# Red Shifts and the expanding universe

1. Sep 15, 2006

### WendyE

I have a question about the light observed from galaxies that are moving away from us due to the expansion of the universe. I understand that we can tell they are moving away because the light from these galaxies is red shifted. I was wondering how this red shift is generated, exactly. Is the light red shifted because the space that the light travels through is expanding? Or is it red shifted for the same reason that a sound source moving away from you is shifted to lower tones?

2. Sep 15, 2006

### GeorgeSol

I'd like to know, too. There seems to be different models to explain it.
Some of the redshifting is caused by the Doppler effect, which, as you mentioned, is similar to sound shifting. This component is due to the motion of the stars as the move about their galaxy and, even more so, the motion of the galaxy about the c.g. of its glactic cluster.

But the more distant galaxies exhibit large redshifts and are mainly due to the expansion of space. Some say the light's wave gets stretched as space expands. I find this difficult to grasp, but it can be useful in mathematical models, apparently, which yield valid results.

3. Sep 16, 2006

### DavidBektas

GeorgeSol is right, the main factor for the redshift varies with distance:

1. For small distances the photon only travels a relatively short time and therefore the redshift due to the expansion of space is small as well. The main factor in this case is the velocity of the object.

2. For large distances it´s the other way around: The redshift due to the expansion dominates the "common" Doppler-Effect due to the velocity.

Another cause for redshift is the gravitational redshift, but I don´t know if that has a considerable effect on the overall redshift (maybe for nearby, massive and relatively slow bodies?).

Last edited: Sep 16, 2006
4. Sep 17, 2006

### Jorrie

Take into account that at small distances, the spectral shifts are blue shift and red shift, i.e., fairly random motions. At larger distances there is only red shift. This supports the cosmological expansion as the cause of the red shift at cosmological distances.

5. Sep 17, 2006

### chronon

From http://math.ucr.edu/home/baez/physics/Relativity/GR/hubble.html
So the two ways of looking at things are essentially the same. Beware of distinctions between redshift due to movement and redshift due to expansion of space: they're the same thing.

My opinion is that the expanding space viewpoint is the wrong way of looking at things, see:
http://www.chronon.org/articles/stretchyspace.html

6. Sep 17, 2006

### DavidBektas

That would imply that the redshift is either due to gravitational or velocity redshift, which leads to a big problem for large distances.

If you interpret the redshift as the "common" Doppler-Shift, you would get velocities v>c for objects at large distances (plus, you would need to explain why there are no blueshifts). From Wiki:

"In very distant objects, v can be larger than c. This is not a violation of the special relativity however because a metric expansion is not associated with any physical object's velocity."

http://en.wikipedia.org/wiki/Hubble_expansion

I hope I didn´t misinterpret you, but I think the stretching of the photon due to the cosmic expansion is the only possible explanation for such enormous redshifts.

7. Sep 17, 2006

### chronon

No, because you would be using a different coordinate system in which all velocities are <c, and you would use the special-relativistic formula for Doppler shifts.
See http://www.chronon.org/articles/milne_cosmology.html

8. Sep 18, 2006

### Jorrie

I'm under the impression that interpreting the red shift by means of special relativistic Doppler shift gives a different answer for things like the distance-red shift relationship than what the cosmological expansion view gives. Cosmological red shift simply says how much space has expanded since the light left the source. Doppler shift, in contrast, says how fast the relative velocity between us and the source was when the light was transmitted. Or do I have it wrong?

9. Sep 18, 2006

### chronon

In the (0,0) case - that is an expanding universe without deceleration due to gravity or a cosmological constant - the two ways of looking at things are equivalent, so that you can consider the redshift as being due to the velocity of recession when the light was emitted, according to the rules of special relativity. If there is non-zero deceleration or acceleration then this will no longer the case - whatever is causing the deceleration or acceleration will also act on the light 'in flight'. However, the difference from the (0,0) case is very small (undetectable before the 1990's), which I don't consider sufficient to give up the view that the redshift is due to recession.

You might like to take a look at http://arxiv.org/abs/gr-qc/9303008 for further information on this matter

Last edited: Sep 18, 2006
10. Sep 18, 2006

### Jorrie

Thanks Chronon, I took a peak, but the idea of cosmological redshift taken as special relativistic Doppler shift still sounds awkward to me, because it implies movement through space. I agree that one can take a given cosmological redshift and say that it is the same as the redshift caused by some velocity v through flat spacetime, but IMO, that does not make it a valid interpretation of cosmological redshift.

11. Sep 19, 2006

### oldman

A paper by Davis and Lineweaver may be useful in this thread. It is at http://arxiv.org/abs/astro-ph/0310808 , and the following extract from it gives the flavour of the matters that are discussed:

A general difficulty I have is that despite the numerous references made in cosmology to "expanding space", I haven't yet come across a believable definition of "space" itself, so I wish people would stop using a concept that is not defined.

Frustrated, I have great sympathy with the views Chronon expresses at: http://www.chronon.org/articles/stretchyspace.html . I've also concocted my own definition of space, namely "Space is what you can swing a cat in". But I'd prefer something more scientific. Any offers?

12. Sep 19, 2006

### Chronos

I still like the rubber sheet analogy. It is a neat way of portraying conservation of energy [energy as a fixed quantity, whereas the volume of space that contains it is variable]. You could tweak the math to conserve space, but energy [redshift] is more measurement friendly.

13. Sep 19, 2006

### SpaceTiger

Staff Emeritus
The Milne cosmology is an actual model of the universe, not just a different coordinate system. It hasn't been consistent with the data for quite some time...

14. Sep 20, 2006

### Garth

Some would disagree.....
A Concordant “Freely Coasting” Cosmology.

Garth

15. Sep 20, 2006

### SpaceTiger

Staff Emeritus
You disagree that the Milne cosmology is more than a change in coordinate system? I don't see what this has to do with your model, other than the fact that both are "coasting".

16. Sep 20, 2006

### Garth

Sorry, I should not have included all your quote!
I was referring to your statement:
and comparing it with the Freely Coasting Model (FCM)

I was not referring to my SCC model, which is different to the Milne model and the FCM of Kolb ( A coasting cosmology ) & the Indian team's interest.

For clarification: Milne is empty, FCM assumes the Milne linear expansion for empirical reasons, but with matter, however, it is not able to suggest a mechanism to deliver this, except Kolb's "K-matter" (DE??)).

Both Milne/FCM and SCC are "Freely Coasting", $R(t) \sim t$; however the Milne model, and the FCM, have k = -1, whereas SCC has k = +1.

They do coincide for BBN where the curvature term is not dominant, however the difference between FCM and SCC is SCC does not fit the Type Ia SN data so readily, therefore it requires these not to be standard candlers, on the other hand, the SCC model is conformally flat and therefore fits the WMAP data better.

I hope I have cleared up the misunderstanding.

Garth

Last edited: Sep 20, 2006
17. Sep 20, 2006

### chronon

Indeed. My argument isn't that that is the way the universe is, but that it is not qualitatively different, i.e. it is close enough to serve as a useful approximation when discussing things like redshift.

In addition to this the Milne cosmology serves as a null hypothesis. Hence it is a useful exercise to see what would allow the data to fit this hypothesis, as is done in the paper quoted by Garth.

18. Sep 20, 2006

### SpaceTiger

Staff Emeritus
The concept of the celestial sphere is useful for qualitative discussions of the night sky, but when people ask about what's actually going on, I don't tell them that the stars are pegged to a glass sphere!

Honestly, though, I can't think of any situation in which the Milne model is useful for conceptualizing the universe we live in. It may be useful for practicing the mathematics of cosmology or understanding how the field developed to its current state, but that's about it.

19. Sep 21, 2006

### Chronos

I agree very much with ST's position. He is not advocating, or denouncing any view, just dealing with the bare facts and only ruling out the most basic bad theories - at least IMO. Scientists are slippery creatures who freely admit their errors within error bars [physics 101]. That assertion still puzzles me. . . and watching judge Judy did not help matters.

I still like Garth's model. He does not beat around the bush or hide behind skirts. He tells it like he sees it, and I respect that. I don't foresee Garth making any excuses if GPB falls short of his predictions. That is the mark of a true scientist, IMO.

20. Oct 23, 2006

### ValenceE

Hello to all,

Again I must state that I'm very much a layman who's always been marveling at our magnificent universe, trying to comprehend what I can about its majesty and my own relationship with it.

I don't always have the scientific knowledge to validate thoughts, ideas and perhaps 'revelations' that come to mind, but I hope, actually I'm sure, that the different forums and all it's participants will help greatly in this quest...

Now here’s an attempt that might just go down in flames, but it’s consequent with what I just said…

Could the red shifts be dependant on both our Earth’s position in our own multi rotational reference frame coupled with the far away galaxies or light sources in their own rotational frame, both moving in opposite directions ?

The velocity of light remains c but we would be moving away from the source as it is moving away from us in our respective rotational frames thus creating a perceived red shift… not necessarily because of expansion.

Does this make any sense?

VE