Cosmological redshift

[Itoero]

What causes cosmological redshift?

Can it be due to gravitation? (gravitational redshift)

[Strange]

It is caused by the difference in scale factor between the source and the observer.

[beecee]

13 minutes ago, Itoero said:

What causes cosmological redshift?

Cosmological redshift is caused by light traversing expanding space.

Quote

Can it be due to gravitation? (gravitational redshift)

Gravity can cause both red and blue shift: As EMR falls into a gravity well, it is blue shifted from our perspective, and as it climbs out of a gravity well it is red shifted...theoretically they cancel each other out.  

[Conjurer]

On 10/25/2018 at 2:34 PM, Strange said:

It is caused by the difference in scale factor between the source and the observer.

I wasn't aware that had ever been proven to have shown to be an exact relation to each other.  It sounds like that would just end up giving you a proper distance.  

[beecee]

33 minutes ago, Conjurer said:

I wasn't aware that had ever been proven to have shown to be an exact relation to each other.  It sounds like that would just end up giving you a proper distance.  

Cosmological redshift is attributable to the expansion of the universe, which is the expansion of spacetime itself, and Its interpretation as a distance depends on the general mainstream cosmological model in use. 

https://arxiv.org/abs/1107.4433

Redshift and distances in a ΛCDM cosmology with non-linear inhomogeneities

Motivated by the dawn of precision cosmology and the wealth of forthcoming high precision and volume galaxy surveys, in this paper we study the effects of inhomogeneities on light propagation in a flat ΛCDM background. To this end we use exact solutions of Einstein's equations (Meures & Bruni 2011) where, starting from small fluctuations, inhomogeneities arise from a standard growing mode and become non-linear. While the matter distribution in these models is necessarily idealised, there is still enough freedom to assume an arbitrary initial density profile along the line of sight. We can therefore model over-densities and voids of various sizes and distributions, e.g. single harmonic sinusoidal modes, coupled modes, and more general distributions in a ΛCDM background. Our models allow for an exact treatment of the light propagation problem, so that the results are unaffected by approximations and unambiguous. Along lines of sight with density inhomogeneities which average out on scales less than the Hubble radius, we find the distance redshift relation to diverge negligibly from the Friedmann-Lemaitre-Robertson-Walker (FLRW) result. On the contrary, if we observe along lines of sight which do not have the same average density as the background, we find large deviations from the FLRW distance redshift relation. Hence, a possibly large systematic might be introduced into the analysis of cosmological observations, e.g. supernovae, if we observe along lines of sight which are typically more or less dense than the average density of the Universe. In turn, this could lead to wrong parameter estimation: even if the Cosmological Principle is valid, the identification of the true FLRW background in an inhomogeneous universe maybe more difficult than usually assumed.

[Strange]

3 hours ago, beecee said:

which is the expansion of spacetime itself,

Expansion of *space*  

3 hours ago, Conjurer said:

I wasn't aware that had ever been proven to have shown to be an exact relation to each other. 

The theoretical predictions were made nearly 100 years ago. They were first confirmed a few years later and have been repeatedly confirmed to greater accuracy many times since. That is why the Big Bang model is accepted. 

[Itoero]

On ‎25‎/‎10‎/‎2018 at 10:46 PM, beecee said:

Cosmological redshift is caused by light traversing expanding space.

Is there evidence that only expanding space lengthens waves?

[Silvestru]

7 minutes ago, Itoero said:

Is there evidence that only expanding space lengthens waves?

I guess Gravity kind of does too.

Photons of light are not technically affected by large gravitational fields; instead space and time itself become distorted around incredibly massive objects and the light simply follows this distorted curvature of space

https://en.wikipedia.org/wiki/Gravitational_lens

[Strange]

15 minutes ago, Itoero said:

Is there evidence that only expanding space lengthens waves?

There two other possible causes: the Doppler effect (relative motion) or gravitational redshift. (The Pound-Rebka experiment neatly combines those two: https://en.wikipedia.org/wiki/Pound–Rebka_experiment)

[Itoero]

Ok, but are relative motion and the expanding of space not basically the same thing?

On ‎8‎/‎11‎/‎2018 at 1:06 PM, Silvestru said:

Photons of light are not technically affected by large gravitational fields; instead space and time itself become distorted around incredibly massive objects and the light simply follows this distorted curvature of space

https://en.wikipedia.org/wiki/Gravitational_lens

A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer. Can't this matter cause gravitational redshift?

I know I'm alone in this but the doppler effect has no explanatory value. It categorizes wave behavior but it doesn't explain how waves change. Gravitational redshift does explain how wave behavior  changes.

[Strange]

18 minutes ago, Itoero said:

Ok, but are relative motion and the expanding of space not basically the same thing?

Absolutely not. There is no proper motion involved (or at least, not significant). And if you try and model cosmological red-shift as the Doppler effect, you get the wrong results.

18 minutes ago, Itoero said:

A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer. Can't this matter cause gravitational redshift?

Gravity could cause redshift of light from the galaxy (I'm not sure if that is large enough to be measurable). But not of light passing the galaxy - it will be blue shifted as it approaches and then redshifted by the same amount as it leaves.

18 minutes ago, Itoero said:

I know I'm alone in this but the doppler effect has no explanatory value.

It explains the changing pitch of cars as they approach and recede. It explains how radar speed detectors work.

18 minutes ago, Itoero said:

It categorizes wave behavior but it doesn't explain how waves change.

What!? It is the simplest explanation of frequency change there is. Even children can grasp the full details.

You don't really mean that you don't understand it. Do you? Surely I have misunderstood you.

 

[Itoero]

17 hours ago, Strange said:

What!? It is the simplest explanation of frequency change there is. Even children can grasp the full details.

You don't really mean that you don't understand it. Do you? Surely I have misunderstood you.

Yes but it doesn't explain how frequency changes. In order to know how the frequency (energy) changes, you need to measure/observe how the wave interacts with the medium it is in

 

17 hours ago, Strange said:

18 hours ago, Itoero said:

 

Absolutely not. There is no proper motion involved (or at least, not significant). And if you try and model cosmological red-shift as the Doppler effect, you get the wrong results

It's not the same thing? But expanding of space causes relative motion...they are IMO correlated. The BB causes expanding of space and the way matter (galaxies) behave due to the expanding of space is called relative motion.

 

[Strange]

33 minutes ago, Itoero said:

Yes but it doesn't explain how frequency changes.

Of course it does. It is trivially obvious. Anyone who can count can understand it.

33 minutes ago, Itoero said:

In order to know how the frequency (energy) changes, you need to measure/observe how the wave interacts with the medium it is in

It has nothing to do with "interaction". It is just a matter of counting. Can you do that?

33 minutes ago, Itoero said:

But expanding of space causes relative motion

No it doesn't.

33 minutes ago, Itoero said:

The BB causes expanding of space and the way matter (galaxies) behave due to the expanding of space is called relative motion.

No it isn't.

33 minutes ago, Itoero said:

It's not the same thing?

Absolutely not.

Quote

We use standard general relativity to illustrate and clarify several common misconceptions about the expansion of the Universe. To show the abundance of these misconceptions we cite numerous misleading, or easily misinterpreted, statements in the literature. In the context of the new standard Lambda-CDM cosmology we point out confusions regarding the particle horizon, the event horizon, the ``observable universe'' and the Hubble sphere (distance at which recession velocity = c). We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests. Attempts to restrict recession velocities to less than the speed of light require a special relativistic interpretation of cosmological redshifts. We analyze apparent magnitudes of supernovae and observationally rule out the special relativistic Doppler interpretation of cosmological redshifts at a confidence level of 23 sigma.

https://arxiv.org/abs/astro-ph/0310808

If it were relative motion, then the recession velocities could not exceed the speed of light.

Interpreting it as Doppler shift does not work.

[Strange]

!

Moderator Note

An off-topic hijack with a personal theory has been hidden. There is a thread in Speculations.

 

[Itoero]

On ‎14‎/‎11‎/‎2018 at 5:58 PM, Strange said:

Of course it does. It is trivially obvious. Anyone who can count can understand it.

I did learn how to count a couple days ago. But math doesn't 'prove' anything. The idea that only relative motion changes frequency need to be experimentally proven. The path the wave travels from its source  need to be studied. How can people know the doppler effect is not due to gravitation?

 

 

Edited November 23, 2018 by Itoero

[Strange]

15 minutes ago, Itoero said:

I did learn how to count a couple days ago. But math doesn't 'prove' anything. The idea that only relative motion changes frequency need to be experimentally proven. The path the wave travels from its source  need to be studied.

Here is an explanation for children, with pictures: https://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect

If that is too advanced, I can try and find something simpler.

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How can people know the doppler effect is not due to gravitation?

Because it happens with constant gravity. 

Another simple diagram: 

 

And a couple here for moving source and moving observers: https://www.bestchoice.net.nz/physics/870/p14943.htm

 

[studiot]

There is such a phenomenon as the gravitational redshift that emerges from general relativity.

https://www.google.co.uk/search?source=hp&ei=WmP4W7KJI8WyrgTFxKeICA&q=gravitational+redshift&oq=gravitational+red&gs_l=psy-ab.1.0.0l10.1798.8426..11134...1.0..0.998.4240.5-1j4......0....1..gws-wiz.....0..0i131j0i10.ikVzzXRkppQ

[Itoero]

On ‎23‎/‎11‎/‎2018 at 8:56 PM, Strange said:

Here is an explanation for children, with pictures: https://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect

If that is too advanced, I can try and find something simpler.

Is it so hard to understand that observing wavebehavior doesn't give info on how they change?  In order to know what in the medium changes the wave, you need to observe/measure the path the wave travels which is difficult/impossible since when you measure/observe a wave, you stop the wave.

 

On ‎23‎/‎11‎/‎2018 at 8:56 PM, Strange said:

Because it happens with constant gravity. 

So motion = constant gravity….seriously?

Do you know about a gravit. field?

 

On ‎23‎/‎11‎/‎2018 at 9:33 PM, studiot said:

There is such a phenomenon as the gravitational redshift that emerges from general relativity.

Yes, it means how motion changes gravitational field which causes the redshift. Why isn't gravitational shift what causes the observable doppler shift?

Motion alters gravitational field which lengthens or shortens waves

[Strange]

9 minutes ago, Itoero said:

Is it so hard to understand that observing wavebehavior doesn't give info on how they change?

It is incomprehensible, why you can't understand something so simple and obvious.

You move towards the source and therefore you see more peaks and troughs per unit tine = higher frequency. You move away and you see fewer wavefronts per unit time = lower frequency.

This has nothing to do with gravity or particle interactions or any other nonsense like that. It is purely about how frequently you encounter the wavefronts.

I cannot believe there is anyone on the planet who is not able to understand that trivially obvious fact. 

Let's try this analogy. When you were little did you ever hold a stick and run it along a fence made up of lots of metal posts close together: dink dink dink dink ...

 Think of those posts as the wavefront. As you move faster the frequency of the links increases. If you move slower, it decreases. Your movement is not creating gravity to distort the fence. You are just encountering the "wavefront" more or less slowly.

Sheesh. I'm sure I could explain this to primary school children more easily.

16 minutes ago, Itoero said:

So motion = constant gravity….seriously?

If you and the police car/radar gun or whatever are on the same stretch of road then the gravity is the same, yes. Moving at a constant speed won't change that.

17 minutes ago, Itoero said:

Yes, it means how motion changes gravitational field which causes the redshift.

Gravitational redshift has nothing to do with the relative motion between the source and the observer. It happens even if they are both stationary. I just cannot understand your confusion.

Quote

Motion alters gravitational field which lengthens or shortens waves

Citation needed.

[swansont]

!

Moderator Note

Discussion on Doppler shift has been split

https://www.scienceforums.net/topic/117157-doppler-shift-split-from-cosmological-redshift/

 

 

On 11/23/2018 at 2:40 PM, Itoero said:

I did learn how to count a couple days ago. But math doesn't 'prove' anything. The idea that only relative motion changes frequency need to be experimentally proven. The path the wave travels from its source  need to be studied. How can people know the doppler effect is not due to gravitation?

Because the doppler effect has been measured in outer space, where gravity is much smaller, and the equation which predicts it still works.  

On 11/14/2018 at 11:25 AM, Itoero said:

 It's not the same thing? But expanding of space causes relative motion...they are IMO correlated. The BB causes expanding of space and the way matter (galaxies) behave due to the expanding of space is called relative motion.

 

You can have relative motion without the expansion of space.  I hope that's obvious. They are not the same thing.

[Ghideon]

Maybe I'm entering the discussion too late to add value but I realised that both @Strange and @Itoero can be "correct".

On 2018-11-25 at 5:34 PM, Strange said:

It is incomprehensible, why you can't understand something so simple and obvious.

You move towards the source and therefore you see more peaks and troughs per unit tine = higher frequency. You move away and you see fewer wavefronts per unit time = lower frequency.

It seems unlikely that the simple description should be impossible to understand and then I read this:

On 2018-11-14 at 5:25 PM, Itoero said:

you need to measure/observe how the wave interacts with the medium it is in

Maybe wrong analogy is used? Lets try to create something where dopler and the statement above makes sense.

Imagine that you don't know if you are above or below water. Of course not a common situation but useful for this thought experiment. You hear a sound that you know have a certain frequency from the source in air. A measurement at your location now shows that the frequency does not match the known source frequency. In this case you cannot know if the frequency has changed due to the fact that you and the source may be under water or if you and source are above water but moving relative to one another. In this case I think you have to "measure/observe the medium". Does this make sense?

I'm just trying to help, ignore the post if it only complicates the matter.

 

[Carrock]

1 hour ago, Ghideon said:

.....Imagine that you don't know if you are above or below water. Of course not a common situation but useful for this thought experiment. You hear a sound that you know have a certain frequency from the source in air. A measurement at your location now shows that the frequency does not match the known source frequency. In this case you cannot know if the frequency has changed due to the fact that you and the source may be under water or if you and source are above water but moving relative to one another. In this case I think you have to "measure/observe the medium". Does this make sense?....

Er... no it doesn't make sense.

With no relative movement of the media, a sound wave (entering) water has the same frequency as it does in air.

(As sound travels faster in water its wavelength in water at any given frequency is longer than in air.)

[Ghideon]

4 hours ago, Carrock said:

water has the same frequency as it does in air

Your remark is correct, I was wrong. I did not think clearly and I misinterpreted an article I read about perceived pitch. My suggested analogy is incorrect.

Edited November 30, 2018 by Ghideon

[Eise]

On 11/8/2018 at 12:57 PM, Itoero said:

Is there evidence that only expanding space lengthens waves?

On 11/23/2018 at 8:40 PM, Itoero said:

The idea that only relative motion changes frequency need to be experimentally proven.

(Bold by me).

I'll give it a try... I will repeat arguments already given, but maybe it helps to see the different causes of redshift all together in comparison.

Strange already said that in the case of light there are more possible causes of redshift. So I have no idea why there should be only one cause for redshift. (There is of course just one cause for cosmological redshift.)

1. Doppler effect when source and observer are moving away from each other
2. light 'climbing' out of a gravity field
3. expanding space

The first one should be the simplest to understand: when moving away from the source, the succession of e.g. wave peaks slows down for the observer. This means that for the observer he measures less peaks per second, so the frequency is lower than the frequency at the source. Logically then also the wavelength becomes longer. Compare when you sit in a train and you pass a railway crossing with sound signals. First, when nearing the crossing, you hear a higher pitch, and when you have passed the crossing, the pitch lowers. There is of course an important difference between the Doppler effect for sound and for light: for sound there is a medium, for light there isn't. So the formulas differ.

A typical case for the second cause of redshift is a (heavy) star. Say we look at the spectral lines of hydrogen that glows at the surface of the star. When we, from a far distance, measure the frequencies of these spectral lines, they are redshifted. A simple explanation would be: the light loses energy when climbing out of the gravity field of the star. But be careful! When we look at the light of a distant source, passing a strong gravity field, the following will happen: 'falling' into the gravitational field, the light will be blueshifted, because it gains energy. However, when climbing out of the gravitational field, it loses exactly the same amount of energy again, so gets redshifted exactly the same amount as it was blueshifted before. So as an endresult, we will see the frequency of the light of the remote source exactly the same as if the gravitational field was not there at all. Only when the observer is in the gravitational field it will see a difference in frequency: but it will be blueshifted. From this it already follows that gravitational fields that light passes on its way, make no difference in frequency. (It can change the path of the light, that is gravitational lensing.)

The third case is due solely to the expansion of space itself. As space is stretching, the light 'stretches' with it, and the wavelength of the light becomes longer, and so its frequency lower.

I think you really should read the article Strange linked too.

[studiot]

3 hours ago, Eise said:

On 08/11/2018 at 11:57 AM, Itoero said:

Is there evidence that only expanding space lengthens waves?

On 23/11/2018 at 7:40 PM, Itoero said:

The idea that only relative motion changes frequency need to be experimentally proven.

(Bold by me).

I'll give it a try... I will repeat arguments already given, but maybe it helps to see the different causes of redshift all together in comparison.

Strange already said that in the case of light there are more possible causes of redshift. So I have no idea why there should be only one cause for redshift. (There is of course just one cause for cosmological redshift.)

1. Doppler effect when source and observer are moving away from each other
2. light 'climbing' out of a gravity field
3. expanding space

 

I'd like to add to your Doppler list.

We also have what is known as The Thermal Doppler Effect, whre the observer is at rest in rest frame, but the source oscillates in that frame.

We have a further type of Doppler called the transverse Doppler effect in Special Relativity, due to the viewing angle of the observer, even when the relative velocity is zero.

Both of these have been experimentally confirmed.

 

Following on from the discussion about the influence of transmission medium it is important to distinguish between light, where the speed is fixed, and other waves, where the speed is variable.

A good example of the variable speed is the so called helium speech experienced by divers breathing specialist atmospheres, due to the change of density and therefore speed in the medium.