. As long as there is speratation between the two objects you will have redshift. Galaxies to the left and right would appear not to have any redshift at all. The will be travelling with us towards the direction of the expansion.

This is not he case when we analyse the data all galaxies we find red shift left, right, up, down, forward ,back ,and inside out. We are expanding in all directions. What you propose russt would indicate that the galaxy is expanding in one direction. But ours isnt. So sorry Russt wrong universe

Ken G
2006-Jan-03, 06:28 AM
I hope this was clear, RussT, from Grant Hutchison's post, but if not, keep in mind that your thinking that the middle galaxy would be blueshifted has to do with your thinking that redshifts and blueshifts are caused by some kind of absolute direction of motion. But the main lesson of relativity is that such absolute motion does not exist, and neither is the concept of redshift or blueshift absolute. Observed motion is actually a relationship between the object and the observer, and different observers will in general have different relationships with the same object, i.e., they will have different relative motions and one may perceive the object as redshifted while another sees it as blueshifted. In the current example, both galaxy 8 and our galaxy see galaxy 4 as redshifted, but more interestingly, both the direction galaxy 4 appears to be moving, and the direction to the galaxy itself, are opposite. As Grant said, motion is relative. That's what distinguishes relativity from, say, the theory of sound waves-- the latter does have an absolute frame, the frame of the medium carrying the sound. And although sounds can also be Doppler shifted in different ways for different observers, there you do have an objective sense of the wavelength embedded in the medium, while you don't get that for light.

grant hutchison
2006-Jan-03, 02:56 PM
I hope this was clear, RussT, from Grant Hutchison's post ...To a large extent I seem to have simply restated Fortunate's argument from a previous post. :sad: I had somehow missed that post as I followed the thread.

Grant Hutchison

Ken G
2006-Jan-03, 03:42 PM
Then I hope RussT will get the right idea from any or all of these posts. It is certainly a common question, so we will feel good if we have successfully conquered it. RussT?

RussT
2006-Jan-04, 02:14 AM
The galaxy eight view: Looking back from galaxy eight, our galaxy appears to be receding, so they see us to be as redshifted as we see them. Galaxy four, which is moving away from us more slowly than galaxy eight, is lagging behind galaxy eight (and therefore receding from it), so galaxy eight inhabitants will see galaxy four redshifted, too, but less so than our galaxy.

Okay, I see why the light would be redshifted, because galaxy 8 is moving away from galaxy 4 faster, the light would take longer to get to galaxy 8.

However, Ken, I still have a problem with galaxy 4 receding from us, and the "appearance" to galaxy 8 that it is receding from it. Sorry guys, but this sounds much more like "adhering" to the Cosmological Principle...that space looks the same from all views, rather than it having to do with relativity.

I do believe that everything is expanding away from us (so not to worry), but this has helped because I can now see a way that the original expansion (The Hubble Constant) can be a more linear everything expanding away, and then the Cosmological Constant (Increasing Variable) expansion would be the galaxies (I should say, the clusters) 'spreading apart' kind of additional expansion... So Thanks.

ZaphodBeeblebrox
2006-Jan-04, 05:41 AM
Okay, I see why the light would be redshifted, because galaxy 8 is moving away from galaxy 4 faster, the light would take longer to get to galaxy 8.

However, Ken, I still have a problem with galaxy 4 receding from us, and the "appearance" to galaxy 8 that it is receding from it. Sorry guys, but this sounds much more like "adhering" to the Cosmological Principle...that space looks the same from all views, rather than it having to do with relativity.

I do believe that everything is expanding away from us (so not to worry), but this has helped because I can now see a way that the original expansion (The Hubble Constant) can be a more linear everything expanding away, and then the Cosmological Constant (Increasing Variable) expansion would be the galaxies (I should say, the clusters) 'spreading apart' kind of additional expansion... So Thanks.
No Appearance Necessary ...

As Grant Said, They ARE, All Moving Apart, It Looks, As it Is!!!!

grant hutchison
2006-Jan-04, 08:11 AM
Okay, I see why the light would be redshifted, because galaxy 8 is moving away from galaxy 4 faster, the light would take longer to get to galaxy 8.At each successive moment, the light has longer to travel than it had in the previous moment. That's what causes the redshift. And if you think about that, it means that galaxy four really, truly is receding from galaxy eight, because the space between the two galaxies is expanding, just as it is between us and galaxy four.

Grant Hutchison

Ken G
2006-Jan-04, 11:06 AM
Note there is also another cause of the redshift, acting concurrently with the effect Grant describes. That is the slower rate of flow of time in galaxy 4 from the perspective of galaxy 8, or from our galaxy for that matter. That's where the relativity comes in. I mention this, even though I know most are aware of it, only because it is so fascinating!

ZaphodBeeblebrox
2006-Jan-04, 09:04 PM
Note there is also another cause of the redshift, acting concurrently with the effect Grant describes. That is the slower rate of flow of time in galaxy 4 from the perspective of galaxy 8, or from our galaxy for that matter. That's where the relativity comes in. I mention this, even though I know most are aware of it, only because it is so fascinating!
This Is, One Area, Where it's COMPLETELY, Counter-Intuitive ...

If a Galaxy, Appears to Be, Receding From us, at 96.82% of The Speed of Light, we Will See Time There, Assuming we Had a Strong Enough Telescope, Moving at One Quarter Speed ...

However, If an Intelligent Species There, Turned a Telescope, On us, They'd See us, Moving at One Quarter Speed, Instead!

Where it Gets, Really Weird, Is If a 3rd Galaxy, Was Half Way, Between Us, they'd See Both us, And, The 2nd Galaxy, Moving at Only, Half Speed!

Sam5
2006-Jan-05, 01:42 AM
I don’t think that’s “counter-intuitive” at all. It’s a basic result of the Doppler effect.

Remember, if the train is moving away from us and sounds its whistle for 1 second, we will hear the whistle tone as being lower AND we will hear the tone as lasting longer than 1 second. This was known back in the 1840s. The same thing happens with light and with light signals from ticking clocks. I don’t see anything “counter-intuitive” about this at all.

AstroSmurf
2006-Jan-05, 12:29 PM
I don’t think that’s “counter-intuitive” at all. It’s a basic result of the Doppler effect.
Yes, but you're (as usual) ignoring the time dilation part of relativistic Doppler shift. Even if a galaxy/spaceship/whatever is moving perpendicularly to our line of sight towards it we'll see time running slower for it. Straightforward Doppler shift predicts no slowdown at all in that situation.

Doppler shift is direction-dependant. Time dilation isn't.
(edited a stupid typo)

Ken G
2006-Jan-05, 02:19 PM
Yes, AstroSmurf is echoing the point I was making, when talking about large frequency shifts the co-called "Doppler" effect is really two effects happening concurrently, one a straightforward application of time-of-flight effects, which sounds like what Sam5 is talking about, and the other a very counterintuitive monkeying with time and space, which is the aspect ZaphodBeeblebrox was referring to. No doubt this continues to hold in general relativity and the expansion of the universe.