Cosmological redshift

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What causes Cosmological redshift?
Can it be due to Compton scattering with free electrons in the corona/atmosphere?https://en.wikipedia.org/wiki/Corona
 

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  • #2
Ibix
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What causes Cosmological redshift?
The metric expansion of space "stretches" photons in flight. That's not very rigorous, but I don't know how to state it rigorously at B level.
Can it be due to Compton scattering with free electrons in the corona/atmosphere?https://en.wikipedia.org/wiki/Corona
No. The corona isn't uniform across the sky, nor is the Sun's position in the sky constant. So you'd expect to see a strong angular dependence and seasonal variation to any effects related to the Sun. We don't.
 
  • #3
CWatters
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Have you googled Cosmological redshift?

Why do you think it might be due to CS in the atmosphere of the star?
 
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The metric expansion of space "stretches" photons in flight. That's not very rigorous, but I don't know how to state it rigorously at B level.
And you should understand that's just an explanation.
The stretching of photons imply they 'lose' energy...how can a photon just lose energy?
I don't mean the sun. Those free electrons are present in the atmosphere of other stars as well.
Have you googled Cosmological redshift?

Why do you think it might be due to CS in the atmosphere of the star?
Blueshift and redshift is due to inelastic scattering of photons.
Free electrons cause inelastic scattering.
 
  • #5
kimbyd
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What causes Cosmological redshift?
Can it be due to Compton scattering with free electrons in the corona/atmosphere?https://en.wikipedia.org/wiki/Corona
It can't be compton scattering. Compton scattering dims bluer light while emitting light at a spread of lower wavelengths.

Redshift is observed by looking at emission lines. Within each atom, the electrons can only have certain specific energies. When the electrons get excited and then cool back down, they emit photons of very specific wavelengths. These leave bright "emission lines" at specific wavelengths. The pattern of emission lines is very specific to the particular atom. When we look at far-away galaxies, the preferred method to measure their redshift is to look for these precise patterns of emission lines, and they are shifted (typically to the red side of the spectrum). Compton scattering can dim an emission line, but it can't shift it to another wavelength.

To see just how different the emissions lines can look, and how unique they are to the specific atoms, see the image at the bottom of this link:
http://astronomy.nmsu.edu/geas/lectures/lecture19/slide02.html

(This link also describes the related process that results in absorption lines)

Sometimes astronomers use a cruder method so that they can use the same amount of telescope time to look at many more galaxies. That cruder method can be fooled by compton scattering. But the "gold standard" is always to look at the specific emission lines, and the cruder method (known as photometric redshift) is always calibrated using emission lines, and a lot of work goes into making sure that astronomers aren't fooled by its limitations.
 
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  • #6
phinds
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The stretching of photons imply they 'lose' energy...how can a photon just lose energy?
By just losing it. Your very understandable confusion arises because you think you can extrapolate a local effect (energy is conserved) to cosmological distances, where in fact it doesn't work. That is, energy is NOT conserved on cosmological scales. **

Sean Carroll on "Energy is not conserved"
http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

**EDIT: This is true for the universe that we live in. If we lived in a universe where space-time was static, then energy would be conserved throughout, they way it is locally in our universe.
 
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CWatters
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And you should understand that's just an explanation.
The stretching of photons imply they 'lose' energy...how can a photon just lose energy?
I don't mean the sun. Those free electrons are present in the atmosphere of other stars as well.

Blueshift and redshift is due to inelastic scattering of photons.
Free electrons cause inelastic scattering.
Redshift typically varies with distance from the star, why/how would scattering at the source effect that?
 
  • #8
kimbyd
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By just losing it. Your very understandable confusion arises because you think you can extrapolate a local effect (energy is conserved) to cosmological distances, where in fact it doesn't work. That is, energy is NOT conserved on cosmological scales. **

Sean Carroll on "Energy is not conserved"
http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

**EDIT: This is true for the universe that we live in. If we lived in a universe where space-time was static, then energy would be conserved throughout, they way it is locally in our universe.
Also, you can interpret it as just a version of the Doppler effect: the photon is redshifted because the source is moving away from us.

It gets a little bit complicated in the context of an expanding universe, because you have to think about the entire path the photon travels, rather than just the emitter's velocity at the time the photon was emitted. But if you set up a chain of imaginary observers along the path of the photon, with each observer moving along with the overall expansion, they'll each see the photon slightly redshifted compared to the previous observer's view because the previous observer is moving away from them. Those little redshifts all add up to exactly what we observe.

Caveat: General Relativity is really, really weird, so the above picture is only one way of looking at the situation. There are lots of other ways of looking at it, all equally valid.
 
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By just losing it. Your very understandable confusion arises because you think you can extrapolate a local effect (energy is conserved) to cosmological distances, where in fact it doesn't work. That is, energy is NOT conserved on cosmological scales.
I'm not talking about 1 photon. In all cosmological redshift they observe red light while it's actually white. And if energy is not conserved on cosmological scale then there is no reason we observe redshifted light...then we should observe white light.

can't be compton scattering. Compton scattering dims bluer light while emitting light at a spread of lower wavelengths.
But it can create red light and inverse compton scattering blue light.
 
  • #10
phinds
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In all cosmological redshift they observe red light while it's actually white. And if energy is not conserved on cosmological scale then there is no reason we observe redshifted light...then we should observe white light.
Not sure what your thought process is that leads you to this erroneous conclusion, but you would be well served to consider that thousands of physicists have looked at this and are all in agreement with Caroll. Do you really think you've figured it out where they have not?

As kymbyd pointed out, GR is weird.
 
  • #11
kimbyd
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I'm not talking about 1 photon. In all cosmological redshift they observe red light while it's actually white. And if energy is not conserved on cosmological scale then there is no reason we observe redshifted light...then we should observe white light.
I do recommend reading Carroll's blog post, which describes what I'll say here in different words. It's not that there isn't any conservation law at all. General Relativity follows a different conservation law (technical name: conservation of stress-energy). This conservation law forces energy to change in a predictable, consistent way under certain conditions.

One way to understand this is that all conservation laws come from symmetries of nature (this is known as Noether's theorem). When you have a system that looks the same (in a certain mathematical way) at different locations, then momentum is conserved. If you have a system that looks the same when you rotate it, then angular momentum is conserved. If you have a system that looks the same at different times, then energy is conserved.

General Relativity throws a wrench into energy conservation because its space-time is not static: it is a dynamic entity. It makes no sense that energy could be conserved in a theory where time itself can vary depending upon the matter configuration. Theories in flat space-time don't have this issue: because of the static time coordinate, they can always come up with some kind of potential energy that keeps energy conserved. You just can't do that in General Relativity except in certain scenarios. In general, energy changes in predictable ways based upon how space-time is curved.

But it can create red light and inverse compton scattering blue light.
But you can't create emission lines that way. Compton scattering takes incoming blue light and emits a spread of wavelengths in a variety of directions. The overall effect when looking at a source through intervening gas is that incoming blue light has been reduced, while a much fainter and redder glow is produced (it's fainter because most of it gets deflected in a different direction). There's no noticeable effect on emission lines.
 
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  • #12
timmdeeg
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..then we should observe white light.
The cosmological redshift is due to increasing distances between emission and absorption of the photon. In flat Minkowski spacetime we talkt about Doppler redshift in this case (increasing distances). In FRW spacetime we have no global inertial system like in Minkowski spacetime but it is legitimate to consider an accumulation of infinitesimal redshifts as @kimbyd pointed out in #8.h
 
  • #13
George Jones
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In all cosmological redshift they observe red light while it's actually white.
Even thought I am not sure what you mean, I want to make a comment.

A hot body does emit light of all colours, but the different colours are emitted with different intensities. The colour with the greatest intensity depends on the temperature of the body. This is called a thermal spectrum. It can be shown mathematically that light emitted with a thermal spectrum by a cosmologically distant object at temperature ##T## will be seen us to have a thermal spectrum, but thermal spectrum that we see will be the same a for an object that has a temperature lower than ##T## (i.e., expansion of the universe takes one thermal spectrum to another thermal spectrum), so we will a different colour that peak intensity. Even light of all colours, if it is thermal, is noticeably redshifted!
 
  • #14
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There is no such colour 'White'.
White is the average of wavelengths perceived by humans.
However there are specific wavelengths which characterize the presence of particular elements.
It is the shift of those narrow bands which determine red shift of cosmologically moving objects.
 
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  • #15
CWatters
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As I understand it light from distant stars is so red shifted it's well beyond the red end of what humans can see and call "red".
 
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  • #16
timmdeeg
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No redshifted means shifted to longer wavelengths independent of what humans call red. So the redshifted radiation may well be invisible for humans.
 
  • #17
kimbyd
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No redshifted means shifted to longer wavelengths independent of what humans call red. So the redshifted radiation may well be invisible for humans.
Galaxies with reasonably-high redshifts are often blue to the eye. Galaxies with lots of active star formation were more common a few billion years ago. When a galaxy, or a part of a galaxy, is actively forming stars, most of the light we see comes from the most massive stars. Even though the massive stars are relatively few in number, they are so much brighter than the smaller stars around them that they make up most of the light. But those bright stars don't last very long, sometimes only tens of millions of years. The longer-lived stars are dimmer and redder, so over time galaxies get dimmer and redder.

So, lots of far-away galaxies that have redshifted a bunch look more blue, while lots of nearby galaxies that haven't redshifted much at all look more red.

Quite a lot depends upon the specific galaxy, though. There are lots of red galaxies far away, and some blue galaxies relatively close by.
 
  • #18
timmdeeg
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Galaxies with reasonably-high redshifts are often blue to the eye.
In this case the corresponding emission lines should be in the ultraviolet range, not sure which electronic transitions are involved here. Whereas the visible emission spectrum of such galaxies is depending on ##z## shifted to infrared wavelenghts and hence is invisible to us, correct?
 
  • #19
kimbyd
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In this case the corresponding emission lines should be in the ultraviolet range, not sure which electronic transitions are involved here. Whereas the visible emission spectrum of such galaxies is depending on ##z## shifted to infrared wavelenghts and hence is invisible to us, correct?
The primary Hydrogen Lyman-##\alpha## line is in the UV range, so, yes!

But they appear blue to the eye mostly because the very bright stars are just ridiculously hot, so this is more about basic temperature than anything. And since temperature and redshift have a trivial relationship, the appearance of blue is directly related to that temperature.

A thermal spectrum starts to look blue to the eye at around 7500K or so. Thus, if a galaxy at redshift ##z=0.3## appears blue, then its brightest stars are at least ##7500K \times 1.3 = 9750K##. The hottest stars can have temperatures around 40,000K.

Anyway, to see this effect visually, I recommend taking a look at the Hubble Ultra Deep Field:
https://www.spacetelescope.org/images/heic0611b/

Most of the large galaxies (which will also usually be closer galaxies) are between white and orange in color. Note that the spiral galaxies tend to be lighter in color than the elliptical galaxies (spiral galaxies have a lot more star formation, because they have more gas and dust spread throughout). If you look at the tiny galaxies in the background, though, there are quite a few of them that are very blue. Not all, for sure. But many are. And their small appearance isn't because they're physically small, but because they're far away.

You can't know the redshifts of these galaxies from this image, of course, but I think it provides a nice visual picture of the effect.
 
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  • #20
timmdeeg
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T If you look at the tiny galaxies in the background, though, there are quite a few of them that are very blue. Not all, for sure. But many are. And it isn't because they're small, but because they're far away.
Yes indeed, its easy to see them. Thanks for your explanations.
 
  • #21
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Have a look at the book 'Seeing Red' by Halton Arp for a nice well presented alternative view of the origins of redshift.
 
  • #22
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Many People think cosmo redshft is due to the expansion of the universe. But what does this mean? Their is no evidence that only motion lengthens waves.
Isn't it possible that motion enable scattering processes and interaction with gravit. field which causes redshift?
 
  • #23
Bandersnatch
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Have a look at the book 'Seeing Red' by Halton Arp for a nice well presented alternative view of the origins of redshift.
Arp's ideas might have been considered an alternative maybe 50 years ago. As evidence kept mounting in favour of BB cosmology, he stuck to his guns, accusing mainstream cosmologists of dogmatic rigidity - which is what the majority of Seeing Red is about. The irony must have been lost on him.
 
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  • #24
Bandersnatch
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Many People think cosmo redshft is due to the expansion of the universe. But what does this mean?
It means that if you integrate Doppler shift between a string of infinitesimal points along the path of light travelling through expanding space, you get cosmological redshift.

Their is no evidence that only motion lengthens waves.
You've got this backwards. One needs to propose a mechanism to account for the observables (precise mathematical formulation of the physical process, not 'maybe it's X') to at least a degree comparable to the cosmological redshift. Nobody says expansion is the only possible cause of redshift - in fact, other causes are known, such as gravitational redshift. But it's the only cause that could be shown to account for the observables in a satisfactory manner. And by now there's a lot of observables.
 
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  • #25
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Arp's ideas might have been considered an alternative maybe 50 years ago. As evidence kept mounting in favour of BB cosmology, he stuck to his guns, accusing mainstream cosmologists of dogmatic rigidity - which is what the majority of Seeing Red is about. The irony must have been lost on him.
I understand that Arp’s work has not been confirmed but then two patches to the standard model, dark matter and energy have not been confirmed yet either. The book is from 20 years ago and has been take seriously by Hoyle, Narlikar, Geoffrey and Margaret Burbidge and many other reputable scientists. It may not offer solutions but I think it presents some absolutely lovely questions which are still relevant. Give it a read.

I think this fits here:

"Of course, if one ignores contradictory observations, one can claim to have an "elegant" or "robust" theory. But it isn't science." ~ Halton Arp
 

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