Red Shifts and the expanding universe

[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?

 

[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.

 

[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?).

 

[Jorrie]

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...

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.

 

[chronon]

From http://math.ucr.edu/home/baez/physics/Relativity/GR/hubble.html

Are the light waves "stretched" as the universe expands, or is the light doppler-shifted because distant galaxies are moving away from us?

In a word: yes.
In two sentences: the Doppler shift explanation is a linear approximation to the "stretched light" explanation. Switching from one viewpoint to the other amounts to a change of coordinate systems in (curved) spacetime.

See also http://www.astro.ucla.edu/~wright/cosmology_faq.html#MX

Are galaxies really moving away from us or is space just expanding?


This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift.

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

 

[DavidBektas]

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

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.

 

[chronon]

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).

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

 

[Jorrie]

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

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?

 

[chronon]

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?

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

 

[Jorrie]

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

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.

 

[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:

In special relativity, redshifts arise directly from velocities. It was this idea that led Hubble in 1929 to convert the redshifts of the “nebulae” he observed into velocities, and predict the expansion of the universe with the linear velocity-distance law that now bears his name. The general relativistic interpretation of the expansion interprets cosmological redshifts as an indication of velocity since the proper distance between comoving objects increases. However, the velocity is due to the rate of expansion of space, not movement through space, and therefore cannot be calculated with the special relativistic...

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?

 

[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.

 

[SpaceTiger]

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

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...

 

[Garth]

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...

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

Garth

 

[SpaceTiger]

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

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".

 

[Garth]

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".

Sorry, I should not have included all your quote!
I was referring to your statement:

It hasn't been consistent with the data for quite some time...

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

 

[chronon]

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...

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.

 

[SpaceTiger]

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.

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.

 

[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.

 

[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 dependent 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

 

[fgosborn]

Hello ValenceE, I am new to this forum. I have been reading about Halton Arp and his theories about red shifts, if you do a search on him and also the electric universe model you might find it interesting. I was looking for some discussion on these models, but have yet to find any.

 

[SpaceTiger]

Hello ValenceE, I am new to this forum. I have been reading about Halton Arp and his theories about red shifts, if you do a search on him and also the electric universe model you might find it interesting. I was looking for some discussion on these models, but have yet to find any.

You won't find much discussion of these models on PF, as they have long since been dismissed by mainstream science.

 

[fgosborn]

You won't find much discussion of these models on PF, as they have long since been dismissed by mainstream science.

Whats PF? Is that in reference to Halton Arp or Electric Universe (sorry, not the sharpest tool in the shed)?

 

[SpaceTiger]

Whats PF?

Physics Forums.

Yes, I was referring to both of those models.

 

[fgosborn]

Electric Universe

<<

Physics Forums. >>

<<Yes, I was referring to both of those models.

>>

Thanks, I'm probably missing something obvious then, but;

I’ve read material that plasma makes up 99% of all matter in the universe. That plasma remains electrically charged in space. That it isn’t a perfect conductor so the magnetic fields are not frozen or locked (have only a vague idea what that means).
If I understand the issue, it’s that modern Astronomy accepts a gravity-dominated universe. But it’s based on the assumption that matter is electrically neutral. So gravity would dominate. But if this is not so, then gravity must be a secondary influence. If electromagtic fields dominate, driven by Birkland currents and Z pinch effects, there are would be no missing mass problems or black holes, neutron stars, dark matter ect.

In Halton Arp’s photographs he shows high red shift quasars connected to low red shift galaxies

 

[SpaceTiger]

That plasma remains electrically charged in space.

The presence of positive and negative charges in electromagnetism causes large-scale plasmas to indeed be neutral, rendering electrostatic forces insigificant for driving the motions of celestial bodies (particularly stars, planets, etc.). Magnetic fields, however, are still important driving forces in the interstellar and intergalactic media.

In Halton Arp’s photographs he shows high red shift quasars connected to low red shift galaxies

It was long ago shown that there is no excess of high-redshift quasars around low-redshift galaxies. The photographs you saw were just chance alignments.

 

[Mike2]

I have to wonder how much of the redshift is due to photons from earlier times having to climb out of a deeper gravity well because the universe was denser when they were emitted. What's this effect called again?

 

[SpaceTiger]

I just received a book in the mail by Donald E. Scott, The Electric Sky and he discusses gravitational lensing and Einstein’s Cross and how that the four quasars that surround the galactic core are one gravitationally lensed distant quasar located at a far distance (based on red shift). But the idea is based on perfect alignment of Earth the galaxy core and the distant quasar. It was pointed out that only two images should be present. For four images it would require four objects in perfect alignment; the Earth the galaxy the two distant quasars.

The author seems to be stuck on the simple point mass models of gravitational lensing. In more complex potentials, near-perfect alignment is not necessarily required to produce four images. As I already said, fgosborn, this is not the place to discuss either the electric universe or Halton Arp's models. Please take such discussions elsewhere on the web.

I have to wonder how much of the redshift is due to photons from earlier times having to climb out of a deeper gravity well because the universe was denser when they were emitted. What's this effect called again?

I'm not aware of any such effect. It would be very small, if it even existed.

 

[fgosborn]

Please take such discussions elsewhere on the web.


I searched the archives and found this link posted:

http://www.tim-thompson.com/grey-areas.html

Wish I had done that before I ordered the book. I think I'll refocus my studies.

 

[hellfire]

I have to wonder how much of the redshift is due to photons from earlier times having to climb out of a deeper gravity well because the universe was denser when they were emitted. What's this effect called again?

In a denser past the universe was also homogeneous and isotropic on large scales and therefore the photons were not forced to climb out of any potential wells against any special direction. Local inhomogeneities, however, may lead to a redshift or blueshift of photons emitted beyond them.