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## Who is moving

Lets say I am responding to the gravitational field of a galaxy. I have a one foot stick. Some guy whizzes by me, parallel to my motion at 1/2 half the speed of light compared to me. He has 1 foot stick. How long does he estimate my stick to be and how long do I estimate his stick to be?

Do we each see each other as moving 1/2 the speed of light?

2. You estimate each others sticks to be ~10.4 inches

Note: when doing thought experiments it is generally easier to use the metric system. A 1m stick would appear as ~866mm (You don't have to convert the a base 10 number to a base 12 effectively.)

3. I don't know?

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When one gets closer and closer to the edge of our hubble sphere, would the galaxies appear smaller and smaller and more and more difficult to see with our telescopes? Beyond our hubble sphere, if it exists, are the galaxies moving away from us faster than the speed of light?

5. Originally Posted by Copernicus
When one gets closer and closer to the edge of our hubble sphere
Im having a hard time correlating your use of "one" (the one moving) and "our". Is the telescope moving?

But; an important point to note is that the hubble sphere is different for anyone at a different location. The hubble sphere moves with the user, and is dialated compared to the one who is not moving.
I'm not sure how the one moving views the dialation, but if they stop, then the sphere is different.

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Originally Posted by NEOWatcher
Im having a hard time correlating your use of "one" (the one moving) and "our". Is the telescope moving?

But; an important point to note is that the hubble sphere is different for anyone at a different location. The hubble sphere moves with the user, and is dialated compared to the one who is not moving.
I'm not sure how the one moving views the dialation, but if they stop, then the sphere is different.
Our hubble sphere, would be meant to be from earth. Anything farther out than13.75 billion light years.

7. Originally Posted by Copernicus
Our hubble sphere, would be meant to be from earth. Anything farther out than13.75 billion light years.
That still doesn't clear up the "one who is close to the edge".
That one is at the center of his own hubble sphere and views us as on the edge of his.

8. Originally Posted by Copernicus
When one gets closer and closer to the edge of our hubble sphere, would the galaxies appear smaller and smaller and more and more difficult to see with our telescopes? Beyond our hubble sphere, if it exists, are the galaxies moving away from us faster than the speed of light?
They aren't moving. Space is expanding. Space is expanding at the same rate there as it is here.

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Originally Posted by WayneFrancis
They aren't moving. Space is expanding. Space is expanding at the same rate there as it is here.
So is there no length contraction of galaxies near the edge of earths hubble sphere compared to lengths of the milkway?

10. Originally Posted by Copernicus
When one gets closer and closer to the edge of our hubble sphere, would the galaxies appear smaller and smaller and more and more difficult to see with our telescopes? Beyond our hubble sphere, if it exists, are the galaxies moving away from us faster than the speed of light?
When someone here examines distances from here that are closer and closer to the edge of our hubble sphere, galaxies appear smaller and smaller and more difficult to see with our telescopes. This is correct.

When one examines galaxies with redshifts larger than those at the edge of our Hubble sphere, we assume they were apparently receding from here faster than the speed of light, due to the expansion of the universe.

The reason we can see these galaxies is because the Hubble sphere is also receding from us, and can catch up and overtake the light from galaxies apparently receding faster than light. That light was emitted in earlier times, when the universe was smaller and those galaxies were closer to us, but it took a long time for our Hubble sphere to catch up with their light.

What all this means is, once we look at galaxies with redshifts larger than those on the edge of our Hubble sphere, those galaxies appear to get larger and larger again the further back in time we look!

To see what I am referring to, check out the section on the angular diameter - redshift test which is about halfway down the following webpage.
http://www.astr.ua.edu/keel/galaxies/obscosmo.html

Or for a simpler explanation you could look here:
http://www.atlasoftheuniverse.com/redshift.html

11. Originally Posted by Copernicus
So is there no length contraction of galaxies near the edge of earths hubble sphere compared to lengths of the milkway?
No, there isn't. Those galaxies aren't moving. The expansion of the universe does not work like Special Relativity.

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Originally Posted by speedfreek
When someone here examines distances from here that are closer and closer to the edge of our hubble sphere, galaxies appear smaller and smaller and more difficult to see with our telescopes. This is correct.

When one examines galaxies with redshifts larger than those at the edge of our Hubble sphere, we assume they were apparently receding from here faster than the speed of light, due to the expansion of the universe.

The reason we can see these galaxies is because the Hubble sphere is also receding from us, and can catch up and overtake the light from galaxies apparently receding faster than light. That light was emitted in earlier times, when the universe was smaller and those galaxies were closer to us, but it took a long time for our Hubble sphere to catch up with their light.

What all this means is, once we look at galaxies with redshifts larger than those on the edge of our Hubble sphere, those galaxies appear to get larger and larger again the further back in time we look!

To see what I am referring to, check out the section on the angular diameter - redshift test which is about halfway down the following webpage.
http://www.astr.ua.edu/keel/galaxies/obscosmo.html

Or for a simpler explanation you could look here:
http://www.atlasoftheuniverse.com/redshift.html
Very interesting. Do others on this board agree?

13. Under relativity -- any flavor of relativity -- the laws of physics in all inertial reference frames are identical, so it's impossible to tell who is moving without external information. One observer may consider themself "privileged" -- being bound to a galaxy may do that -- but that's just a personal prejudice.

So, we have two observers in relative motion: U and H. Either H or U may, with equal validity, consider the other one or themself to be moving. When either measures the other's one-meter long stick, they'll come up with the same values. Similarly, if each measures the other's second, they'll come up with identical values. In SR and GR, the value they'll get for the other party's meter and second will differ from the value they get for their own meter or second.

So who's moving? Who do you want to be moving? Either choice is equally valid.

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Originally Posted by Copernicus
Very interesting. Do others on this board agree?
Yes; that is all standard GR cosmology.

Edit: Misattributed the quote.
Last edited by ctcoker; 2012-Jun-29 at 04:17 AM.

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Originally Posted by ctcoker
Yes; that is all standard GR cosmology.
I was wondering if others agreed with the website that was cited.

http://www.atlasoftheuniverse.com/redshift.html

16. Originally Posted by Copernicus
I was wondering if others agreed with the website that was cited.

http://www.atlasoftheuniverse.com/redshift.html
Well, aside from the fact that the author is using old figures that have since been revised downward. But the principles are sound.

In reference to the angular diameter - redshift relationship, both the websites I cited are saying the same thing.

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Originally Posted by speedfreek
Well, aside from the fact that the author is using old figures that have since been revised downward. But the principles are sound.

In reference to the angular diameter - redshift relationship, both the websites I cited are saying the same thing.
I was not aware that the angular diameter was increased and the light intensity was decreased. That is phenomenal information. Why isn't it talked about more?

18. Originally Posted by Copernicus
I was not aware that the angular diameter was increased and the light intensity was decreased. That is phenomenal information. Why isn't it talked about more?
Because, whilst it is a prediction of the Big-Bang theory, it is actually quite difficult to confirm through observation. It would be easy to confirm if all galaxies were always the same size, but in reality we have to make predictions based on our various models of galaxy formation.

I think the jury is still out as to whether we have confirmed it or not.

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Originally Posted by speedfreek
Because, whilst it is a prediction of the Big-Bang theory, it is actually quite difficult to confirm through observation. It would be easy to confirm if all galaxies were always the same size, but in reality we have to make predictions based on our various models of galaxy formation.

I think the jury is still out as to whether we have confirmed it or not.
Sounds like this particular set of current data doesn't support the Big-Bang theory, or am I misinterpreting. Please explain what parts are supported by this data? Thanks

20. Originally Posted by Copernicus
Sounds like this particular set of current data doesn't support the Big-Bang theory, or am I misinterpreting. Please explain what parts are supported by this data? Thanks
I don't understand why you would think that. The problem is that we need to know the absolute size of a galaxy in order tell how far away it is by how large it looks. In the early universe, were galaxies the same size as they are today? How do galaxies form?

The data corresponds to certain models of galaxy formation, so it confirms the expansion if galaxies form that way, but we are not sure they do yet, so the jury is still out.

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Originally Posted by speedfreek
I don't understand why you would think that. The problem is that we need to know the absolute size of a galaxy in order tell how far away it is by how large it looks. In the early universe, were galaxies the same size as they are today? How do galaxies form?

The data corresponds to certain models of galaxy formation, so it confirms the expansion if galaxies form that way, but we are not sure they do yet, so the jury is still out.
At this point, I would just say that big bang theory predicts.

red shifting is accepted. Red shifting should help us determine a trend for size of galaxies as they go back into time. It would seem to me that this trend would be pretty clear cut. If it is not clear cut, then there is a problem with the theories. Tell me where I am wrong?

22. Originally Posted by Copernicus
At this point, I would just say that big bang theory predicts.

red shifting is accepted. Red shifting should help us determine a trend for size of galaxies as they go back into time. It would seem to me that this trend would be pretty clear cut. If it is not clear cut, then there is a problem with the theories. Tell me where I am wrong?
You're wrong because this is not about evidence that contradicts BBT, but one prediction of BBT that is technically difficult to verify, and with other scientific issues aside from BBT. But there have been other predictions that have been verified, like the CMB. And of course, BBT was developed in the first place because of the observational evidence. Nobody has managed to come up with an alternative that has a better fit to the totality of evidence.

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## Data of galaxy brightness and width as distance increases from earth.

Are there graphs that show the observed intensity of galaxy brightness as distance increases from earth?
Are there graphs that show the observed galaxy width as distance increases from earth?

24. Oh, if only it were that simple! Here is an example of the real science:

http://arxiv.org/abs/astro-ph/0106566
The Tolman Surface Brightness Test for the Reality of the Expansion. IV. A Measurement of the Tolman Signal and the Luminosity Evolution of Early-Type Galaxies

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Originally Posted by speedfreek
Oh, if only it were that simple! Here is an example of the real science:

http://arxiv.org/abs/astro-ph/0106566
The Tolman Surface Brightness Test for the Reality of the Expansion. IV. A Measurement of the Tolman Signal and the Luminosity Evolution of Early-Type Galaxies
I'm glad they ruled out tired light. But I am astonished that there is not a simple way to plot observed average brightness of galaxies verses distance from earth. I don't really see why we wound need a galaxy formation model to do this. Is there no way no quantify observed brightness?

26. Originally Posted by Copernicus
So is there no length contraction of galaxies near the edge of earths hubble sphere compared to lengths of the milkway?
Since it isn't a SR effect I would say no. There is no time dilation either. There are thought experiments that can be done that will show that cosmic expansion only causes an appearance of time dilation, like with super nova light curves. But this isn't a SR effect but purely a Doppler effect. With resent posts showing even relativistic length contraction is actually cancelled out optically wise. IE the contracted object has the illusion that it isn't really contracted.
This is my understanding at least.

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Originally Posted by WayneFrancis
Since it isn't a SR effect I would say no. There is no time dilation either. There are thought experiments that can be done that will show that cosmic expansion only causes an appearance of time dilation, like with super nova light curves. But this isn't a SR effect but purely a Doppler effect. With resent posts showing even relativistic length contraction is actually cancelled out optically wise. IE the contracted object has the illusion that it isn't really contracted.
This is my understanding at least.
That is nice to know. I wasn't aware of this. Lends credence to actual expansion of universe.

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Originally Posted by Copernicus
I'm glad they ruled out tired light. But I am astonished that there is not a simple way to plot observed average brightness of galaxies verses distance from earth. I don't really see why we wound need a galaxy formation model to do this. Is there no way no quantify observed brightness?
There is (astronomers do it all the time), but such a plot simply isn't very useful most of the time. We really have only a vague idea of how far away most galaxies are, and since galaxies come in all different shapes, sizes, and brightnesses, we need to know how far away they are to nail down how bright they really are. We need to know how bright they really are in order to make a plot of apparent brightness v. distance useful.

Now, there are other ways to estimate a galaxy's brightness (you can, for instance, look at how quickly the stars are circling the center in a spiral galaxy to get its mass, and then use the Tully-Fisher relation to get the luminosity), but they have their own pitfalls and usually are not more precise than a few percent at best. In short, measuring most anything related to distance and true brightness in astronomy is very, very difficult.

Oh, and I noticed that no one ever answered the second part of your original question (does each observer see the other as moving at 1/2 c?). The answer to that is no, they don't. Since each observer sees themselves as at rest, you need to use relativistic velocity addition. When you do this, the answer for two observers approaching each other at 1/2 c is that each sees the other as moving at 4/5 c relative to themselves.

29. Originally Posted by ctcoker
Oh, and I noticed that no one ever answered the second part of your original question (does each observer see the other as moving at 1/2 c?). The answer to that is no, they don't. Since each observer sees themselves as at rest, you need to use relativistic velocity addition. When you do this, the answer for two observers approaching each other at 1/2 c is that each sees the other as moving at 4/5 c relative to themselves.
As I read his statement, he was just saying someone else had a relative velocity of 1/2 c compared to the the observer, so no relativistic velocity addition required. Quoting:

"Some guy whizzes by me, parallel to my motion at 1/2 half the speed of light compared to me."

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Oh, you're right. In that case, yeah, each sees the other as moving at 1/2 c.

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