Is redshift unreliable as a measuring tool?

  • Thread starter azzkika
  • Start date
  • #1
60
0

Main Question or Discussion Point

If there are 2 objects emitting light to each other and the light fills the spacetime between them and then that spacetime expands the waveform becomes stretched. However, if the middle of the spacetime between the 2 objects has yet to have light enter it from either object and that spacetime expands, the light will take longer to reach the objects from each other but the wavelength will not have been stretched.

I am no expert but that is my understanding. Please let me know if i am just stupid, or could redshift in fact be entirely unreliable?
 

Answers and Replies

  • #2
marcus
Science Advisor
Gold Member
Dearly Missed
24,738
785
...
I am no expert but that is my understanding. Please let me know if i am just stupid, or could redshift in fact be entirely unreliable?
I'll give you my opinion. What you say doesn't sound stupid. I believe that redshift is not entirely reliable as a handle on distance. It is only as reliable as the model we use to derive estimates of distance from redshift measurement.

this is the mainstream cosmo model. It is supported by a lot of different data of different kinds that fit together surprisingly well. So it's pretty convincing and, absent solid evidence that there's something wrong, people trust it provisionally and use it. It is based on Gen Rel geometry which seems to work slightly better than Euclidean esp over large distances.

There are other measures of distance that one can use to compare with what you get from redshift using the accepted math model. The model so to speak harmonizes the different measures of distance, makes sense out of the different handles we have on the universe layout.

But there is no absolute reason the model HAS to be right.
===============

The physical picture of a spatial medium that stretches lightwaves is not accepted. So the physical reasoning you gave should be kept in the category of a visual image or a metaphor. Largescale geometry itself changes, distances* can change, but I know of no completely satisfactory simple explanation that fits with the physical intuition we acquire experiencing static earthbound geometry.

Other consequences of Gen Rel, besides expansion, are also unintuitive, like the GPS time effect. But Gen Rel has been checked to high accuracy as a model of how geometry behaves and the unintuitive stuff doesn't go away.

Probably we eventually need better basic intuition about geometry. Perhaps a quantum field theoretical understanding of space and time will someday resolve some of these puzzles and calm the feeling of amazement.

*even the distances between objects which appear to be stationary with respect to the background of ancient light
 
  • #3
60
0
I'll give you my opinion. What you say doesn't sound stupid. I believe that redshift is not entirely reliable as a handle on distance. It is only as reliable as the model we use to derive estimates of distance from redshift measurement.

this is the mainstream cosmo model. It is supported by a lot of different data of different kinds that fit together surprisingly well. So it's pretty convincing and, absent solid evidence that there's something wrong, people trust it provisionally and use it. It is based on Gen Rel geometry which seems to work slightly better than Euclidean esp over large distances.

There are other measures of distance that one can use to compare with what you get from redshift using the accepted math model. The model so to speak harmonizes the different measures of distance, makes sense out of the different handles we have on the universe layout.

But there is no absolute reason the model HAS to be right.
===============

The physical picture of a spatial medium that stretches lightwaves is not accepted. So the physical reasoning you gave should be kept in the category of a visual image or a metaphor. Largescale geometry itself changes, distances* can change, but I know of no completely satisfactory simple explanation that fits with the physical intuition we acquire experiencing static earthbound geometry.

Other consequences of Gen Rel, besides expansion, are also unintuitive, like the GPS time effect. But Gen Rel has been checked to high accuracy as a model of how geometry behaves and the unintuitive stuff doesn't go away.

Probably we eventually need better basic intuition about geometry. Perhaps a quantum field theoretical understanding of space and time will someday resolve some of these puzzles and calm the feeling of amazement.

*even the distances between objects which appear to be stationary with respect to the background of ancient light
What got me thinking (sorry no link I cannot find it), was a set of blogs of astronomers debating redshift and some were advocating it as wrong given individual stars having totally different redshifts from their host galaxy. I wondered if this was possible due to the stars having developed long after the other stars in the galaxy and spacetime having stretched the light from the galaxy more with spacetime expansion prior to the origination of newer stars in the same galaxy. If this is possible in theory then redshift cannot be accurate if there's no way to determine if spacetime has expanded before light entered it.
 
  • #4
marcus
Science Advisor
Gold Member
Dearly Missed
24,738
785
... I wondered if this was possible due to the stars having developed long after the other stars in the galaxy and spacetime having stretched the light from the galaxy more with spacetime expansion prior to the origination of newer stars in the same galaxy. If this is possible in theory...
That isn't possible in theory. The light we get today, from a galaxy, has all traveled approximately the same length of time and has been redshifted approximately the same.

The age of individual stars in the galaxy does not affect the overall travel time of their light.
 
  • #5
zonde
Gold Member
2,941
213
What got me thinking (sorry no link I cannot find it), was a set of blogs of astronomers debating redshift and some were advocating it as wrong given individual stars having totally different redshifts from their host galaxy.
If they were talking about redshift of individual stars that must be about very close galaxies. And in that case that discussion should be about local velocity effects and interpretations not about cosmological expansion.

I wondered if this was possible due to the stars having developed long after the other stars in the galaxy and spacetime having stretched the light from the galaxy more with spacetime expansion prior to the origination of newer stars in the same galaxy. If this is possible in theory then redshift cannot be accurate if there's no way to determine if spacetime has expanded before light entered it.
Redshift depends from age of light not from age of star so to say. So it's hard to take your considerations seriously.
 
  • #6
60
0
That isn't possible in theory. The light we get today, from a galaxy, has all traveled approximately the same length of time and has been redshifted approximately the same.

The age of individual stars in the galaxy does not affect the overall travel time of their light.
It is not the travel time, rather if space expansion occurred with light from a galaxy that redshifted the light and then a star developed after the expansion or the rate of expansion had changed, that would account for anomolous readings of stars to their inherent galaxy.
 
  • #7
60
0
If they were talking about redshift of individual stars that must be about very close galaxies. And in that case that discussion should be about local velocity effects and interpretations not about cosmological expansion.


Redshift depends from age of light not from age of star so to say. So it's hard to take your considerations seriously.
Maybe I misunderstand redshift. I was under the impression redshift occurs due to wavelengths stretching when spacetime expands and has nothing to do with age.
 
  • #8
zonde
Gold Member
2,941
213
Maybe I misunderstand redshift. I was under the impression redshift occurs due to wavelengths stretching when spacetime expands and has nothing to do with age.
Redshift occurs as wavelength of light is stretched between it's emission and absorption. The longer the period between emission and absorption the more is light redshifted.
 
  • #9
Chronos
Science Advisor
Gold Member
11,408
738
Fortunately, we have a clear choice here: redshift is due either to expansion of the universe [science as we know it], or some version of the tired light fairy. I believe fairies are less reliable than science.
 
Last edited:
  • #10
60
0
Redshift occurs as wavelength of light is stretched between it's emission and absorption. The longer the period between emission and absorption the more is light redshifted.
So it has nothing to do with source travelling towards or away from viewer then? Would it not be blue shifted if travelling towards us and redshifted if travelling away regardless of how long it took to reach us?
 
  • #11
zonde
Gold Member
2,941
213
So it has nothing to do with source travelling towards or away from viewer then? Would it not be blue shifted if travelling towards us and redshifted if travelling away regardless of how long it took to reach us?
Of course source traveling toward viewer is blueshifted and source traveling away from viewer is redshifted.
But independently how do you interpret redshift there is general observation that distance (as determined independently from redshift) and redshift are correlated i.e. the more the redshift the more the distance to the source. Well, except for small deviations from average that are attributed to "local" speeds.
 
  • #12
856
12
Fortunately, we have a clear choice here: redshift is due either to expansion of the universe [science as we know it], or some version of the tired light fairy. I believe fairies are less reliable than science.
So a photon which has travelled for 13Billion years has no chance of encountering an effect which changes its frequency? There is attenuation of light as well as dispersion from a narrow beam to a wide beam, but these cannot affect frequency? How about a single photon which cannot be attenuated any further by the medium?


The only way of reducing its energy is to reduce its frequency. There are many processes where light is absorbed by a material and ejected with a different wavelength. One that might be familiar to you is fluorescence, where light of higher energy /higher wavelength is absorbed (e.g. green), then emitted at a lower wavelength (e.g. red). The photon is not really being 'slowed down', but the wavelength in is different than the wavelength out.


I am sure that this has been fully considered by CMBR physicists and I am probably babbling, I havent had my first coffee :)
 
  • #13
Chronos
Science Advisor
Gold Member
11,408
738
A photon does not 'lose' energy when redshifted, the energy is merely time dilated.
 
  • #14
856
12
A photon does not 'lose' energy when redshifted, the energy is merely time dilated.

What is time dilated photon energy? Is it another way of saying that the frequency or energy that the photon has is lower when it interacts with the matter in our spatial frame of reference. Trying to remember the terms used in my special relativity class in 1980!

Does anyone at all still consider Tired Light as a possible explanation for red shift?
 
  • #15
zonde
Gold Member
2,941
213
What is time dilated photon energy? Is it another way of saying that the frequency or energy that the photon has is lower when it interacts with the matter in our spatial frame of reference. Trying to remember the terms used in my special relativity class in 1980!
If you interpret redshift as Doppler effect then you have two effects because of time dilation - photon to photon frequency is reduce resulting in intensity drop and individual photon frequency is reduced as well resulting in even more intensity drop.
Summary effect however is drop in intensity at four order compared to redshift. Other two additional orders come from aberration if I am not mistaken.

Not very sure how to treat intensity drop associated with redshift in case of coordinate expansion but it seems that you should have the same drop of 4 orders.

Does anyone at all still consider Tired Light as a possible explanation for red shift?
Searching arxiv.org for "tired light" gave me two papers:
http://arxiv.org/abs/astro-ph/9904131" [Broken]
http://arxiv.org/abs/0707.3351" [Broken]

Not too much considering that it is most obvious alternative to Big Bang.
 
Last edited by a moderator:

Related Threads on Is redshift unreliable as a measuring tool?

Replies
5
Views
2K
  • Last Post
Replies
2
Views
714
Replies
9
Views
818
Replies
9
Views
2K
  • Last Post
3
Replies
56
Views
9K
Replies
3
Views
1K
Replies
16
Views
15K
Replies
7
Views
1K
Top