Experimental evidence on cosmological redshift

[zrek]

TL;DR Summary

I'm looking for argues how the scientists figured out that the strecthing spacetime causes the redshift and not other possible explanations.

The space expands due the dark energy, so the light wavelength also stretches during its journey between galaxies, causing a measurable redshift.
How can I argue that this is the right reasoning for the measured redshift? What if someone states that the photon maybe simple loses its energy over long distances, so this is why its wavelength increases? Is there some evidence against this approach?

 

[Bandersnatch]

There's a number of arguments, summarised e.g. here: https://www.astro.ucla.edu/~wright/tiredlit.htm
'Tired light' is the general term for the proposition that you describe.

I'm fond of the the Tolman Surface Brightness Test for how intuitive it is (only mentioned in passing in the link above, but see the paper linked therein):
If the light were to lose energy by some mechanism that is only dependent on the distance of travel, the surface brightness of distant objects would fall in inverse proportion to the redshift $L \propto (z+1)^{-1}$. If the cause is expansion of space, the surface brightness reduces by $L \propto (z+1)^{-4}$. The extra three (z+1) factors appear because in expanding space there is a reduction in photon density. One (z+1) factor is due to the time delay between consecutive wave peaks increasing the faster the object is receding (each peak is emitted from a bit further back than the preceding one); i.e. it's a radial effect, along the line of sight. The remaining two (z+1) factors come from the changing perceived size of the observed object between emission and reception; i.e. one (z+1) for each dimension perpendicular to the line of sight.

In other words, in non-expanding universe we'd see any typical galaxy sending the same amount of photons per unit surface, regardless of redshift. Twice the surface area would always mean twice the photons. In an expanding universe we expect the galaxies to be sending significantly less photons per unit area the more distant they are, as their image is 'diluted' by the expansion .

The same argument can be applied to the CMBR, as explained in the link.

 

[Orodruin]

Summary:: I'm looking for argues how the scientists figured out that the strecthing spacetime causes the redshift and not other possible explanations.

The space expands due the dark energy,

Apart from the post by @Bandersnatch , this quoted statement contains a common misconception: The expansion of space is not caused by dark energy. The universe could perfectly well expand without dark energy and did so being matter dominated for a long long time. Only relatively recently has dark energy started to dominate. What dark energy does cause is accelerated expansion.

 

[Ibix]

The space expands due the dark energy

This isn't correct, by the way, as Orodruin says. Space expands in a universe without dark energy, but the expansion rate and the way it changes over time is different is affected by dark energy. Looking at the history of expansion, then, tells us about dark energy.

 

[zrek]

There's a number of arguments, summarised e.g. here: https://www.astro.ucla.edu/~wright/tiredlit.htm
'Tired light' is the general term for the proposition that you describe.

I'm fond of the the Tolman Surface Brightness Test for how intuitive it is (only mentioned in passing in the link above, but see the paper linked therein):
If the light were to lose energy by some mechanism that is only dependent on the distance of travel, the surface brightness of distant objects would fall in inverse proportion to the redshift $L \propto (z+1)^{-1}$. If the cause is expansion of space, the surface brightness reduces by $L \propto (z+1)^{-4}$. The extra three (z+1) factors appear because in expanding space there is a reduction in photon density. One (z+1) factor is due to the time delay between consecutive wave peaks increasing the faster the object is receding (each peak is emitted from a bit further back than the preceding one); i.e. it's a radial effect, along the line of sight. The remaining two (z+1) factors come from the changing perceived size of the observed object between emission and reception; i.e. one (z+1) for each dimension perpendicular to the line of sight.

In other words, in non-expanding universe we'd see any typical galaxy sending the same amount of photons per unit surface, regardless of redshift. Twice the surface area would always mean twice the photons. In an expanding universe we expect the galaxies to be sending significantly less photons per unit area the more distant they are, as their image is 'diluted' by the expansion .

The same argument can be applied to the CMBR, as explained in the link.

Thank you, sounds interesting and surprising.

 

[zrek]

This isn't correct, by the way, as Orodruin says. Space expands in a universe without dark energy, but the expansion rate and the way it changes over time is different is affected by dark energy. Looking at the history of expansion, then, tells us about dark energy.

Thanks for the correction. However I'd add that the other non-accelerating effects of the expansion would not cause the spacetime structure itself expandig, only making the distance bigger between the objects in space (I mean the objects are traveling by them in the space and not the space itself is growing), so by that the wavelength of the light would not change, and the redshift would be not because of the stretching of space, but the doppler effect of the moving objects. Correct me if I'm wrong.

 

[Orodruin]

Thanks for the correction. However I'd add that the other non-accelerating effects of the expansion would not cause the spacetime structure itself expandig, only making the distance bigger between the objects in space (I mean the objects are traveling by them in the space and not the space itself is growing), so by that the wavelength of the light would not change, and the redshift would be not because of the stretching of space, but the doppler effect of the moving objects. Correct me if I'm wrong.

You are wrong. The only way to have distances growing by "objects moving in space" is in the Milne universe, which is actually a coordinate patch on Minkowski space and you can transform the Milne coordinates to the typical Minkowski ones - or just consider it locally in local Minkowski coordinates.

Also, there is no real distinction between cosmological redshift and Doppler shift. There is just general redshift. What is typically referred to as "cosmological" redshift is a particular coordinate interpretation for comoving observers but the underlying cause can be traced back to the same mathematics. See https://www.physicsforums.com/insights/coordinate-dependent-statements-expanding-universe/

 

[PeroK]

Correct me if I'm wrong.

You're wrong! Redshift is redshift. Dark energy just changes (increases) the amount of redshift over time.

 

[zrek]

You are wrong. The only way to have distances growing by "objects moving in space" is in the Milne universe, which is actually a coordinate patch on Minkowski space and you can transform the Milne coordinates to the typical Minkowski ones - or just consider it locally in local Minkowski coordinates.

Also, there is no real distinction between cosmological redshift and Doppler shift. There is just general redshift. What is typically referred to as "cosmological" redshift is a particular coordinate interpretation for comoving observers but the underlying cause can be traced back to the same mathematics. See https://www.physicsforums.com/insights/coordinate-dependent-statements-expanding-universe/

"...only way to have distances growing by "objects moving in space"" ... Well, this is a very surprising for me, because I've always heard that some objects may travel from us quicker than light, and it is possible because the space stretches itself. And the space stretch itself cause a specific wavelength increase of the light. But since the topic currently is not this debate, I just wanted to express my surprise and not discussing this in details here.

 

[Orodruin]

Well, this is a very surprising for me, because I've always heard that some objects may travel from us quicker than light, and it is possible because the space stretches itself.

Uhm ... I do not think you are reading my post correctly.

I am saying that the only way you can have a universe where the redshift between comoving observers can be explained as a pure Doppler shift is the Milne universe. The reason for this is that it is just Minkowski space and the comoving observers are just inertial observers traveling from the same spacetime origin. If you go to Minkowski coordinates, none of the objects are traveling faster than light. In fact, in no situation is any object traveling faster than light. In an expanding universe in Robertson-Walker coordinates, distances may grow faster than the speed of light but this does not mean that anything is traveling faster than the speed of light. It is also a coordinate dependent statement that is predicated on using Robertson-Walker coordinates.

Edit: Note. This is all a discussion that is quite significantly above B-level. A deeper knowledge will be required to make appropriate sense out of it.

 

[Ibix]

Well, this is a very surprising for me, because I've always heard that some objects may travel from us quicker than light, and it is possible because the space stretches itself. And the space stretch itself cause a specific wavelength increase of the light.

Yes, more or less. But this expansion, including faster-than-light recession, happens with or without dark energy. This fact follows from the Friedmann equations if you have any non-zero mass or energy density. Orodruin did not say otherwise.

 

[JimJCW]

Summary:: I'm looking for argues how the scientists figured out that the strecthing spacetime causes the redshift and not other possible explanations.

The space expands due the dark energy, so the light wavelength also stretches during its journey between galaxies, causing a measurable redshift.
How can I argue that this is the right reasoning for the measured redshift? What if someone states that the photon maybe simple loses its energy over long distances, so this is why its wavelength increases? Is there some evidence against this approach?

I think the expansion of space is not an observed fact; it is a hypothesis in the ΛCDM (Big Bang) model. A cosmological model, Big Bang or others, needs to explain the observed µ vs. z data,

(See the thread, How well do cosmological models explain the observed µ vs. z data?)

It will be interesting to see how well the expression given in Post #2 by @Bandersnatch can explain the above figure:​

 

[phinds]

I think the expansion of space is not an observed fact

Seriously? So, you believe that objects at the edge of our observable universe that are receding from us at about 3c are actually MOVING faster than the speed of light rather than receding in an expanding universe?

 

[Orodruin]

Seriously? So, you believe that objects at the edge of our observable universe that are receding from us at about 3c are actually MOVING faster than the speed of light rather than receding in an expanding universe?

Both of those seem like coordinate dependent statements to me ...

That said, the LambdaCDM model is a pretty good fit to cosmological data such as redshift-luminosity etc and that is coordinate independent.

 

[phinds]

Both of those seem like coordinate dependent statements to me ...

OK, how is the fact that the distance between objects is increasing faster than c co-ordinate dependent? That is, what am I missing?

 

[Ibix]

OK, how is the fact that the distance between objects is increasing faster than c co-ordinate dependent? That is, what am I missing?

The distance to an object now is $\int\sqrt{|g|}dx$ where the integral is carried out along a surface of constant time. But what "a surface constant time" is depends on your coordinate choice. There's a natural choice (constant cosmological time) in FLRW spacetimes, but you aren't required to use it.

The redshift is the directly measured invariant.

 

[Orodruin]

OK, how is the fact that the distance between objects is increasing faster than c co-ordinate dependent? That is, what am I missing?

Distance is coordinate dependent (or rather dependent of the foliation of spacetime into simultaneities) so how distance changes is therefore coordinate dependent by construction.

See also https://www.physicsforums.com/insights/coordinate-dependent-statements-expanding-universe/

 

[PeterDonis]

I think the expansion of space is not an observed fact; it is a hypothesis

More precisely, it is the only hypothesis that anyone has found that can explain all of the observed data on redshifts, luminosities, angular sizes, features of the CMB, etc.

(Even more precisely, "expansion of space" is a general property of a class of GR models, and this class of models is the only class of models that anyone has found that can explain all of the observed data in cosmology.)

in the ΛCDM (Big Bang) model.

$\Lambda CDM$ is not the same as "Big Bang"--the latter is a much more general class of models. $\Lambda CDM$ includes a number of additional hypotheses besides "expansion of space" (which is a feature of all "Big Bang" type models), such as "dark energy density is constant" and "cold dark matter". Those additional hypotheses could end up having to be modified, without changing "expansion of space" at all.

 

[PeterDonis]

It will be interesting to see how well the expression given in Post #2 by @Bandersnatch can explain the above figure:

View attachment 298348

Don't we already know it explains it quite well? See post #8 in the previous thread you linked to.

 

[Bandersnatch]

@phinds, you don't 'see' anything moving faster than light. Or expanding. All you can directly observe is redshifts, and brightness, and sizes, and some secondary effects like the shape of luminosity curves, that you then fit a model to. That's where Jim was coming from, even if the conclusion he draws is far too strong.

It will be interesting to see how well the expression given in Post #2 by @Bandersnatch can explain the above figure:

The figure plots distance modulus - i.e. magnitudes. There can be no factor from the changing surface brightness showing in there, since for magnitudes it doesn't matter how diffuse an object is. It's the total amount of incoming light that counts. E.g. Andromeda Galaxy has apparent magnitude of ~3.5, but it's barely visible to the naked eye because of how spread out the incoming light is.
Whether the time delay factor shows in there or not, I don't know. But the Tolman test can be seen directly in the Lubin & Sandage paper from the link in post #2, as well as in the CMB prefactor section (same effect, different target) of the same link.

I think the expansion of space is not an observed fact; it is a hypothesis in the ΛCDM (Big Bang) model.

While it's true that expansion as such is not a direct observable, it's status is not that of just a hypothesis. The expansion (or contraction) of space is a prediction of the current-best theory of gravity. The reality of expansion is then validated by observations matching the predictions (off the top of my head: the existence and features of the CMBR, the lyman-alpha forest, the shape of SN1a light curves, the aforementioned Tolman surface brightness test). Additionally, expansion is not subordinate to the LCDM model - the model is subordinate to the expansion. While you can argue the specifics of the LCDM - is that L constant or not?; is that DM cold or not?; should that DM be modified gravity instead? what are the precise values of these parameters? - there's little room left for arguing against the expansion at this stage. It's way past being a hypothesis.

 

[JimJCW]

$\Lambda CDM$ is not the same as "Big Bang"--the latter is a much more general class of models. $\Lambda CDM$ includes a number of additional hypotheses besides "expansion of space" (which is a feature of all "Big Bang" type models), such as "dark energy density is constant" and "cold dark matter". Those additional hypotheses could end up having to be modified, without changing "expansion of space" at all.

I will be more careful when writing something like, “… in the ΛCDM (Big Bang) model.” According to Lambda-CDM model,

The ΛCDM … model is a parameterization of the Big Bang cosmological model …​

​

It is frequently referred to as the standard model of Big Bang cosmology …​

​

 

[Orodruin]

I will be more careful when writing something like, “… in the ΛCDM (Big Bang) model.” According to Lambda-CDM model,

The ΛCDM … model is a parameterization of the Big Bang cosmological model …​

​

It is frequently referred to as the standard model of Big Bang cosmology …​

​

Still not accurate.

 

[JimJCW]

More precisely, it is the only hypothesis that anyone has found that can explain all of the observed data on redshifts, luminosities, angular sizes, features of the CMB, etc.

(Even more precisely, "expansion of space" is a general property of a class of GR models, and this class of models is the only class of models that anyone has found that can explain all of the observed data in cosmology.)

As you pointed out in Post #19, the ΛCDM model, based on the hypothesis of expansion of space, can explain the observed µ vs. z data very well:

Please let me make the following demonstration:

Sorrell’s 2009 paper reported the following µ vs. z relation based on a non-expanding universe model:

As shown in Fig. 2 of the paper, the result can explain the observed supernova µ vs. z data reasonably well up to z = 2. But, as you commented in Post #10 of the thread, How well do cosmological models explain the observed µ vs. z data?, it may not work as well beyond z = 2.

Sorrell emphasized in the paper that his model has no free adjustable parameters. Maybe we can treat n and H₀ in the above equation as parameters and determine their values using least square method by fitting the above equation to the observed µ vs. z data. (The ΛCDM model has several parameters.) The result is given by

The calculated curve is compared with the observed data as shown below:

The above figure suggests that the modified Sorrell model can fit the observed µ vs. z data as well as the ΛCDM model can. Of course, work is still needed to interpret the result.

 

[PeterDonis]

Sorrell emphasized in the paper that his model has no free adjustable parameters. Maybe we can treat n and H0 in the above equation as parameters

Are there any actual papers that do this? Or is it just your personal theory?

 

[JimJCW]

Are there any actual papers that do this? Or is it just your personal theory?

Please see Sorrell’s 2009 paper, Misconceptions about the Hubble recession law.

 

[Vanadium 50]

Tired light.
Tired hypothesis.
https://en.wikipedia.org/wiki/Tired_light

Wait...I think I posted this the last time you trotted out tired ight. Or maybe it was the time before that. Or the time before that.

 

[PeterDonis]

Please see Sorrell’s 2009 paper, Misconceptions about the Hubble recession law.

The model described in that paper, as you yourself noted in the very passage from your post that I quoted, has no adjustable parameters. I am asking about your suggestion that one could treat $n$ and $H_0$ as parameters. Is there any reference for that?

 

[JimJCW]

The model described in that paper, as you yourself noted in the very passage from your post that I quoted, has no adjustable parameters. I am asking about your suggestion that one could treat $n$ and $H_0$ as parameters. Is there any reference for that?

Treating n and H₀ as parameters is my conjecture. The result shown in the figure is interesting. As I said, ‘work is still needed to interpret the result.’

 

[PeterDonis]

Treating n and H₀ as parameters is my conjecture.

Then it is off limits for discussion here. If you get it published in a peer-reviewed paper, then it can be used as a basis for PF discussion.

 

[JimJCW]

While it's true that expansion as such is not a direct observable, it's status is not that of just a hypothesis.

. . . there's little room left for arguing against the expansion at this stage. It's way past being a hypothesis.

Big Bang is the prevailing model of the universe. What you are saying is believed by most people. However, the idea that the observable universe expanded from a tiny point of Planck length (1.6×10^-35 m) to the present radius of 4.4×10²⁶ m (46.5 Gly) is so overwhelming that some people may have traces of uncertainties back in their minds.

The following articles related to expansion of space [have been deleted by the moderators since they are not valid references.]

 

[PeterDonis]

The following articles

Are not valid references.

 

[PeroK]

Big Bang is the prevailing model of the universe. What you are saying is believed by most people. However, the idea that the observable universe expanded from a tiny point of Planck length (1.6×10^-35 m) to the present radius of 4.4×10²⁶ m (46.5 Gly) is so overwhelming that some people may have traces of uncertainties back in their minds.

A lot of scientific discoveries are overwhelming. That a single fertilised egg can develop into a fully grown human being. The evolution of life including intelligent, conscious humans.

That the solar system evolved from a cloud of dust. Even that there are trillions of galaxies each with hundreds of billions of stars.

It's not a good argument against a theory simply to say that the theory is mind boggling.

 

[JimJCW]

Big Bang is the prevailing model of the universe. What you are saying is believed by most people. However, the idea that the observable universe expanded from a tiny point of Planck length (1.6×10^-35 m) to the present radius of 4.4×10²⁶ m (46.5 Gly) is so overwhelming that some people may have traces of uncertainties back in their minds.

The following articles related to expansion of space [have been deleted by the moderators since they are not valid references.]

Please see Wendy Freedman’s 2021 paper, Measurements of the Hubble Constant: Tensions in Perspective, in The Astrophysical Journal.

 

[Vanadium 50]

Why are you arguing with yourself?

 

[JimJCW]

Why are you arguing with yourself?

If I seem to be not committed, it’s because I wish to find the truth.