[saspinski (OP)]

1) While we used to say that sky is blue, and Gagarin said that the earth is blue, it seems more correct to say: air is blue.

2) When we see a mountain not too far the green of vegetation or the ochre of the soil are evident. Far away mountains are bluish.

3) Radio telescopes can see the color of the space, that is CMB.

4) Distant galaxies are reddish.

5) Photons from distant galaxies are scattered by cosmic powder of different materials and sizes, lowering its wavelength

6) That scattering, considering the reflexion in all directions, also results in the CMB color.

7) It is not necessary to assume the expansion of the universe to explain either the CMR or redshift of galaxies.

By  Occam's razor the expansion of the universe is not necessary, while it is a possible explanation, as the hypothesis of creation by God.

[evan_au]

1. air is blue

Dust particles in the air scatter blue light more effectively than red light, due to Rayleigh scattering. This scattering changes the direction that light travels, but not its wavelength.

This means that blue light is likely to be seen at large angles away from the Sun (ie across the whole sky), and red light is more likely to be seen at small angles from the Sun (ie at sunset and dawn).

So it would be just as valid to say that "air is red".

3) Radio telescopes can see the color of the space, that is CMB.

A perfect vacuum does not absorb nor emit photons. So space has no color.

However, space is filled with various atoms and molecules at low density, and these can be observed with radio telescopes, such as the Hydrogen line, or spectral lines from Oxygen, Helium, Carbon Monoxide, etc. This line spectrum is a specific pattern of wavelengths that are characteristic of particular substances. We can produce these substances in the lab, measure their spectral lines, and recognize the pattern of these spectral lines when we look into space.

The Cosmic Microwave Background Radiation is something different from a line spectrum - it is "black-body radiation", covering a wide part of the spectrum. It is emitted by something that is opaque (ie not space, which is transparent), and it is possible to measure the effective temperature of the object that emitted it. Today, that effective temperature is -270C (2.7K).

5) Photons from distant galaxies are scattered by cosmic powder of different materials and sizes, lowering its wavelength

It is true that dust clouds in space do redden the light from stars beyond it, just as the air reddens the light from the Sun behind it.

See the discussion of a line spectrum above - we can tell what pattern of wavelengths were received, we can deduce the chemicals that emitted it, and tell what wavelength was emitted. We can see that light from distant galaxies has changed in wavelength, and we can tell that the light was not scattered, since scattering does not change the wavelength.

You do not need to go very far to see this effect on Earth - when you are passed by an emergency vehicle, the siren sounds lower in pitch when it is moving away from you, compared to when it is approaching; this is called the Doppler effect. You can't claim that this is due to scattering by dust in the air, since the air has basically the same composition when the siren is moving towards you as when it is moving away.

The CMBR can't be emitted by space today, but if space was hotter and denser in the past, it becomes opaque at around 3000C. The CMBR is thought to be a Doppler-shifted version of the last radiation emitted when the universe was much hotter and denser. The fact that the effective temperature is much lower than when it was emitted reflects a huge Doppler shift.

7) It is not necessary to assume the expansion of the universe to explain either the CMR or redshift of galaxies.

Sure - there are some other niche theories around (but not very popular ones).

What is necessary is that you understand that Doppler shift and Rayleigh scattering are distinct effects, with distinct properties.

• Astronomers know that both effects exist, and take great pains to disentangle them.
• Spectroscopy provides an effective way to disentangle Doppler shift and Rayleigh scattering for many sources.
• So astronomers are certain that galactic red shift is Doppler shift, and is not Rayleigh scattering.

I suggest you read the links above, and then address your question again.

[saspinski (OP)]

Thanks for your answer.

Do you know any article or book comparing and explaining how to disentangle dust scattering from doppler effect? If astronomers take great pains to disentangle them, they have probably written a lot about it.

While visually blue, our daily sky has a range of wavelength similar to blackbody radiation curve.
It is not so perfect as CMB, but the later has all types of dust materials to be scattered during billions of years and that can smooth the curve.

[evan_au]

Do you know any article or book comparing and explaining how to disentangle dust scattering from doppler effect?

This was the first reference listed by Google. It mentions the potential confusion, and then goes on to show how spectroscopy avoids any confusion in sources that exhibit a line spectrum, like dust clouds, stars and galaxies:
http://autocww.colorado.edu/~toldy2/E64ContentFiles/AstronomyAndSpace/Redshift.html


It is harder for blackbody radiation, which doesn't exhibit a line spectrum.

While visually blue, our daily sky has a range of wavelength similar to blackbody radiation curve.

The light in our sky comes from the Sun, which has a blackbody temperature of around 5500K.
The blue light in the daytime accentuates the blue end of the spectrum, due to Rayleigh scattering at large angles from the Sun's position, so it is not a blackbody spectrum.
The reddish light at dawn and sunset accentuates the red end of the spectrum, due to Rayleigh scattering of the blue light, so it is not a blackbody spectrum, either.
Put them together, and you have something like blackbody radiation.
In addition to the Sun's blackbody spectrum, there are superimposed emission and absorption lines that let us measure the chemical content of the Sun and its velocity relative to the Earth (via Doppler effect). It is possible to do this with such precision that it is possible to measure "earthquake waves" on the Sun via the techniques of Helioseismology.


So Sunlight exhibits blackbody radiation, spectral lines and Doppler shift, all of interest to astronomers. For astronomers, the air just gets in the way, and they routinely subtract out its effects. Fortunately, the atmosphere is pretty transparent at optical frequencies.

It is not so perfect as CMB, but the later has all types of dust materials to be scattered during billions of years and that can smooth the curve.

I think you are confusing two things:
the small variations in the CMB across the sky are measured in millionths of a degree K. This looks like a very pure blackbody radiation spectrum in all directions.
however, the raw data collected by CMB survey telescopes has major variations across the sky. In the plane of the galaxy there are many stars with surface temperatures ranging from 1000K to 10000K. There are dust clouds with temperatures ranging from 10K to 100K, and planets somewhere in between. Other galaxies also make their own contributions.
The very pure published map is the result after astronomers have gone to immense trouble to subtract out all of these other sources. This includes measuring at very many frequencies, determining interfering sources, and subtracting them.
So rather than intervening dust clouds smoothing the spectrum, each dust cloud superimposes its own thermal spectrum (and line spectrum) that introduces extra bumps in the spectrum that must be estimated and removed.


For the sort of processing needed to remove these distortions, see the instruments on the Planck  spacecraft: https://en.wikipedia.org/wiki/Planck_(spacecraft)


Fortunately, away from the plane of the galaxy, there is far less dust and stars obscuring the view, and this gives a clearer view of the CMB across most of the sky.

[zx16]

Suppose we humans had eyes which didn't give colour-vision.  So that we couldn't distinguish any colours. Or even recognise the idea of "colour". So that our brains perceived no sharp intrinsic difference between "red" and "blue". But only provided a smooth gradation of grayness in the Universe.

How would that affect our view of the Universe?  Would we be so intensely concerned about the "red-shift" in the spectra of distant galaxies?

[chiralSPO]

Suppose we humans had eyes which didn't give colour-vision.  So that we couldn't distinguish any colours. Or even recognise the idea of "colour". So that our brains perceived no sharp intrinsic difference between "red" and "blue". But only provided a smooth gradation of grayness in the Universe.

How would that affect our view of the Universe?  Would we be so intensely concerned about the "red-shift" in the spectra of distant galaxies?

It might have taken longer to learn about the electromagnetic spectrum if we could not differentiate between frequencies of light with our eyes. However, I think it would eventually be discovered. After all, most of the electromagnetic spectrum is invisible to us, but we have discovered gamma, x-ray, UV, IR, THz, microwave and radio EM radiations. This handicap would also likely not have interfered with our discovery of the Doppler effect, as it is very apparent by sound. (except for very extreme examples, it is not possible to detect red-shift or blue-shift by eye.)

[evan_au]

Suppose we humans had eyes which didn't give colour-vision. ...How would that affect our view of the Universe?

We would not have noticed the similarity of the planet Mars and the star Antares (or "rival of Mars"), both of which are significant for their orangeness.

Spectral lines were first studied in detail by Fraunhofer. He basically examined the Sun's spectrum with a microscope, revealing a few dozen spectral lines.

Effectively the microscope dissected the visible spectrum into perhaps 1000 bands, which is far finer than the approximately 7 colours that our eyes see in a rainbow. So I suspect that monochromatic vision would not have affected spectroscopy significantly. In fact:

• an astronomer trying to do spectroscopy before the invention of photography would have had to rely on the monochromatic rod cells in his eye.
• even after the invention of photography, most astronomical photographic film is black-and-white
• modern astronomical CCDs are mostly monochrome. It is only when you put filters in front of the CCD (discarding 2/3 of the incoming light) that you can construct a color image, by taking 3 exposures. (Or by using wavelength-selective splitters you can divide the light amongst 3 monochrome CCDs.)

To do really high-resolution spectroscopy for discovery of extrasolar planets or helioseismology, you effectively divide the spectrum into millions or even billions of bands, which are totally invisible to the human eye.

[saspinski (OP)]

While different materials have typical spectral lines, that does not mean that the emission from bulks of them show that lines.
For example, we can not get (it would be great if it was possible) a chemical analysis of a plate of steel being hot rolled using some optical device. It is necessary to take a small piece and "burn" it in a spectrometer. I don´t know in detail how a spectrometer works, but it need to take the atoms out of the cristal lattice of the metal to be able to identify its spectral lines.

So, the H, He or other individual atoms in the stars show the spectral lines of that elements, but a small dust powder in the space formed by trillions of atoms, at some temperature, will emit a near blackbody radiation typical to its temperature, in the same way that a hot rolled plate does in a rolling mill.

If the average temperature of the space dust is about 2.7K, those particles will emit and absorve radiation, forming our black body universe, after correcting, as you said, all influences of spectral lines of elements from stars, or individual atoms in the space.

[evan_au]

a small dust powder in the space formed by trillions of atoms, at some temperature, will emit a near blackbody radiation typical to its temperature,

It is expected that relatively high-melting point substances like silicon dioxide, iron, nickel and carbon will tend to form dusty aggregates in cold dust clouds.

However, low melting-point substances like the predominant hydrogen and helium tend to have considerable amounts of free-floating atoms and ions, even in cold dust clouds. These atoms exhibit a line spectrum whch gives away the presence of the dust cloud. The 21cm hydrogen line is especially useful in this regard, because the electron/proton spin transition that produces it is extremely rare. This allows it to pass through quite dense gas clouds, and has been used to map the dusty spiral arms of the Milky Way galaxy.

Having determined an outline of the structure of our galaxy, more precise dust cloud surveys are now taking place using other molecular species like carbon monoxide.

If the average temperature of the space dust is about 2.7K

It is rare to find dust clouds cooler than 2.7K. If one formed, it would heat up to 2.7K via the effects of the CMB alone.

However, the coldest and densest dust clouds (also called molecular clouds)are also the birthplace of new stars. These heat up the dust clouds, and strong stellar winds disperse them. The hot stars embedded in these cold dust clouds can be seen with infra-red telescopes.

[saspinski (OP)]

I understand that if a known spectra (say H) is identified from a far away galaxy, except for a shift to lower frequencies, we assume doppler effect as the reason for the shift.
Because photon energy is proportional to frequency, the loss of energy could be caused by the speed of the galaxy away from us. 

But when a photon interact to a metal releasing an electron, it loses some energy (necessary to take out the electron from its bonding state).  If all photons from the galaxy loses the same amount of energy by that process (interaction to bulk dust) the loss of energy would preserve the spectra pattern, adding the shift.

If we think of millions, or billions of years of travelling, the probability of photoelectric interaction to solid dust particles in the space would increase to distant galaxies.

All we need to accept is that the space is not so empty, in the same way that our atmosphere has much more than air molecules. It may be difficult to detect its concentration in the space because most of them will be orbiting earth or the sun at high relative speeds to any spaceship. One possibility could be placing a very polished metal surface out of ISS for example. And after some years, look for an increase in the roughness, similar to a shot blasting process. But it could take centuries instead of years to see any change. 

[evan_au]

the probability of photoelectric interaction to solid dust particles in the space would increase

In Einstein's deductions about the photoelectric effect (which won him a Nobel Prize), he showed that light energy was quantized; below a certain energy, no electrons were ejected, but when the photon energy increased above a certain threshold frequency, electrons are ejected with increasing kinetic energy.

• In the photoelectric effect, the electron absorbs the entire photon energy, or it does not absorb the photon. It doesn't absorb a proportion of the photon's energy.
• For most substances, the threshold frequency is in the ultraviolet range (although some materials have been used for visible-light photomultiplier tubes). So most materials in space will not redden visible or infra-red radiation via the photoelectric effect.
• If it does not produce the photoelectric effect, the photon may be subject to other effects like Rayleigh scattering or simple absorption.
• One of the more common dust materials in interstellar dust clouds is carbon black (like soot). It absorbs light very effectively across a wide part of the electromagnetic spectrum, rather than emit electrons via the photoelectric effect. The photon absorption causes the dust cloud to heat up above the temperature of the CMB. The cloud itself then becomes visible via its blackbody radiation.
• The presence of a dust cloud may also be detected by comparing visible and infra-red observations; UV and visible light is absorbed much more effectively than longer wavelength infra-red radiation (absorption is not as effective if the dust particle size is smaller than half the photon wavelength).

So I don't see how:
1) the photoelectric effect with its threshold frequency and total photon absorption
could emulate
2) Doppler red shift, which produces a proportional shift in frequency across the whole electromagnetic spectrum without absorbing any photons.
See: https://en.wikipedia.org/wiki/Photoelectric_effect#Emission_mechanism

[saspinski (OP)]

Yes, photoelectric effect is not a good candidate for the job of draining energy from galaxies photons.

If there was another method to measure the recession speed of galaxies, to compare with the doppler effect calculation, the latter would be above any reasonable doubt.

For example: the hubble telescope (or another device in orbit) gets the spectra of a galaxy now and again six months later. It would be chosen so that the difference in the relative speed between the two measurements is maximum (2 x earth orbital speed to the sun).
If the spectra pattern shows a difference as expected by the two galaxy recession speeds, and the experimental error is well below that pattern difference, then the redshift is caused by doppler effect.
If there is no difference, the photons loses its energy by some (perhaps still unkown) process. 


Well, thinking again it doesn´t proof anything. Of course there would be a difference because even if there was no recession speed, there would be a speed difference and the doppler effect between the 2 speeds of the experiment. 

[evan_au]

to compare with the doppler effect calculation

The Doppler effect is a basic characteristic of all wave motion - gamma rays, light, microwaves, sound and ocean waves.

I hope you don't doubt its existence? Just speed down the road, and Doppler shift will cost you real money!

get the spectra of a galaxy now and again six months later

This would prove that Doppler shift happens due to the motion of the Earth. It says nothing about the motion of galaxies.

We know that Doppler shift happens in our Solar System, because space agencies allow for it when tracking spacecraft (and use it to measure the radial velocity of spacecraft).

• We know that Doppler shift happens within our galaxy, because astronomers use it to detect extrasolar planets, and to measure the velocity of stars in the arms of the Milky Way galaxy.
• We know that Doppler shift happens between us and other galaxies, due to galaxy rotation curves. This shows that stars on one side of the galaxy are approaching us, while stars on the other side of the galaxy are receding from us (relatively speaking)*.
• Most importantly, we know that some galaxies are moving towards us, giving them a blue shift, especially the Andromeda galaxy. All of the mechanisms that have been described in this thread that make light "redder" don't explain how a few galaxies are "bluer".

*Galaxy rotation curves are not without their mystery. There appears to be more mass in the galaxy than we can see with conventional telescopes, leading many cosmologists to suggest the existence of "Dark Matter". Regardless of the state of Dark Matter, the rotation curves show that the Doppler effect works between us and distant galaxies, and that red shift is not due to intervening dust clouds; the alternative hypothesis would be that dust clouds selectively redden or "bluen" just one side of every galaxy.

[saspinski (OP)]

I have no doubt over the reality of doppler effect, and its use to estimate the relative speed of stars to us.

I only say that without another way to measure galaxies speeds, any conclusion based on redshift shoud be viewed "cum grano salis".

Logically speaking, if a source of light approaches us or moves out of us, there will be a frequency shift. What also means that, if there is no frequency shift there is none of that movements. But I can not conclude from that proposition that, if there is a frequency shift then there is any of that movements.

In old disk players, if we had for any reason a slower revolution speed, all the tunes or voices would be played at lower frequencies, while the device was stable, not moving.

About dark matter: if we had only the results that you mentioned: the speed of peripherical stars are greater than expected by gravitational theory, it could be only the case that the galaxies are expanding. But we have another way to estimate the mass of galaxies: gravitational lensing. And it confirms that there are more stuff there than we can detect. Suppose we had all that measurements made by Vera Rubin and others but no General Relativity. The conclusion could be: galaxies are not static but are expanding.

[evan_au]

I have no doubt over the reality of doppler effect, and its use to estimate the relative speed of stars to us.
I only say that without another way to measure galaxies speeds, any conclusion based on redshift shoud be viewed "with a grain of salt"

Galaxies are made of stars.
If you are happy to measure the speed of stars using Doppler shift, surely you are happy to measure the speed of galaxies with Doppler shift?

Stars (and the galaxies they form) emit a line spectrum, so this is an easy way to distinguish Doppler shift from other effects like scattering and absorption by intergalactic dust clouds.

In old disk players, if we had for any reason a slower revolution speed, all the tunes or voices would be played at lower frequencies

If the rpm of the player (its timebase) were reduced, all the sounds move to lower frequencies.
A similar change in timebase can occur in general relativity, due to intense gravitational fields and/or high-velocity relative motion.

• It takes a very intense gravitational field to produce a detectable change in timebase (this is sometimes called "Einstein shift", since it comes from his theory). But the vast majority of stars don't have a sufficiently "deep" gravitational well to produce a significant effect on the scale of a galaxy - the effect is most pronounced in white dwarf stars, the tiny glowing cinders of burnt-out stars.
• High velocities can produce an additional redshift (or blueshift) in addition to the Doppler effect. In practice, astronomers use this "Relativistic Doppler shift" for high velocities (eg >1% of the speed of light). But the dominant trend seen by Hubble and confirmed by later studies is that more distant galaxies are moving away from us more rapidly than closer galaxies.

[Kryptid]

Can redshift be measured via cosmic ray emissions from galaxies? If the redshift of charged particle radiation is the same as that of photons, then it must be due to metric expansion of space, since that should increase their wavelengths by the same amount. If it is caused by dispersion in the intergalactic medium, then the two redshift measurements would disagree since different particles would be affected differently.

[evan_au]

Can redshift be measured via cosmic ray emissions from galaxies?

At one time it was thought that cosmic rays were electromagnetic in nature, with higher energy than gamma rays.

We now know that cosmic rays are atomic nuclei (mostly protons), accelerated to relativistic velocities by high-energy events in the galaxy (or, presumably, other galaxies).

As a massive object, an atomic nucleus cannot be accelerated to reach the speed of light, and so it does not display a redshift in the same way that electromagnetic radiation does.

In addition, as a charged particle, its direction is deflected by the Earth's magnetic field, and magnetic fields in our galaxy; if it came from another galaxy, it would be affected by magnetic fields in the source galaxy, and any magnetic field in intergalactic space. The Sun's solar wind (and solar cycle) are known to affect the intensity of cosmic rays arriving at Earth.


Cosmic rays are thought to originate from the Sun, from the crab nebula (a pulsar which is a supernova remnant), from the direction of the galactic core, and supernovas. But it is really hard to pin down cosmic rays which come from a particular galaxy, to compare it with the redshift of that galaxy.

Even if we could pin down a particular cosmic ray as coming from a particular galaxy, cosmic rays are rare, and it would be incredibly rare to detect two from the same galaxy. And we can't measure cosmic ray energy very accurately, as we mostly detect them after they have collided with multiple atoms in the upper atmosphere.

So I'm afraid that any comparison between galactic redshift spectra and cosmic ray energies is beyond our current technology.
https://en.wikipedia.org/wiki/Cosmic_ray#Sources_of_cosmic_rays

[Kryptid]

Can redshift be measured via cosmic ray emissions from galaxies?

At one time it was thought that cosmic rays were electromagnetic in nature, with higher energy than gamma rays.

We now know that cosmic rays are atomic nuclei (mostly protons), accelerated to relativistic velocities by high-energy events in the galaxy (or, presumably, other galaxies).

As a massive object, an atomic nucleus cannot be accelerated to reach the speed of light, and so it does not display a redshift in the same way that electromagnetic radiation does.

In addition, as a charged particle, its direction is deflected by the Earth's magnetic field, and magnetic fields in our galaxy; if it came from another galaxy, it would be affected by magnetic fields in the source galaxy, and any magnetic field in intergalactic space. The Sun's solar wind (and solar cycle) are known to affect the intensity of cosmic rays arriving at Earth.


Cosmic rays are thought to originate from the Sun, from the crab nebula (a pulsar which is a supernova remnant), from the direction of the galactic core, and supernovas. But it is really hard to pin down cosmic rays which come from a particular galaxy, to compare it with the redshift of that galaxy.

Even if we could pin down a particular cosmic ray as coming from a particular galaxy, cosmic rays are rare, and it would be incredibly rare to detect two from the same galaxy. And we can't measure cosmic ray energy very accurately, as we mostly detect them after they have collided with multiple atoms in the upper atmosphere.

So I'm afraid that any comparison between galactic redshift spectra and cosmic ray energies is beyond our current technology.
https://en.wikipedia.org/wiki/Cosmic_ray#Sources_of_cosmic_rays

I see. In that case, I'd say to compare different wavelengths of electromagnetic radiation then. All wavelengths should be equally affected by metric space expansion but unequally effected by scattering (since any hypothetical dust would have different optical properties at different wavelengths).

[guest39538]

1) While we used to say that sky is blue, and Gagarin said that the earth is blue, it seems more correct to say: air is blue.

2) When we see a mountain not too far the green of vegetation or the ochre of the soil are evident. Far away mountains are bluish.

3) Radio telescopes can see the color of the space, that is CMB.

4) Distant galaxies are reddish.

5) Photons from distant galaxies are scattered by cosmic powder of different materials and sizes, lowering its wavelength

6) That scattering, considering the reflexion in all directions, also results in the CMB color.

7) It is not necessary to assume the expansion of the universe to explain either the CMR or redshift of galaxies.

  By  Occam's razor the expansion of the universe is not necessary, while it is a possible explanation, as the hypothesis of creation by God.

NO, the air is not ''blue'', the air is transparent and has not enough permeability to compress the light length to a wave length of around 400 nm.   The sky is ''blue'' , not of my opinion of a Rayleigh scattering but of the ionisation layer of the atmosphere offering greater permeability and resistance to the light length to compress the light to a wave of 400 nm.   Red sky at night being angle related of light and released wave compression and force by angle.


We as humans only observe temporal change of c between 400-700nm.

[saspinski (OP)]

Air is blue and clouds are white. Sea is blue and wave foam is white. I believe that in both cases the white color results from the aggregation of several wavelengths reflected by the air-water surfaces, where the wavelength is a function of the surface thickness.

I can not imagine an experience that could say if galaxies redshift results from their recession speed or from a loss of energy by the photons, after travelling millions of years.
Both ideas need "ad hoc" entities: some stuff that is continually created and excerces a pressure to expand the universe in spite of gravity, or some stuff that turns the space a dissipation field, that drains energy from photons. Both very thin, but effective because are everywhere in the universe.