expansion of the universe

[ragasssd]

please forgive me if I'm missing something. They conclude the universe is expanding based on red shift of distant galaxies. But isn't it possible that some other particles or properties in intersteller/galactic space we don't know about is causing the light to change wavelength? Can't some material, like dust particles, be causing light from more distant galaxies to red shift?

[primummobile]

One thing is that we also see blue shifted objects, although very few. M31 (the Andromeda Galaxy) is a notable example.

[Strange]

This was one of the first theories for red shift and has not survived testing: http://en.wikipedia.org/wiki/Tired_light

One problem is that those sort of interactions would cause other effects, such as blurring. Another is that we observe an equal time dilation of distant objects.

[John Jaksich]

please forgive me if I'm missing something. They conclude the universe is expanding based on red shift of distant galaxies. But isn't it possible that some other particles or properties in intersteller/galactic space we don't know about is causing the light to change wavelength? Can't some material, like dust particles, be causing light from more distant galaxies to red shift?

Please don't apologize. When Edwin Hubble and Milton Humison measured the Redshifts of galaxies in the early 1920s to 1930s--they did so using spectroscopic techniques that the so-called dust particles of which you speak would not have been sensitive enough to detect. Only in the case where the measurements are in the 1 to 5 nanometers of a spectral scan----and this would occur on the molecular scale---primarily. I am unsure of what "tired light" is all about?---but it does sound somewhat suspect to me?? I don't know.

[John Jaksich]

One thing is that we also see blue shifted objects, although very few. M31 (the Andromeda Galaxy) is a notable example.

Is it not the case that the two galaxies are moving toward one another --so the observed spectrum would be "blue shifted" just the same.

[primummobile]

Is it not the case that the two galaxies are moving toward one another --so the observed spectrum would be "blue shifted" just the same.

Yes. That was the point. M31 is approaching us. So it is blue shifted. But other galaxies that lie in roughly the same line of sight as M31 are red-shifted. It's a simplistic answer but offers clear evidence that the red shift is not caused by any intergalactic particles the light is travelling through. And the blue shift is very small compared to the red shift we see of more distant galaxies.

[John Jaksich]

Yes. That was the point. M31 is approaching us. So it is blue shifted. But other galaxies that lie in roughly the same line of sight as M31 are red-shifted. It's a simplistic answer but offers clear evidence that the red shift is not caused by any intergalactic particles the light is travelling through. And the blue shift is very small compared to the red shift we see of more distant galaxies.

I thought that the OP asked if "dust" could be in effect a "cause of blue" coloration---I must have mistaken how to answer it----maybe it was a homework question?

[primummobile]

I thought that the OP asked if "dust" could be in effect a "cause of blue" coloration---I must have mistaken how to answer it----maybe it was a homework question?

No, that's the way I took it too. But if galaxies in the same line of sight as M31 are shifted red, the light is passing through the same "dust" for at least the last two million light years of the journey. Why would one be red and the other blue?

[primummobile]

Or for that matter, why would we only observe around 16 blue-shifted objects out of the billions of objects we can see?

[John Jaksich]

Or for that matter, why would we only observe around 16 blue-shifted objects out of the billions of objects we can see?

Good point--well said!

[ctcoker]

The simple answer for why it couldn't be dust is that dust reddens the light*, but does not and cannot redshift spectral lines; lines are how we determine redshift, not photometric color information. This dust (or any other material, for that matter), if there were enough of it to extinct the UV and optical portions of the spectrum, would then reemit in the far IR as a blackbody and be plainly visible that way.

*It does so by preferentially scattering and absorbing bluer light; most astrophysical dust is much more transparent in the IR than optical or UV wavelengths, and is almost completely transparent in the far IR.

[George]

The supernovae studies, which demonstrate the acceleration of the expansion, also serve to refute a redshift due to some sort of photon interaction with gas and dust.

[George]

*It does so by preferentially scattering and absorbing bluer light; most astrophysical dust is much more transparent in the IR than optical or UV wavelengths, and is almost completely transparent in the far IR.

ngc3314a probably has answered this for me from the ancient world of the Bad Astronomy forum, but is this scattering almost completely addressed in Rayleigh Scattering?

[John Jaksich]

ngc3314a probably has answered this for me from the ancient world of the Bad Astronomy forum, but is this scattering almost completely addressed in Rayleigh Scattering?

On a nanometer scale and smaller ---you would be able to measure "so-called" interactions---but aren't these dust particles on the order of parsecs---

To me, at least, that would not make any sense in terms of the distances involved.

[George]

On a nanometer scale and smaller ---you would be able to measure "so-called" interactions---but aren't these dust particles on the order of parsecs---

To me, at least, that would not make any sense in terms of the distances involved.

My question is likely more shallow than you might think. Since most of the gas and dust are less than the size of the wavelength of blue, or violet, light then I assume Rayleigh Scattering would account for nearly all the reddening we see. I may be underestimating absorption, admittedly. Of course, IR does not scatter that much, at least not by the Rayleigh Scattering account.

[John Jaksich]

My question is likely more shallow than you might think. Since most of the gas and dust are less than the size of the wavelength of blue, or violet, light then I assume Rayleigh Scattering would account for nearly all the reddening we see. I may be underestimating absorption, admittedly. Of course, IR does not scatter that much, at least not by the Rayleigh Scattering account.

George:

I understand your point---(I think)---to me at least, I try to envision on a test-tube scale first with varying gradients of concentration---IR spectra are shown to shifted to varying positions of red or blue from "theoretical" spectra ---all which are dependent upon concentration or molecule (--or atom type) or both.

Maybe I am barking at the moon? I am not attempting to go tangential on the OP.

[George]

George:

I understand your point---(I think)---to me at least, I try to envision on a test-tube scale first with varying gradients of concentration---IR spectra are shown to shifted to varying positions of red or blue from "theoretical" spectra ---all which are dependent upon concentration or molecule (--or atom type) or both.

Maybe I am barking at the moon? I am not attempting to go tangential on the OP.

I am unclear what you mean by IR shifting to red or blue positions. Gas and dust along the light path will scatter light in accord with Rayleigh Scattering. What I don't know is by how much relative to other scattering or absorbing (and emission) processes, which may be more the area you are considering.

[John Jaksich]

I am unclear what you mean by IR shifting to red or blue positions. Gas and dust along the light path will scatter light in accord with Rayleigh Scattering. What I don't know is by how much relative to other scattering or absorbing (and emission) processes, which may be more the area you are considering.

I am getting the impression we are talking past one another, also. What I meant by my statements is that this so-called interstellar dust from the OP plays no significant role in the Red-shifting of galaxies (or in the case of the Andromeda galaxy--a blue shift).

I am not arguing against the fact of Rayleigh scattering.

When I went on to cite my hastily put together "thought-experiment "--I obviously left out some salient details (I apologize for it). Let me attempt to re-state it concretely: The OP cited a so-called Interstellar Dust as being a source of an (?) interfering Red-shift in the spectra of accelerating galaxies (?). I attempted to illustrate that for reasons that whether this dust was (i.e. PAHs) or hydrogen or some other species (i.e. CO)---the density within a given volume of space would dictate how and to what extent these species would only be responsible for their own spectral signature and not have a measurable effect upon the spectrum of the receding galaxies.


I was attempting (in varying degrees ineptitude?) to explain to the OP how it is not possible, within the given experiments of Hubble and Humason, to measure any coupling of dust scattering to the redshifts of receding galaxies.

[Reality Check]

Another reason that any scattering being a source of the observed redshift of galaxies is that scattering blurs images. But was can get sharp images of galaxies to quite high z. See Errors in Tired Light Cosmology.

[George]

When I went on to cite my hastily put together "thought-experiment "--I obviously left out some salient details (I apologize for it). Let me attempt to re-state it concretely: The OP cited a so-called Interstellar Dust as being a source of an (?) interfering Red-shift in the spectra of accelerating galaxies (?). I attempted to illustrate that for reasons that whether this dust was (i.e. PAHs) or hydrogen or some other species (i.e. CO)---the density within a given volume of space would dictate how and to what extent these species would only be responsible for their own spectral signature and not have a measurable effect upon the spectrum of the receding galaxies.

Thanks for clarity, and it's a good point. I was only looking at reddening and if ctcoker might have some insight on it. It is sorta on topic, but not by much.

I was attempting (in varying degrees ineptitude?) to explain to the OP how it is not possible, within the given experiments of Hubble and Humason, to measure any coupling of dust scattering to the redshifts of receding galaxies.[/QUOTE]

[ctcoker]

ngc3314a probably has answered this for me from the ancient world of the Bad Astronomy forum, but is this scattering almost completely addressed in Rayleigh Scattering?

Not really. The wavelength^-4 dependence of Rayleigh scattering is not generally applicable to the detailed scattering/absorption properties of ISM dust. In addition to the fact that the grains are not generally several hundred times smaller than the wavelegnths they scatter, absorption and emission features due to the composition of the dust are very prominent.

A good first approximation of the stuff is that it mostly consists of graphite and silicate grains with sub-micron sizes, with PAHs (polycyclic aromatic hydrocarbons) making up the very small end (couple tens of angstroms in size). Absorption tends to be as important as scattering, if not more so; if you had a handful of ISM dust in your hand, it'd look and feel like black fireplace soot or coal dust.

There have been many papers written on the subject; a good place to start might be Mathis, Rumpl, and Nordsieck (1977). Look also for papers by Bruce Draine, Laor, Zubko, Weingartner, and others. Ignore most of the verbiage and look at the plots.

[George]

Not really. The wavelength^-4 dependence of Rayleigh scattering is not generally applicable to the detailed scattering/absorption properties of ISM dust. In addition to the fact that the grains are not generally several hundred times smaller than the wavelegnths they scatter, absorption and emission features due to the composition of the dust are very prominent.

This is surprising, though I have not heard that Rayleigh scattering required sizes to be hundreds of times less than the wavelength.

A good first approximation of the stuff is that it mostly consists of graphite and silicate grains with sub-micron sizes, with PAHs (polycyclic aromatic hydrocarbons) making up the very small end (couple tens of angstroms in size). Absorption tends to be as important as scattering, if not more so; if you had a handful of ISM dust in your hand, it'd look and feel like black fireplace soot or coal dust.

That's interesting, and the color certainly favors absorption of starlight, I suspect. This would seem to also expalin the low albedos of KBO's and, no doubt, Oort objects (once they too are observed).

There have been many papers written on the subject; a good place to start might be Mathis, Rumpl, and Nordsieck (1977). Look also for papers by Bruce Draine, Laor, Zubko, Weingartner, and others. Ignore most of the verbiage and look at the plots.

Thanks much for this, and I will read them. I didn't want to get too far off topic, so this is a helpful direction from here.

[ctcoker]

This is surprising, though I have not heard that Rayleigh scattering required sizes to be hundreds of times less than the wavelength.

Rayleigh scattering requires two things: that the scattering particles be much smaller than the wavelength of light being scattered (so, tens to hundreds of times smaller at least), and that the index of refraction of the particles is close to 1. The first can be true in the UV, optical, and NIR (but often is not), while the second is strongly violated.

[George]

Rayleigh scattering requires two things: that the scattering particles be much smaller than the wavelength of light being scattered (so, tens to hundreds of times smaller at least), and that the index of refraction of the particles is close to 1. The first can be true in the UV, optical, and NIR (but often is not), while the second is strongly violated.

I don't doubt that most of the atmospheric scattering is due to the much smaller cross sections (molecules instead of particles), but even particles also can scatter in accordance with Rayleigh's law.

Craig Bohren's, Atmospheric Optics (Fig. 8), demonstrates this, where even 100nm sized particles follow Rayleigh scattering. I think this is consistent with fat globules in water tanks, deeper red sunsets with greater number of dust particles due to a hot turbulent day, etc.

I am, however, still in the learning mode with this, so don't take this in a way that seems too "teachy". I am just curious if I'm off track or not.

[ctcoker]

Molecules are particles for the purposes of Rayleigh scattering - the treatment is exactly the same. The oil droplets mentioned by Bohren are also optically soft (low refractive index, do not have a tendency to scatter light at very large angles, especially when immersed in water), unlike graphite and silicate grains. Dust particles in the air are also not going to follow Rayleigh scattering; they tend to be larger than ISM grains, and thus will tend to act more like Mie scatterers. Even when small, they will behave differently from the oil droplets in water.