I How do we know that red-shift is caused by expansion?

[tkyoung75]

TL;DR Summary

Filtration / diffusion of light can cause red-shift, so how did physicists conclude that redshift is due to expansion.

Hi, I am not a cosmologist, but this question has been bugging me for some time.
I am an engineer, and I own an aquarium, at least I owned an aquarium with an awesome LED light.
Under certain conditions, I found that when the blue night light was on, organics in the water would fluoresce, green, and penetration would be reduced.
How do we know that a similar phenomena, not necessarily organic has not occurred to light from the far field. Flouresence, filtration, or some other action on light of shorter wavelengths?

 

[PeroK]

What research have you done into modern cosmological models?

Note that you've opened this as an A-level thread, which implies you are looking for responses at postgraduate research level. If you haven't studied cosmology at undergraduate level, then it probably should be a B-level.

 

[Orodruin]

First of all, there really is nothing in intergalactic space that would fluoresce like that so there is that. Cosmological redshift also fits very well with observed effects. Fluorescence would not be able to reproduce an overall shift of the entire spectrum, nor could it explain the Lyman alpha forest.

Other things than standard cosmological effects are also being considered generally when looking at cosmology as a whole. A similar situation occurs for the accelerating expansion where dimming by interstellar dust has been considered as a possible explanation of the relative dimness of supernovae, which is the main evidence for accelerating expansion.

 

[EigenState137]

Greetings,

Fluorescence would not be able to reproduce an overall shift of the entire spectrum

That is fundamental. All of the spectral features--from UV to microwave--are shifted the same in the case of the redshift. That fact is of great assistance in the assignments of the spectrum.ES

 

[Vanadium 50]

Filtration / diffusion of light can cause red-shift

That is simply not true.

If you look at the Wikipedia page on Redshift you will see this picture

The spectral features move. Filters don't do this. Neither does fluorescence.

 

[Drakkith]

How do we know that a similar phenomena, not necessarily organic has not occurred to light from the far field. Flouresence, filtration, or some other action on light of shorter wavelengths?

1. Spectral lines themselves are redshifted. Any absorption and reemission effect would obliterate the spectral lines instead of preserving them. So you end up with the same spectral 'fingerprint' that certain elements or molecules emit/absorb, except that it has been stretched. There is no other known mechanism that explains this other than redshift.

2. Distant stars, galaxies, and other objects that emit a thermal spectrum, that is, a spectrum that is produced by virtue of an object being hot. Their spectrums are observed to be identical to a non-redshifted spectrum except that it has been 'stretched', which is exactly what we would expect when something is redshifted.

3. Any kind of fluorescence would be incredibly obvious when we look at the spectra of objects in space since the spectrum of a fluorescing object looks like nothing else in nature. We don't see this.

 

[EigenState137]

Greetings,

the spectrum of a fluorescing object looks like nothing else in nature

I do not think that statement is true.ES

 

[Drakkith]

I do not think that statement is true.

It's possible that I am mistaken. Do you have a counterexample?

 

[EigenState137]

Greetings,

Fluorescence is nothing more than the radiative decay of some set of bound excited states under electric dipole selection rules. The term is generally associated with population of those bound excited states via a radiation source. In condensed media, non-radiative relaxation channels can be fast relative to radiative decay such that the initial population distribution of excited states is significantly altered. That is not the case in low density gas phase systems, particularly with atoms that lack vibrational and rotational degrees of freedom.

The emission spectrum from an emission nebula or HI cloud that is excited by the continuum of a nearby star should therefore look very much like the emission spectrum of a similar object undergoing radiative recombination. The bound excited states are the same, the selection rules are the same. Population distributions and thus relative line strengths, and lineshapes might be different but those would be more subtle effects. Indeed the principal difference is the energy spectrum of the excitation source—does it populate predominately bound states, or is it sufficiently energetic to photoionize the system.

If we agree that astrophysical systems such as nebulae and HI or HII clouds are indeed “objects”, and if we agree that radiative recombination can indeed yield the observed emission spectra associated with such objects, then the spectrum of a fluorescing object can indeed look very much like something else in nature. The difference is in the excitation process, not in the radiative decay process.ES

 

[Drakkith]

My apologies, I was thinking of phosphorescence, not simple fluorescence.

 

[tkyoung75]

Thank you all for taking the time to answer my question and providing insight.
Sorry for posting as a post graduate topic. I can see now that it is quite fundamental.
Regards.

 

[EigenState137]

Greetings,

My apologies, I was thinking of phosphorescence, not simple fluorescence.

No apology necessary.

Phosphorescence is indeed a different beast. The closest thing I can think of to phosphorescence would be the electric quadrupole (E2) and magnetic dipole (M1) emission lines associated with collisionally pumped, low-lying metastable states such as [OII], [OIII], [NII], and [SII]. Not phosphorescence, but forbidden transition.ES

 

[artis]

I think @EigenState137 said it best in post #4 as well as @Orodruin in post #3.
If it was just the visible spectrum we are talking about but it's also the CMBR and in each case we get the same result.

@tkyoung75
And even if the intergalactic space was filled with fluorescent dust we would still get similar results to those of the existing and already mentioned "Lyman alpha forest" , because even if the same phosphor is used depending on the distance from Earth it's emission peak would differ so in the end you would get not one but multiple ones, the further the cloud the lower the frequency of the light.PS. Now that I think of it, this would be a cool way to make those older mercury vapor fluorescent tubes seem more natural, they emit multiple peaks but if you redshift those peaks you could smear out the spectrum of the lamp. There is just one problem, this implies your ceiling lamp needs to recede from you close to the speed of light and multiple lamps would need different receding speeds in order to make the spectrum linear.