# The Naked Scientists Forum

### Author Topic: Why does red-shift mean the Universe is expanding?  (Read 3398 times)

#### Thebox

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##### Why does red-shift mean the Universe is expanding?
« on: 16/07/2016 08:10:00 »
I am well aware of the Dopppler, but I have a slight misunderstanding in how red-shift could possibly mean expansion.

When light slows down, it is quite evidently that the wave compresses to form its wavelength,

The faster light travels  the more light permeates and is a ''straighter wave''

Blue is slower than red, but white is faster than blue and red,

So red must be pulling rather than expanding because red is slower than white.

if it were expanding it would still be white.

p.s im off fishing , look forward to returning to your replies.

« Last Edit: 16/07/2016 13:20:03 by chris »

#### PhysBang

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #1 on: 16/07/2016 14:09:34 »
OK, so it is clear that thebox doesn't actually care about the real answer, since they have made it clear that they are only interested in their own personal view on the world. However, in the interests of those who might care, let's have a simple explanation.

Redshift could mean that everything is merely moving away from us and that we are seeing a doppler effect on light. Light doesn't slow down, but as an object emits light gets farther away, the peaks between the waves of emitted light get farther apart than they would normally be so the light gets shifted to a longer wavelength.

But there are other explanations. In General Relativity, we can describe the way to measure distances of space in the universe as a standard Euclidean geometry with one difference: the average distance between objects increases over time. (This works at the hugest scales, but not within galaxies because gravity holds galaxies together.) This leads to no motion between galaxies, since they do not change there position, but their distance changes and so this introduces a Doppler-like effect on the light.

One big advantage of the latter description over the former is that the latter does not require us to imagine that the Earth is the centre of the universe. In the latter description, every point of space is the same.

There are other advantages, too, but this gets into the entire science of cosmology and thus can get technical.

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #2 on: 16/07/2016 16:17:50 »
As PhysBang said, cosmological redshift means that galaxies are moving away from us. Observations of these galaxies led to the discovery that they aren't simply moving away from us but that, in general, each galaxy is moving away from every other galaxy. It's as if the galaxies were dots on a rubber sheet and the rubber sheet was being uniformly stretched in every direction. The mere presence of cosmological redshift itself does not imply that space itself is expanding. It just means that the universe, defined by the collection of galaxies, is spreading out. Why we think that space itself is expanding is for other reasons such as the fact that the general theory of relativity (GR) coupled with observations of cosmological redshift tells us that distant galaxies are moving away from us at speeds faster than the speed of light. That can only happen if space itself is expanding. GR is used to construct models of the universe and the model which describes the universe is one that leads to the Friedman metric and equations. For more on this please see: https://en.wikipedia.org/wiki/Metric_expansion_of_space

The expansion of space means that the distance between different points in space increases with time. The analogy that is commonly used to explain this phenomena is the surface of a sphere. Please keep in mind that the analogy is between our three-dimensional space and the surface of the balloon. It would be going outside the analogy to think that of the surface of the sphere as being embedded in a higher dimensional space. So when the surface of the sphere increases as the sphere itself gets larger the distance between any two points on the sphere increases. This means that as time goes on there is more and more surface area on the surface of the sphere. 2D beings living on the surface would be able to make measurements of the amount of space they had and would determine that there is more and more space as time increases. If our universe is a closed universe then there would be an ever increasing amount of space in the universe.
« Last Edit: 16/07/2016 16:24:33 by PmbPhy »

#### PhysBang

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #3 on: 16/07/2016 16:40:39 »
As PhysBang said, cosmological redshift means that galaxies are moving away from us.
I very carefully did not say this.

The idea of "the curvature of spacetime" in General Relativity is really important, despite how some people like to diminish it. The galaxies (or rather galaxy superclusters) of the standard cosmological model are not moving, they are staying still. It is the distance between these objects that is getting larger. This is an effect of the shape of spacetime as it changes over cosmological time, not the result of a force.

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #4 on: 16/07/2016 21:45:47 »
Quote from: PhysBang
I very carefully did not say this..
I was referring to this comment Redshift could mean that everything is merely moving away from us - So sue me.

#### evan_au

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #5 on: 17/07/2016 00:00:29 »
Quote from: TheBox
Blue is slower than red, but white is faster than blue and red
It is not quite clear what is being discussed here, since the sentence contains no nouns, only adjectives.

However, just to be clear: In the vacuum of space, blue light, red light and white light all travel at the same speed, which we call c.

If we regard the light emitted by a star as "white light", consisting of many wavelengths:
- If this star is observed by an astronomer who is moving away from the star, he will see the wavelengths of this light as being longer than when they were emitted, so the light of the star appears redder due to doppler shift.
- If this star is observed by an astronomer who is moving towards the star, she will see the wavelengths of this light as being shorter than when they were emitted, so the light of the star appears bluer due to doppler shift.

Quote
I have a slight misunderstanding in how red-shift could possibly mean expansion
If you look at distant galaxies (as Edwin Hubble did), and notice that the more distant galaxies tended to have greater redshift, then you conclude that more distant galaxies are expanding away from us.

If you see that the expansion is roughly proportional to distance, then you conclude that these distant galaxies are also expanding away from each other. ie the whole visible universe is expanding.

This does not mean that blue shift is impossible; the "nearby" Andromeda galaxy is part of our local cluster of galaxies and has a blue shift; it is moving towards our Milky Way, and is expected to collide in about 5 billion years. This is interpreted as a local motion within our local galaxy cluster, and does not disprove the general expansion of the universe.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #6 on: 17/07/2016 12:26:34 »
I am now more confused than I started out .  OK, the permitivity of a medium slows light down , the waves become more compressed the slower the speed?

The light that passes through glass is more ''red'' than light that passes through brick?

added- ''in a race, the ''red'' tip is faster than a ''blue'' tip when relative to passing through something?
« Last Edit: 17/07/2016 12:52:41 by Thebox »

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #7 on: 17/07/2016 12:41:18 »
Quote from: evan_au
If we regard the light emitted by a star as "white light", consisting of many wavelengths:
- If this star is observed by an astronomer who is moving away from the star, he will see the wavelengths of this light as being longer than when they were emitted, so the light of the star appears redder due to doppler shift.
- If this star is observed by an astronomer who is moving towards the star, she will see the wavelengths of this light as being shorter than when they were emitted, so the light of the star appears bluer due to doppler shift.
To be more precise, astronomers must first break the starlight up into a spectrum so that they can identify the emission and absorption lines. The frequency of that light is compared to the spectra of what the starlight should be if the galaxy wasn't moving relative to us. Using that data the astronomers then determine what recession velocity it takes to do that and that's the velocity of the galaxy!

Quote from: evan_au
Quote
I have a slight misunderstanding in how red-shift could possibly mean expansion
If you look at distant galaxies (as Edwin Hubble did), and notice that the more distant galaxies tended to have greater redshift, then you conclude that more distant galaxies are expanding away from us.
The redshift means that the object emitting that light is moving away from us. When a source of light moves away from the observer that observer no sees the light as its stretched out due to the source moving away from us just like the tone of the whistle of a train decreases when it changes from a higher pitch to a lower pitch as the train approaches you and then passes you and is then moving away.

Quote from: evan_au
If you see that the expansion is roughly proportional to distance, then you conclude that these distant galaxies are also expanding away from each other. ie the whole visible universe is expanding.
I recommend exercising caution on interpreting the meaning of the relative motion of galaxies. I think people get confused on this point because its difficult to understand that its actually the space itself that is stretching out rather than the object getting further away in space. Recall my analogy with the sphere.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #8 on: 17/07/2016 12:55:34 »

The redshift means that the object emitting that light is moving away from us. When a source of light moves away from the observer that observer no sees the light as its stretched out due to the source moving away from us just like the tone of the whistle of a train decreases when it changes from a higher pitch to a lower pitch as the train approaches you and then passes you and is then moving away.

The Doppler effect I understand of sound, why do we compare this effect of light to sound ?

Couldn't the Doppler shift of sound and the redshift of light be two different things?

Doesn't white light passing through a Prism red-shift? blue-shift? etc

How is the Phase velocity shift  of the temporal distortion of the spectral colour red different to the Hubble observed red-shift of light?

« Last Edit: 17/07/2016 12:58:19 by Thebox »

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #9 on: 17/07/2016 13:55:48 »
Quote from: Thebox
The Doppler effect I understand of sound, why do we compare this effect of light to sound ?
Because each has the essential characteristics of what the Doppler effect is. Both light and sound waves move at a given speed. We use the letters v for sound and c for light. Both have a wavelength L, frequency f and time period R, related to each other in the same exact relationship; c(or v) = L/T = Lf

Quote from: Thebox
Couldn't the Doppler shift of sound and the redshift of light be two different things?
That question is meaningless. To make it meaningful you need to state what you mean by "different things" because the Doppler effect is not a "thing".

Quote from: Thebox
Doesn't white light passing through a Prism red-shift? blue-shift? etc
[/quite]
No. The amount of deflection of a beam of light passing through a prism depends on the frequency of the light.

Quote from: Thebox
How is the Phase velocity shift  of the temporal distortion of the spectral colour red different to the Hubble observed red-shift of light?
I'm sorry but I don't know what that sentence means

#### evan_au

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #10 on: 18/07/2016 22:47:10 »
Quote from: TheBox
I am now more confused than I started out .  OK, the permitivity of a medium slows light down , the waves become more compressed the slower the speed?
Yes, light travels slower when it is traveling through glass. But that is only while the light is traveling through the glass; all the light once again travels at c when it returns to a vacuum (like space).

And when we talk about the wavelength of light, we usually mean the wavelength in a vacuum. When we are talking about a different medium, we normally say so, eg microelectronics is exposed to UV light in a liquid because the wavelength of light is shorter in a liquid than in air, so you can draw finer lines, and fit more transistors on a chip.

Quote
Doesn't white light passing through a Prism red-shift? blue-shift? etc
No. The wavelength of the light before it enters the prism is exactly the same as the wavelength after it exits the prism.

What might be confusing you is that glass is a "dispersive medium", ie some wavelengths travel faster than others.
See: https://en.wikipedia.org/wiki/Dispersion_(optics)

Quote
in a race, the ''red'' tip is faster than a ''blue'' tip when relative to passing through something?
If I understand this correctly, in glass, blue light travels slower than red light, so blue light is bent more when it enters a glass prism, producing a rainbow. (Although there are other materials that work the opposite way, and are used in telecommunications to reassemble laser pulses which smear out in time after they have passed through an optical fiber.)

Quote
The light that passes through glass is more ''red'' than light that passes through brick?
Light doesn't propagate through a brick.

But in very general terms, for transparent materials, a more-dense material like air, water or glass affects the wavelength more than a less-dense material like a vacuum.

But there are many exceptions (eg diamond). You can look at a table of refractive indices.
https://en.wikipedia.org/wiki/List_of_refractive_indices

But be aware that in many materials, the refractive index is dependent on wavelength.

Quote
Phase velocity [vs] Hubble observed red-shift of light?
As I understand it, the Phase Velocity becomes important when you have a dispersive medium.

However, cosmological redshift is viewed through the vacuum of space, which is not a dispersive medium. (Otherwise we would see stars with colored streaks, like a rainbow or a spectrograph.)

Phase velocity does not affect the Hubble redshift.
See: https://en.wikipedia.org/wiki/Phase_velocity

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #11 on: 21/07/2016 15:13:59 »
I've done some more thinking on this issue and came to a different understanding that I had before. Before I was wrong. I erroneously thought that by "expanding universe" that cosmologists were speaking of expanding space and that cosmological redshift was evidence of that. The correct understanding is that by an "expanding universe" galaxies are all moving away from each other. That is to say that if you were in an observer in one galaxy and observed another galaxy then you'd determine that light from that galaxy was redshifted and as such was moving away from you. You'd then determine through observations that the speed of the galaxy was a linear function of the distance to the galaxy.

It should be noted that the expansion of space is consistent with this determination but is not proved by it. Especially since nothing can actually be proved in science. So in the past when I claimed that cosmological redshift implied that space was expanding, I was wrong. It means that the universe is expanding. That space is expanding is another issue. It would take a force other than gravity to restrain a galaxy from moving away from ours even if it was moving at constant speed. Also, if space wasn't expanding then no galaxy would move away from us at speeds faster than the speed of light.

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#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #12 on: 23/07/2016 08:13:13 »

Yes, light travels slower when it is traveling through glass. But that is only while the light is traveling through the glass; all the light once again travels at c when it returns to a vacuum (like space).

Yes, the glass has more permitivity than the vacuum of space and space allows light to permeate freely.

Quote
Doesn't white light passing through a Prism red-shift? blue-shift? etc
Quote from: Evan
No. The wavelength of the light before it enters the prism is exactly the same as the wavelength after it exits the prism.

Huh?  the white light going in comes out different colours, i.e different wavelengths, I can see red and blue among the colours, how is the red output not the same as redshift?

Quote
The light that passes through glass is more ''red'' than light that passes through brick?
Quote from: Evan
Light doesn't propagate through a brick.

Visible light doesnt propagate through a brick, what about x-ray light and such?

Quote
However, cosmological redshift is viewed through the vacuum of space, which is not a dispersive medium. (Otherwise we would see stars with colored streaks, like a rainbow or a spectrograph.)

At least you understand the ''gin-clear'' space, but according to science we don't actually view through anything do we because that is not how sight works...!

« Last Edit: 23/07/2016 08:15:29 by Thebox »

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #13 on: 23/07/2016 08:18:00 »
I've done some more thinking on this issue and came to a different understanding that I had before. Before I was wrong. I erroneously thought that by "expanding universe" that cosmologists were speaking of expanding space and that cosmological redshift was evidence of that. The correct understanding is that by an "expanding universe" galaxies are all moving away from each other. That is to say that if you were in an observer in one galaxy and observed another galaxy then you'd determine that light from that galaxy was redshifted and as such was moving away from you. You'd then determine through observations that the speed of the galaxy was a linear function of the distance to the galaxy.

Yes Pete that is correct like I have said many times before , there is no evidence of space expanding, the evidence is of the distance between Galaxies is increasing.

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #14 on: 23/07/2016 09:29:18 »
Quote from: Thebox
Yes Pete that is correct like I have said many times before , there is no evidence of space expanding, the evidence is of the distance between Galaxies is increasing.
I know what you said but you keep ignoring or forgetting what I said. Its  probably because you may not know what the term evidence means. Evidence of a theory is any information that is consistent with that theory. If space was expanding then Galaxies would be moving apart and we'd see each galaxy redshifted, the amount of redshift being determined by the distance that galaxy is from us. The further away the galaxy the greater the redshift. And that's exactly what we see. That means that cosmological redshift is evidence of expanding space.

Your problem has always been your lack of understanding of the scientific method and the philosophy of science. One of the most important points that a scientist learns when they study the philosophy of science is that proof is not part of the scientific method. That's because evidence of a hypothesis or theory can in theoretically be explained in more than one way. That's why cosmological redshift is said to be evidence that space is expanding and its not considered proof that space is expanding.

So before you make anymore of your false claims that there's no evidence that space is expanding, first learn what evidence is.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #15 on: 23/07/2016 13:04:49 »
Quote from: Thebox
Yes Pete that is correct like I have said many times before , there is no evidence of space expanding, the evidence is of the distance between Galaxies is increasing.
I know what you said but you keep ignoring or forgetting what I said. Its  probably because you may not know what the term evidence means. Evidence of a theory is any information that is consistent with that theory. If space was expanding then Galaxies would be moving apart and we'd see each galaxy redshifted, the amount of redshift being determined by the distance that galaxy is from us. The further away the galaxy the greater the redshift. And that's exactly what we see. That means that cosmological redshift is evidence of expanding space.

Your problem has always been your lack of understanding of the scientific method and the philosophy of science. One of the most important points that a scientist learns when they study the philosophy of science is that proof is not part of the scientific method. That's because evidence of a hypothesis or theory can in theoretically be explained in more than one way. That's why cosmological redshift is said to be evidence that space is expanding and its not considered proof that space is expanding.

So before you make anymore of your false claims that there's no evidence that space is expanding, first learn what evidence is.

What? you have  said in previous post you have now realised space is not expanding and the evidence is of objects.
« Last Edit: 23/07/2016 13:07:44 by Thebox »

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #16 on: 23/07/2016 13:26:26 »
Quote from: Thebox
What? you have  said in previous post you have now realised space is not expanding and the evidence is of objects.
Wrong!!! I did not say that. I said that cosmological redshift implies that space is expanding. That was an incorrect assertion. It's evidence of spatial expansion, it doesn't imply it. Please read more carefully next time.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #17 on: 23/07/2016 13:34:58 »
Quote from: Thebox
What? you have  said in previous post you have now realised space is not expanding and the evidence is of objects.
Wrong!!! I did not say that. I said that cosmological redshift implies that space is expanding. That was an incorrect assertion. It's evidence of spatial expansion, it doesn't imply it. Please read more carefully next time.

No Pete, you said -

''I've done some more thinking on this issue and came to a different understanding that I had before. Before I was wrong. I erroneously thought that by "expanding universe" that cosmologists were speaking of expanding space and that cosmological redshift was evidence of that.''

There is no evidence of space itself expanding , the evidence of red shift is of Galaxies.

Pete please talk to me about something, I have a problem with red shift showing ''expansion'', we know that when light slows down it can change frequency, so when we observe red shift that must be light slowing down, now if something is travelling away from something, it is impossible for the ''leader'' to slow down the ''trailer'', the light should not red shift if something is moving away it should remain ''gin-clear'' a maxed out speed?

What permitivity causes the red shift?

#### jerrygg38

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #18 on: 23/07/2016 13:44:18 »

The Doppler effect I understand of sound, why do we compare this effect of light to sound ?

Couldn't the Doppler shift of sound and the redshift of light be two different things?

Light is similar to radar waves which exhibit Doppler effects. A radar source moving toward us will have a higher frequency and a radar source moving away from us will have a lower frequency.
The red shift of the far stars can be viewed from two perpectives. As the universe expands everything expands. the clocks and rulers expand. thus there is a common mode redshift. Yet white light will still appear white. The expansion also has a Doppler shift due to the fact that the distance to the far stars has changed with respect to the expanding universe. Thus it looks like a simple Doppler problem.
To make matters worse space time is non-linear so the best we can do is work out linear approximations to space and time. Einstein's work tends to be a best fit approximation to space and time from a mathematical perspective.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #19 on: 23/07/2016 13:51:17 »

The Doppler effect I understand of sound, why do we compare this effect of light to sound ?

Couldn't the Doppler shift of sound and the redshift of light be two different things?

Light is similar to radar waves which exhibit Doppler effects. A radar source moving toward us will have a higher frequency and a radar source moving away from us will have a lower frequency.
The red shift of the far stars can be viewed from two perpectives. As the universe expands everything expands. the clocks and rulers expand. thus there is a common mode redshift. Yet white light will still appear white. The expansion also has a Doppler shift due to the fact that the distance to the far stars has changed with respect to the expanding universe. Thus it looks like a simple Doppler problem.
To make matters worse space time is non-linear so the best we can do is work out linear approximations to space and time. Einstein's work tends to be a best fit approximation to space and time from a mathematical perspective.

Light can not work the same as sound and the doppler, where is the opposing force coming from to slow down the light to ''red''?

#### Colin2B

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #20 on: 23/07/2016 19:01:43 »
The Doppler effect I understand of sound

Apparently not:

Light can not work the same as sound and the doppler, where is the opposing force coming from to slow down the light to ''red''?
What opposing force, there isn't one in sound Doppler.
Light doesn't slow down for a Doppler shift and it doesn't need to.
You are so eager to push your own theories that you are failing to read and understand what is being said.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #21 on: 23/07/2016 22:01:03 »
The Doppler effect I understand of sound

Apparently not:

Light can not work the same as sound and the doppler, where is the opposing force coming from to slow down the light to ''red''?
What opposing force, there isn't one in sound Doppler.
Light doesn't slow down for a Doppler shift and it doesn't need to.
You are so eager to push your own theories that you are failing to read and understand what is being said.

There is no theory just a question.  If light does not slow down then how do you observe ''red''?

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #22 on: 24/07/2016 00:31:39 »
Quote from: Thebox
No Pete, you said -

''I've done some more thinking on this issue and came to a different understanding that I had before. Before I was wrong. I erroneously thought that by "expanding universe" that cosmologists were speaking of expanding space and that cosmological redshift was evidence of that.''
Then I was correct in the first place. Cosmological redshift is indeed evidence of space expanding. It just doesn't imply it.

the box still has to learn what evidence is and I'm not about to keep correcting you over and over while you ignore what I'm explaining to you and merely repeat erroneous claims.

#### jerrygg38

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #23 on: 24/07/2016 00:45:32 »

Light can not work the same as sound and the doppler, where is the opposing force coming from to slow down the light to ''red''?
You are assuming that red light is slower than blue light. they both move at the same speed of C.  Draw a chain of sine waves on a piece of paper. If you are stationary and the wave flows toward you the light will be white. Now move toward the chain of waves. this will cause the peaks and valleys to happen faster. thus the light will be bluer. Now move away from the chain of waves. The peaks and valleys will happen slower. thus the light turned redder.
The redshift shows that the waves coming toward us come from a source of light that is moving away from us.

#### evan_au

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #24 on: 24/07/2016 05:22:45 »
Quote from: TheBox
Quote from: Evan
The wavelength of the light before it enters the prism is exactly the same as the wavelength after it exits the prism.
Huh?  the white light going in comes out different colours, i.e different wavelengths, I can see red and blue among the colours, how is the red output not the same as redshift?
If you put light of a single color into a prism (eg from a laser), it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming light.

The prism will bend the light by a different angle depending on its color (ie its wavelength and frequency).

So:
• if you shine a red laser into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming red light.
• if you shine a blue laser into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming blue light.
• But the red light will come out at a slightly different angle than the blue light
Now white light (eg from the Sun) is made up of many wavelengths all mixed together (including red and blue).
So:
• If we shine the red light from the Sun into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming red light.
• If we shine the blue light from the Sun into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming blue light.
• But the red light will come out at a slightly different angle than the blue light (or green light). This is how a prism splits up white light from the Sun into its constituent colors.

The rainbow effect has nothing to do with redshift; in redshift, all colors have their wavelength (and frequency) changed compared to the wavelength when it was emitted.

Quote from: TheBox
we know that when light slows down it can change frequency, so when we observe red shift that must be light slowing down
I think you may be confusing frequency and wavelength?

In a vacuum, frequency and wavelength of light are directly related to each other, and the speed is constant = c. The speed of light does not slow down in space.

However, when you consider light traveling through different media (eg glass, as discussed in another recent thread), the speed of light is variable, and is less than c. This can cause the speed of light to slow down temporarily (until it returns to a vacuum, when it returns to its original speed=c).

While light is traveling through glass at <c, its wavelength is less than it is in a vacuum. But its frequency is the same as the frequency in a vacuum.

So, overall, seeing a red shift from a distant galaxy does not imply that light is slowing down.
Quote from: jerrygg38
there is a common mode redshift. Yet white light will still appear white.
A bit of nitpicking here (to reduce confusion for TheBox)...
White light from a star like the Sun has an overall "black body" spectrum, where some frequencies output more power than other frequencies.

Red-Shifted light from the Sun when observed by distant galaxies would appear slightly more red, because the peak of the spectral output is moved down to redder frequencies.

In practice, this form of redshift assumes that an astronomer knows what the peak spectral output was originally; a bit impractical for a very distant star.

Apart from the overall broadband "black-body" spectrum, there are narrow Fraunhofer lines that are absorbed by atoms in the Sun's outer atmosphere. A distant astronomer can work out which atoms these lines come from, and he knows (from laboratory measurements) at what frequencies these absorption lines occurred in the source star.

This allows astronomers to work out the Doppler shift and relative velocity of a distant galaxy; by measuring the velocity of many different galaxies, Hubble was able to deduce cosmological expansion.

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #24 on: 24/07/2016 05:22:45 »