# The Naked Scientists Forum

### Author Topic: Why does red-shift mean the Universe is expanding?  (Read 3340 times)

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #25 on: 24/07/2016 08:02:22 »
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.

Ok , I understand your reasoning for now.

You say white light is a mixture of frequencies, however I am sure we can look at white light in two ways

1)white light is a mixture of frequencies

2)A mixture of frequencies can be created from white light and white light is the ''base'' frequency.

How do we know number 2 does not apply? because to me the ''gin-clear'' appears to be one constant frequency, an equilibrium to sight and 'invisible'' rather than like the NSF white background these words are on, and why do we not call it invisible light because it is definitely not white like the background of NSF?

p.s I have said before about the angular distortion of light entering a prism and I was told this was wrong.

C.O.R.P

Center of radiation pressure delta = delta P  = delta fr where P is pressure and fr is frequency?

is this how a prism works?

« Last Edit: 24/07/2016 09:00:11 by Thebox »

#### evan_au

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #26 on: 24/07/2016 23:16:43 »
Quote from: TheBox
2. A mixture of frequencies can be created from white light and white light is the ''base'' frequency.
How do we know number 2 does not apply?
If you split white light from the Sun into its constituent frequencies with a prism, you will see the individual frequencies to which we give names like red, yellow, green, blue, etc - the colors of the rainbow. Each one of these colors is visibly different from the original white light.

If you use a thermometer, you will also detect additional invisible frequencies like infra-red and ultraviolet past the visible edges of the spectrum.

No matter how you manipulate* one individual frequency of the rainbow by shining it through lenses, reflecting off mirrors, etc, you cannot reconstruct white light. This is because a single frequency does not possess the large variety of frequencies necessary to produce white light.

However, if you take all of the frequencies of the rainbow, and combine them together, you will have something that looks like the original white light. This experiment was first conducted by Isaac Newton.

So white light is not a single "base" frequency, but is the sum of many different frequencies.

*Today we have non-linear crystals that can double or halve the frequency of light; this does change the color of the light, but it will still be visibly different from the original white light.

Quote
NSF white background
What is "NSF"? National Science Foundation?

Quote
C.O.R.P is this how a prism works?
I am afraid that, as usual, I cannot understand the pseudo-geometrical construction in the diagram.

#### PmbPhy

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #27 on: 25/07/2016 13:55:21 »
More on the expansion of the universe. As some of you may know, Alan Guth is a very good friend of mine. I asked him about this and he responded. He said that I could post his response:

Quote from: Pete
I'm studying cosmology right now and am unable to determine whether or not by the expansion of the universe cosmologists mean that all galaxies are receding from all other galaxies or whether they mean that space is expanding. I can easily understand why you say that cosmological redshift implies those galaxies which are redshifted are moving away from us and that, in general, all galaxies are moving away from us and that observers in each galaxy make the same determination and refer to this as the expansion of the universe. But I don't see how the presence of cosmological redshift implies that space is expanding. Can you help clarify this issue for me, please? Thank you.

Quote from: Alan Guth
I think that if you ask 12 cosmologists this question, you'll get 15 different answers.  Whether or not the question is meaningful depends somewhat on what other assumptions one makes.

1) If the total volume of the universe is finite, then there is o way that all the galaxies could get further apart without the volume increasing.  This would apply to a closed universe, but we probably don't live in a closed universe.  There are also ways of defining periodic identifications in an open universe, making the volume finite, and then the volume of such a universe would have to increase with time to be consistent with the expansion that we observe.  But if the volume is infinite, then there is no unambiguous meaning to saying that it is getting larger.

2) Even if the universe is infinite, however, it is still easier to think in a language in which space is expanding.  General relativity allows you to assign coordinates any way you want, but for cosmology it is overwhelmingly simplest to use a coordinate system that expands with the average flow of matter.  In this coordinate system the universe looks (to high accuracy on large scales) homogeneous.  In this coordinate system the average velocity of matter is zero.  The volume contained within any region of coordinate space is getting larger with time.  But if you wanted to be stubborn, you could use a coordinate system that did not expand, but instead matter would be moving through it. In this coordinate system, however, the universe would not look at all homogeneous, and everything would be a mess.  Nobody ever uses such a coordinate system.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #28 on: 25/07/2016 21:40:46 »
Quote from: TheBox
2. A mixture of frequencies can be created from white light and white light is the ''base'' frequency.
How do we know number 2 does not apply?
If you split white light from the Sun into its constituent frequencies with a prism, you will see the individual frequencies to which we give names like red, yellow, green, blue, etc - the colors of the rainbow. Each one of these colors is visibly different from the original white light.

If you use a thermometer, you will also detect additional invisible frequencies like infra-red and ultraviolet past the visible edges of the spectrum.

No matter how you manipulate* one individual frequency of the rainbow by shining it through lenses, reflecting off mirrors, etc, you cannot reconstruct white light. This is because a single frequency does not possess the large variety of frequencies necessary to produce white light.

However, if you take all of the frequencies of the rainbow, and combine them together, you will have something that looks like the original white light. This experiment was first conducted by Isaac Newton.

So white light is not a single "base" frequency, but is the sum of many different frequencies.

*Today we have non-linear crystals that can double or halve the frequency of light; this does change the color of the light, but it will still be visibly different from the original white light.

Quote
NSF white background
What is "NSF"? National Science Foundation?

Quote
C.O.R.P is this how a prism works?
I am afraid that, as usual, I cannot understand the pseudo-geometrical construction in the diagram.

On your Naked scientist forum and you need to ask what NSF meant?

Ok I understand the colours and why we say/think white light is  a mixture of frequencies but I think you missed the point so will say it in an alternative mannerism.

White light is the whole, the whole is a singularity, an individual with no other characteristics than whole, not mixed like a rainbow road but a ''solid'' of one thing, however this one thing can be distorted into things that are different than the whole, special white 'paint'' than when slowed down by obstruction changes from the whole into a lesser white paint , it changes to blue or red but is still actually white, it is just slower white.

#### evan_au

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #29 on: 25/07/2016 22:19:13 »
Quote from: TheBox
On your Naked scientist forum and you need to ask what NSF meant?

Sorry, I missed it...
"NSF white background" is the white background of the Naked Scientists Forum webpage...

Quote
White light is the whole, ... an individual with no other characteristics than whole, not mixed like a rainbow road
That is the point of this discussion - white light does have different characteristics which can be separated out to form a rainbow.

Quote
light is ... a singularity
I suggest that you stop using "singularity", as it always comes out confused.

In this context, you possibly mean something like "an indivisible whole".

#### evan_au

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #30 on: 25/07/2016 22:26:21 »
Quote from: Alan Guth
if you wanted to be stubborn, you could use a coordinate system that did not expand, but instead matter would be moving through it.
So perhaps we could say "In the vicinity of our local galactic cluster, space is getting less dense over time. However, we don't see matter being destroyed - it is just moving outside our vicinity, and the same could be said of most other galaxy clusters in our visible universe."

So for the stubborn, a universal reduction in the local density of matter is equivalent to expansion of the universe.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #31 on: 26/07/2016 08:59:22 »
Quote from: TheBox
On your Naked scientist forum and you need to ask what NSF meant?

Sorry, I missed it...
"NSF white background" is the white background of the Naked Scientists Forum webpage...

Quote
White light is the whole, ... an individual with no other characteristics than whole, not mixed like a rainbow road
That is the point of this discussion - white light does have different characteristics which can be separated out to form a rainbow.

Quote
light is ... a singularity
I suggest that you stop using "singularity", as it always comes out confused.

In this context, you possibly mean something like "an indivisible whole".

1.
singularity - the state, fact, quality, or condition of being singular.

I do not think I misuse the word in any sense, if anything you are not being ambiguous with the word singularity and you only look at the word in this content -

''PHYSICS MATHEMATICS
a point at which a function takes an infinite value, especially in space–time when matter is infinitely dense, such as at the centre of a black hole.''

But yes, an ''indivisable'' whole.

''white light does have different characteristics which can be separated out to form a rainbow.''

Define separate?  coloured light is hardly a separation, it is still light and if not being compressed in wavelength and was allowed to permeate properly is still actually white light.

I do not think we actually separate it, I think we ''manipulate'' it by changing the wavelength/frequency from the wavelength/frequency whole , which to me is a perfect linearity whole  that extends in all directions from source and has an infinite ''wave-length''  and 0 frequency, but only zero because our devices are set up,calibrated at 0 and at an already equilibrium to the environment to begin with.

''

#### puppypower

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #32 on: 26/07/2016 12:14:28 »
If energy or EMR (electromagnetic radiation) is red shifting due to the expansion of the universal space-time, does that mean the EM force is also shifting, since these are two sides of the same coin?

In other words, if we assume dark energy is causing the expansion of universal space-time, where space-time is expanding in all directions at the same time, have the size of atoms increased with increasing space-time? Or are the size of atoms constant and therefore exempt from universal space-time expansion? If exempt, how can the EM ignore dark energy and changes universal space-time? Also, if they are exempt, are the material based space-time contractions we see, sort of an illusion, since the EM based matter/atoms, which defines the phases of matter, is not doing anything by stays constant?

For example, say we had an isolated hydrogen atom, early in the universe, when space-time was far more contracted. Dark energy is acting, causing space-time to expand in all directions. Does the hydrogen atom expand along with space-time, such that the modern hydrogen atoms have additional energy levels due to the larger size? If not and the EM force and atoms are exempt, does the EM force, by being exempt, play an ever increasing role, helping to compensate for gravity and different references, since gravity is not exempt from space-time expansion as reflected by galaxies and can gravity can generate different references?

The red shifted energy we measure is EM energy, so the answer of why red shift is dependent on what happens to the EM force and the size of atoms that make use of that force, as they absorb and emit EMR, as dark energy changes space-time and gravity.
« Last Edit: 26/07/2016 12:17:28 by puppypower »

#### evan_au

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #33 on: 26/07/2016 12:42:07 »
Quote from: TheBox
Quote from: evan_au
''white light does have different characteristics which can be separated out to form a rainbow.''
Define separate?
I mean we can separate white light into its constituent colors (the rainbow) using raindrops in the sky, a glass prism or a diffraction grating.

Quote
I do not think we actually separate it, I think we ''manipulate'' it by changing the wavelength/frequency from the wavelength/frequency whole

The three methods above are all "linear" techniques that do not generate any frequencies that weren't already there in the original white light. They just separate out the frequencies that were already there.

This is distinct from red-shift: if we move relative to the light source, the observed frequencies differ from the original frequencies by a percentage (the Doppler shift) without generating any new frequencies that weren't already there. This red-shifted white light can then be separated out into these new frequencies using a prism.

This is very distinct from "non-linear" optical materials, which manipulate the frequency of the incoming photons to produce photons at frequencies that were not present in the incoming light, typically the sum or difference frequencies. But these materials only work at very high light intensities, such as you can get from a focused laser.

Your cellphone and WiFi devices use non-linear circuits to manipulate microwave frequencies. These non-linear circuits produce sum and difference frequencies that were not present on the antenna. The difference frequency is lower than the frequencies at the antenna, and we can manipulate these lower frequencies more easily to decode the data.

If we were listening to music, we call non-linear effects like speaker overload "distortion", because it (unintentionally) manipulates the audible signal, generating sum and difference frequencies that were not present in the original music recording.
« Last Edit: 26/07/2016 12:57:28 by evan_au »

#### evan_au

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #34 on: 28/07/2016 22:26:06 »
Quote from: TheBox
I do not think we actually separate it, I think we ''manipulate'' it by changing the wavelength/frequency from the wavelength/frequency whole
It may be possible to illustrate the mixture of colors in white light by an example from chemistry: the mixture of chemicals in black crude oil.

You could consider black crude oil as a single thing, an indivisible whole.

However, when you put it into a petroleum refinery, they separate different chemicals from the crude oil by their boiling points. This does not change the nature of the components; if you mixed them together again, you would have the original black crude oil.  https://en.wikipedia.org/wiki/Oil_refinery#The_crude_oil_distillation_unit

This is similar to what happens when you split light into its constituent colors with a prism; it does not change the nature of the components; if you mixed them together again, you would have the original white light.

However, not all of the chemical components of crude oil are equally valuable - there is a great demand for lighter oils to power cars and trucks, for example. So long-chain molecules are reacted with hydrogen to turn them into shorter molecules; this is called "cracking". If you mixed all the components together after cracking, you would find something different from the original black crude oil.
See: https://en.wikipedia.org/wiki/Cracking_(chemistry)

This is similar to what happens with white light when we manipulate it with non-linear techniques. You can end up with color components that are half the wavelength of the original light. If you then mixed all the components together again, you would find something different from the original white light.

In this sense, "White light is a mixture of colors" is similar to the lesson in early chemistry lessons around "Black crude oil is a mixture of chemicals". And the lessons on the difference between a mixture and a chemical reaction.

It's as clear as Black and White!

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #35 on: 30/07/2016 09:53:41 »
Quote from: TheBox
Quote from: evan_au
''white light does have different characteristics which can be separated out to form a rainbow.''
Define separate?
I mean we can separate white light into its constituent colors (the rainbow) using raindrops in the sky, a glass prism or a diffraction grating.

Looking from a different ''angle'' I can perceive this differently, it means the same thing almost but there is a subtle difference I ''see'' from an alternative viewpoint.

Explaining is obviously my weakness, I will try though like I always do.

Let us look at a ''solid'', we can take the ''solid'' and split it down into different particles,  we can say the ''solid'' is a mixture of particles.

Let us look at a tube of smarties, we can say the tube consists of a mixture of colours.

Let us look at a spinning spectral wheel that makes the colours seem visually white when in rotation, we can say that the white is a mixture of colours in motion but the colours do not actually mix.

(Sounds gibberish, I am trying to aid myself looking for the correct wording).

Let us look at the spectral wheel spinning compared to the surrounding space, I can see the now formed mixture of position of the colours as white , I see it is white relative to the reference frame of the surrounding clear space.  But I do not see the surrounding space to be white I see it to be '''gin-clear'' or clear space.

My brain at this stage acknowledges that the changing  position of the colour on the wheel rotating at a certain speed give the illusion of a white wheel. My brain is saying to me RGB ''flicker'',

(Pause in thought to explain that I use to be a silk screen printer, I used 4 colour process, coloured dots overlay to build up a picture, my mind keeps recalling this information about layers).

So far I have flickers and layers in my head and to you all this may seem complete gibberish and irrelevant but I try not to overlook ''things''.

(pause for a cigarette).

How can I explain the un-explainable that is in my head, I have no idea of how to explain it .

Lets try take a 1000w, split it into several different wattages,

We could say the different watt's is a mixture of 1000w,

added - what i am trying to say is that ''white'' light is not  a mixture of frequencies from my viewpoint, visible light (colour) is the same, it is all light and not  mixture of anything, colour is a process of the slowing down of the light by permitivity , refraction and other.

Temporal distortion is colour, we are not splitting the slight, we manipulate the light to make colour, the clear space light is exactly the same as blue light or red light or any other light, but the light in space is not been magnetically distorted by em fields permitivity , absorbsion etc.

aded- if we can imagine light passing through space is a whole and a mono type field, then things affect this mono type field,

Light cannot travel centripetally from a source?

added - sorry for the extended added content. I have just had a thought it may seem off topic but it is not,

A particle emits light, but a particle also absorbs light, so why does a particle not absorb its own light?  does a particle have a light capacitance it can absorb?

hfmax=?

...

added - I still have a problem with red shift, if light is red shifting , i.e  the wavelength is shorter,   then there is an opposing force needed to do this, the galaxies would have to be pushing the light rather than travelling away from the light, something is not making sense or there is something I am not understanding, but either way it seems contradictory.

added- considering Newtons third law where is this diagram in error?

« Last Edit: 30/07/2016 10:39:40 by Thebox »

#### PhysBang

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #36 on: 31/07/2016 14:23:26 »
Again, the problem here seems to be that thebox does not bother to read any physics and merely makes it up himself. That thebox does not then understand science is his fault, not the fault of scientists or science educators.

#### Thebox

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##### Re: Why does red-shift mean the Universe is expanding?
« Reply #37 on: 31/07/2016 16:11:16 »
Again, the problem here seems to be that thebox does not bother to read any physics and merely makes it up himself. That thebox does not then understand science is his fault, not the fault of scientists or science educators.

I do not ''see'' how that answers my question?

I will redefine for you, If an object is moving away from you stretching a piece of elastic, then how can the elastic possibly become ''slack''?

Added - I did you a diagram of the question if you are unsure.

« Last Edit: 31/07/2016 16:14:31 by Thebox »

#### The Naked Scientists Forum

##### Re: Why does red-shift mean the Universe is expanding?
« Reply #37 on: 31/07/2016 16:11:16 »