How does the expansion of the universe stretch light?

[Peter Watkins]

Any galaxy that we can see, must have been traveling at less than light speed when the photons that we now see, departed from it. Therefore, the visible rate of galaxy separation is less than light speed. How then, could expansion "stretch" light? Particularly, if it is the expansion of space that is causing galaxies to separate, rather than their own movement, then between any two galaxies there would be pressure in both direction to effect separation. With perhaps hundreds of galaxies between us and the most distant views, light photons would have to overcome the resistance of the space that is pushing in the opposite direction from its own travel. One can understand that if a galaxy was receding from us at a high rate at the moment that the photons left, then the wave-length of the photons could be slightly stretched.
On the face of it, in order for space to move galaxies, it must have a high degree of viscosity. This, surely, would have the effect of slowing these hard working photons. Why then are they not compressed by, in the case of the most distant views, several billion years of resistance. And, (perhaps one for the physics guys), why do they weave from side to side, describing a wave motion?

 

[Drakkith]

First, the expansion of space in no way puts "pressure" on anything inside. All it means is that the distance between ANY two points in space is always increasing. Light does not have any pressure to fight against as it travels. There is no resistance, no viscocity, nothing.

I believe the idea is that the expansion of space stretches the wavelength out, making the light redder. But I don't know the details of the phenomenon.

Also, light does not weave from side to side. The classic diagrame of a wiggly line merely represents the fact that the electric and magnetic fields that make up the photon are alternating back and forth from positive to negative. Light travels in a straight line, always.

 

[Chalnoth]

Any galaxy that we can see, must have been traveling at less than light speed when the photons that we now see, departed from it.

This is false. There is no unique way to define the velocity of a far-away object, and without a unique definition, the velocity of a far-away object clearly can't obey any hard-and-fast rules.

By the usual definition of recession velocity, most of the galaxies that we can see from Earth are now and always have been receding at faster than the speed of light.

 

[Galron]

The best way to think of the expansion I have seen is this.

t=0=the singularity

.

t>0

1234567890

...

1.2.3.4.5.6.7.8.9.0

...

1..2..3..4..5..6..7..8..9..0

...

1...2...3...4...5...6...7...8...9...0

Essentially all this says is time and space are the same size as they were at the singularity, the only thing that has changed is the graph itself of distance in terms of x,y,z,t or i even.

If you like.

 

[Galron]

First, the expansion of space in no way puts "pressure" on anything inside. All it means is that the distance between ANY two points in space is always increasing. Light does not have any pressure to fight against as it travels. There is no resistance, no viscocity, nothing.

I believe the idea is that the expansion of space stretches the wavelength out, making the light redder. But I don't know the details of the phenomenon.

Also, light does not weave from side to side. The classic diagrame of a wiggly line merely represents the fact that the electric and magnetic fields that make up the photon are alternating back and forth from positive to negative. Light travels in a straight line, always.

It's called doppler shift and is exactly the same for spatial attenuation as it is for sound, light or whatever.

Spoiler

 

[Drakkith]

It's called doppler shift and is exactly the same for spatial attenuation as it is for sound, light or whatever.

Doppler shift is a result of an object moving towards or away from you, and I thought I remember reading that the expansion of space can stretch the wavelength of light out in addition to the Doppler shift from an object. But I'm not 100% sure.

 

[Galron]

Doppler shift is a result of an object moving towards or away from you, and I thought I remember reading that the expansion of space can stretch the wavelength of light out in addition to the Doppler shift from an object. But I'm not 100% sure.

It's an analogy basically only space is moving and you aren't. It's probably best to think of it as a doppler shift in attenuations of space. Ok my brains now leaked out my ears, and will have to go sit down.

 

[Drakkith]

It's an analogy basically only space is moving and you aren't. It's probably best to think of it as a doppler shift in attenuations of space. Ok my brains now leaked out my ears, and will have to go sit down.

I think we should leave doppler shift to moving objects and not try to relate it to the effect of expansion on light. Otherwise things just get messy!

 

[Galron]

I think we should leave doppler shift to moving objects and not try to relate it to the effect of expansion on light. Otherwise things just get messy!

 

[Chalnoth]

The simplest way I can think to say it is that you get some very specific total redshift for faraway objects due to the expansion. How much of that redshift is due to the doppler shift and how much is due to the expansion between us and the far away object is completely arbitrary.

 

[PAllen]

This is false. There is no unique way to define the velocity of a far-away object, and without a unique definition, the velocity of a far-away object clearly can't obey any hard-and-fast rules.

By the usual definition of recession velocity, most of the galaxies that we can see from Earth are now and always have been receding at faster than the speed of light.

This is true, but to add to the point about no unique way to define this, an alternative definition:

parallel transport galaxy's 4-velocity (it doesn't matter what you measure it relative; it is a covariant geometric object) at time of emission along the null-path the light follows to reach Earth (in *any* coordinate system you choose)

produces no superliminal speeds; in fact in produces a relative speed such that conventional SR kinematic doppler fully accounts for the red shift.

[EDIT: I should point out that the reason such an approach is not normally used is that it leads to a difficult distance scale. If you build a ladder of distances using reasonable measurements, extrapolate back to emission time, you find that relative speed by the parallel transport definition is inconsistent with such a distance scale; you would be required to use an adjusted distance scale.]

 

[Peter Watkins]

Thank you for your replies. What I should have said is that the recession rate between us and any observed galaxy cannot have been greater than the speed of light, at the time that the light departed, proven by the simple fact that their light has reached us. How then can a sub-light speed expansion rate stretch light? With regard to "expanding space" these pages have many times stated that it is this phenomenon that is the cause of universal expansion. "Scientific America" states:-"The velocity in Hubbles law is a recession velocity caused by the expansion of space, not a motion through space." It also states that:- "as space expands, light waves get stretched, if the universe doubles in size during the waves' journey, their wavelength doubles and their energy halves."
As an aside, by how much can visible light waves be stretched before they are no longer visible light? How can we be sure that infrared light from distant galaxies is not stretched visible light? Do ultra-violet rays and x-rays and gamma rays also get stretched? Or are they somehow immune?

 

[drollere]

i'm baffled by the confusion here.

the doppler shift is used to describe the expansion related red shift in light: the calculation used to determine the red shift z is simply the doppler formula, applied to measured galactic spectra with the standard scale of wavelength.

to say the red shift is possibly not a doppler shift is like saying a pound of feathers is possibly not really really a pound.

to say that the speed of an object can't be determined because there are many different answers is incorrect. what is your personal distance from the capitol of illinois? does that answer apply to everyone? does that mean the distance to chicago can't be determined?

special relativity theory merely requires the choice of inertial frames of reference. it doesn't say inertial frames become useless.

the oscillations of light are a behavior within an electromagnetic field, not a gravitational field. but i have not seen a clear answer as to how the phenomenon of cosmological red shift is produced.

it appears that when space is gravitationally dominated it keeps its dimensions -- the Earth and our galaxy are not measurably physically expanding, because they are gravitationally bound -- while space that is not gravitationally influenced appears to be expanding, as it were underneath the electromagnetic fields carrying light from distant objects.

i've never seen a source state the minimal value of gravitation needed to produce the transition to a measurable red shift, e.g., as the minimum distance between two solar masses.

 

[Chalnoth]

Thank you for your replies. What I should have said is that the recession rate between us and any observed galaxy cannot have been greater than the speed of light, at the time that the light departed, proven by the simple fact that their light has reached us.

Yes, it absolutely can, and has, as given by the usual definition of recession velocity.

As I mentioned earlier, however, this definition is arbitrary and you can define far-away velocities however you choose. But the recession velocity is sort of the "obvious" velocity that you would write down: it's simply the Hubble expansion rate times the instantaneous distance to the object (the instantaneous distance is the distance given by the time it would take light rays to traverse the distance if you could instantly freeze the expansion to let those light rays do the bouncing).

The way this works is the following: you have a far-away universe emitting light in our direction in the early universe. At the time the light was emitted, the recession velocity of this galaxy was greater than the speed of light, and so as the light moved in our direction, the expansion of our universe carried it away faster than it could approach us.

However, our universe has an expansion rate (hubble parameter) that is slowing down. So, after some amount of time, after the light ray had traversed some distance, eventually the expansion rate slowed enough that the light ray started to make headway against the expansion, finally reaching us billions of years later. But the galaxy that emitted that light was further away still: it wasn't traveling towards us at the speed of light, it was just sitting where it always was among its local galaxies. So even though the expansion rate slowed enough that the light ray could eventually get to us, it didn't need to slow enough for that galaxy to stop receding at faster than the speed of light.

Therefore, there are many galaxies that we can see which always have been and always will be receding at faster than the speed of light.

 

[marcus]

i'm baffled by the confusion here.

It is confusion on your part. Chalnoth's discussion is clear and correct.

the calculation used to determine the red shift z is simply the doppler formula, applied to measured galactic spectra with the standard scale of wavelength.

You sound confused. The redshift is not "determined by a calculation" using doppler formula, as you seem to believe. The redshift z is measured directly from the spectra. z+1 is the ratio by which all wavelengths are expanded. (Answer to Peter Watkins)

to say that the speed of an object can't be determined because there are many different answers is incorrect...

You are mistaken Drollere. There are several different measures of distance. Recession speed (better called recession rate) is the rate that distance to something is increasing. Before specifying a recession rate one should really say WHICH measure of distance one is using. As Chalnoth pointed out the natural measure when discussing Hubble Law expansion is the instantaneous distance (where you imagine freezing the expansion process at a particular moment so you can measure it, by bouncing a radar signal or however you like, and so measure the distance at that moment). The Hubble Law v = HD is stated in terms of that distance. For the law to apply, D is understood to be the distance "now" (at some moment) and v the current rate that distance is expanding.

Thank you for your replies. What I should have said is that the recession rate between us and any observed galaxy cannot have been greater than the speed of light, at the time that the light departed, proven by the simple fact that their light has reached us. How then can a sub-light speed expansion rate stretch light? With regard to "expanding space" these pages have many times stated that it is this phenomenon that is the cause of universal expansion. "Scientific America" states:-"The velocity in Hubbles law is a recession velocity caused by the expansion of space, not a motion through space." It also states that:- "as space expands, light waves get stretched, if the universe doubles in size during the waves' journey, their wavelength doubles and their energy halves."
As an aside, by how much can visible light waves be stretched before they are no longer visible light? How can we be sure that infrared light from distant galaxies is not stretched visible light? Do ultra-violet rays and x-rays and gamma rays also get stretched? Or are they somehow immune?

Chalnoth gave a very clear answer already. As he pointed out the natural definition of distance to use in discussing Hubble Law. It's sometimes called the "proper" distance. I will use that distance and just amplify by saying that:
1. most of the galaxies we observe have redshift z bigger than 1.7.
2. with any such galaxy, the distance to it was already growing > c when the light was emitted. And the distance to is growing faster than c today.

In answer to your question, all wavelengths are expanded by the same ratio z+1. the visible range is .4 to .7 and .5 is green. Suppose z = 4 so that the expansion ratio z+1 = 5 then some invisible UV with wavelength .1 gets stretched to .5, which is visible as green.
and green .5 would be stretched to 2.5 which is infrared.

And some infrared would be stretched out by a factor of 5 and become microwave etc.

should have said is that the recession rate between us and any observed galaxy cannot have been greater than the speed of light, at the time that the light departed,

Yes, it absolutely can, and has, as given by the usual definition of recession velocity.

As I mentioned earlier, however, this definition is arbitrary and you can define far-away velocities however you choose. But the recession velocity is sort of the "obvious" velocity that you would write down: it's simply the Hubble expansion rate times the instantaneous distance to the object (the instantaneous distance is the distance given by the time it would take light rays to traverse the distance if you could instantly freeze the expansion to let those light rays do the bouncing).

The way this works is the following: you have a far-away universe emitting light in our direction in the early universe. At the time the light was emitted, the recession velocity of this galaxy was greater than the speed of light, and so as the light moved in our direction, the expansion of our universe carried it away faster than it could approach us.

However, our universe has an expansion rate (hubble parameter) that is slowing down. So, after some amount of time, after the light ray had traversed some distance, eventually the expansion rate slowed enough that the light ray started to make headway against the expansion, finally reaching us billions of years later. But the galaxy that emitted that light was further away still: it wasn't traveling towards us at the speed of light, it was just sitting where it always was among its local galaxies. So even though the expansion rate slowed enough that the light ray could eventually get to us, it didn't need to slow enough for that galaxy to stop receding at faster than the speed of light.

Therefore, there are many galaxies that we can see which always have been and always will be receding at faster than the speed of light.

 

[drollere]

You sound confused. The redshift is not "determined by a calculation" using doppler formula, as you seem to believe. The redshift z is measured directly from the spectra. z+1 is the ratio by which all wavelengths are expanded.

nope. the wavelength of a specific absorption or emission line in the receding galaxy spectrum is measured. the wavelength of the matching absorption or emission line is measured in a terrestrial laboratory. the ratio is taken, using the 19th century doppler formula. this is called a calculation by most people. what do you call it?

You are mistaken Drollere. There are several different measures of distance.

and therefore? because there are many different measures of distance (z, light years, parsecs, etc.) from many different inertial frames does not mean that the distance "cannot be determined".

 

[PAllen]

nope. the wavelength of a specific absorption or emission line in the receding galaxy spectrum is measured. the wavelength of the matching absorption or emission line is measured in a terrestrial laboratory. the ratio is taken, using the 19th century doppler formula. this is called a calculation by most people. what do you call it?

This ratio is just redshift. The doppler formula is what relates this to velocity. There are two versions: pre-relativity and relativistic. They give different answers. Neither is normally used for cosmologic reshift because a given expanding universe model directly relates redshift to distance instead of velocity. The resulting recession rates are often superluminal.

As I mentioned in an earlier post, there are ways to use use the geometry of given universe model with parallel transport of 4-momentum along a null path that allow treating the recession non-superluminally, and consistent with relativistic doppler formula. This is a mathematically valid but non-standard approach.

and therefore? because there are many different measures of distance (z, light years, parsecs, etc.) from many different inertial frames does not mean that the distance "cannot be determined".

What people are telling you is not about units or frames, but that nobody has billion light year rulers. We measure things like liminosity, redshift, and there is no choice but derive distance in the context of (hopefully well motivated) model. You need a model of standard candles, and of dynamics of spacetime, or you can do nothing with the actually measured quantities. Different assumptions or definitions lead to different distances.

Within GR, even over modest distances, using a distance defined in terms of bouncing light (multiplying time by c) versus proper distance (metric computation) do not yield exactly the same result in the presence of curvature.

 

[marcus]

and therefore? because there are many different measures of distance (z, light years, parsecs, etc.) from many different inertial frames does not mean that the distance "cannot be determined".

I wasn't talking about distance UNITS. In a world with changing geometry, or spacetime curvature, there are different definitions corresponding to different ways of measuring. PAllen said it.

What people are telling you is not about units ... Different assumptions or definitions lead to different distances.

Within GR, even over modest distances, using a distance defined in terms of bouncing light (multiplying time by c) versus proper distance (metric computation) do not yield exactly the same result in the presence of curvature.

Thanks for taking this one up! Here is an example that might help in the case of someone else confused on this point, if they are willing to learn. Just ask them to google "cosmo calculator" and they will get:

http://www.astro.ucla.edu/~wright/CosmoCalc.html

They immediately see various versions of distance! This is for a galaxy with redshift z=3.
Measuring by light travel time the distance to the galaxy is 11.476 billion lightyears. Say 11.5 not to put too fine a point on it.

==quote==
...
The light travel time was 11.476 Gyr.
The comoving radial distance, which goes into Hubble's law, is 6460.6 Mpc or 21.072 Gly.
The angular size distance DA is 1615.1 Mpc or 5.2678 Gly.
...
==endquote==

The estimated proper distance, if you think of freezing the expansion process at this moment and measuring by radar or whatever usual means, is 21 billion lightyears.

And judging by the angle it makes in the sky, the distance is would be only a little bit over 5 billion lightyears. That is the proper distance the thing was when the light was emitted and started on its way to us. (If you had frozen expansion then, instead of now.)

It's obvious you know all this PAllen, I'm just thinking how we can explain things to a confused newcomer, like that there are different meanings of measure, not just different units. There is also that essay by the Princeton guy about the various different measures of distance used in cosmology. Ask them to google "Hogg distance" and they will get the essay titled "Distance Measures in Cosmology". It explains the different measures which the calculator illustrates like "travel time", proper, angular size etc. I suspect you are already familiar with that essay.

 

[Peter Watkins]

Thank you for your replies, baffling though some of them may be for a rapidly fossilising brain. The problem I have is understanding why light is stretched. All regularly observed galaxies will have a set of co-ordinates so that astronomers can find them. These co-ordinates do not show where the galaxy currently is but rather, where it was when the light that is now arriving here, left there. If we were to draw an instantaneous picture of the light path, we would see them reaching our galaxy, and behind the point marked by our co-ordinates would be a light path leading back to the galaxy of origin, however far away that may now be. My point is, what possible influence could that receding galaxy, and the others all around it, above, below, to the side, in front and so on, have upon the departed photons? Stretched by a high recession rate at the moment of leaving I can understand, but how the moving apart of galaxies elsewhere can cause light waves to stretch seems to make no sense.
Nice to see that Coldman now agrees with me that the expansion rate is slowing,(#14), something that I have been saying for 2-3 years now. Also that "expanding space" is simply the increase in distance between galaxies, rather than a mechanism that is causing the separation, (#2).

 

[Chalnoth]

Thank you for your replies, baffling though some of them may be for a rapidly fossilising brain. The problem I have is understanding why light is stretched. All regularly observed galaxies will have a set of co-ordinates so that astronomers can find them. These co-ordinates do not show where the galaxy currently is but rather, where it was when the light that is now arriving here, left there. If we were to draw an instantaneous picture of the light path, we would see them reaching our galaxy, and behind the point marked by our co-ordinates would be a light path leading back to the galaxy of origin, however far away that may now be. My point is, what possible influence could that receding galaxy, and the others all around it, above, below, to the side, in front and so on, have upon the departed photons? Stretched by a high recession rate at the moment of leaving I can understand, but how the moving apart of galaxies elsewhere can cause light waves to stretch seems to make no sense.
Nice to see that Coldman now agrees with me that the expansion rate is slowing,(#14), something that I have been saying for 2-3 years now. Also that "expanding space" is simply the increase in distance between galaxies, rather than a mechanism that is causing the separation, (#2).

Well, yeah, the far away galaxy has no influence whatsoever upon the light once that light is emitted. That's sort of the point. This is what allows us to see some galaxies that are now and always have been receding at faster than light (according to the usual definition of recession velocity).

Anyway, the simplest possible way to look at this is to define your coordinates in such a way that each galaxy is nearly stationary but with the distances between galaxies increase with time. In this view, when the light traverses the universe, it gets stretched right along with the expansion, and that accounts for the redshift. There may be some redshift due to the local motion of the galaxy, but most of it comes from the stretching of space that also stretches out the wavelength of the light as it travels.

 

[marcus]

The problem I have is understanding why light is stretched. the separation...

Good question. I want to reinforce what Chalnoth just said, with some more mental imagery.
You could picture it as a chain of "infinitesimal" doppler shifts all along the way. Chalnoth's picture of a long chain of stepping stones, each one stationary relative to background, which are drifting apart.

At each stepping stone galaxy, an observer can approximate the local situation by a standard "flat" framework that includes the next galaxy and in which the next galaxy actually appears to be moving away, and there is a standard doppler effect.

So the overall redshift is seen as the cumulative effect of many many many standard doppler shifts each calculated in an approximately flat locale.

If this doesn't help, please ignore it

 

[Peter Watkins]

Thank you again. One last problem; looking at the full e/m spectrum it would seem that gamma rays are something like a million time shorter than visible light rays. If a galaxy is far enough away to have it's visible light rays stretched by 25% by the time they arrive here, wouldn't this stretch gamma rays from the same source by a factor of 250,000, meaning that they are no longer gamma rays. And yet they can be seen coming from distances approaching 13 billion light years.

 

[PAllen]

Thank you again. One last problem; looking at the full e/m spectrum it would seem that gamma rays are something like a million time shorter than visible light rays. If a galaxy is far enough away to have it's visible light rays stretched by 25% by the time they arrive here, wouldn't this stretch gamma rays from the same source by a factor of 250,000, meaning that they are no longer gamma rays. And yet they can be seen coming from distances approaching 13 billion light years.

Any wavelength is shifted (expanded) by the same factor. A z factor of 1 converts blue light to red light and a gamma ray to a somewhat less energetic gamma ray. A z factor of 9 (around the biggest observed) still only converts most gamma rays to less energetic gamma rays or x-rays (a 10 mev gamma becomes a 1 mev gamma).

 

[Galron]

The simplest way I can think to say it is that you get some very specific total redshift for faraway objects due to the expansion. How much of that redshift is due to the doppler shift and how much is due to the expansion between us and the far away object is completely arbitrary.

Quite.

 

[Galron]

i'm baffled by the confusion here.

the doppler shift is used to describe the expansion related red shift in light: the calculation used to determine the red shift z is simply the doppler formula, applied to measured galactic spectra with the standard scale of wavelength.

to say the red shift is possibly not a doppler shift is like saying a pound of feathers is possibly not really really a pound.

to say that the speed of an object can't be determined because there are many different answers is incorrect. what is your personal distance from the capitol of illinois? does that answer apply to everyone? does that mean the distance to chicago can't be determined?

special relativity theory merely requires the choice of inertial frames of reference. it doesn't say inertial frames become useless.

the oscillations of light are a behavior within an electromagnetic field, not a gravitational field. but i have not seen a clear answer as to how the phenomenon of cosmological red shift is produced.

it appears that when space is gravitationally dominated it keeps its dimensions -- the Earth and our galaxy are not measurably physically expanding, because they are gravitationally bound -- while space that is not gravitationally influenced appears to be expanding, as it were underneath the electromagnetic fields carrying light from distant objects.

i've never seen a source state the minimal value of gravitation needed to produce the transition to a measurable red shift, e.g., as the minimum distance between two solar masses.

Yeah but the point is you can understand the question even though you might not understand why it was asked: perspective wise.

No doubt when you were learning this these questions ran through your mind, unless you were a genius, is what I mean.

They might of done genius or not. But meh you get my point.

 

[Jocko Homo]

My point is, what possible influence could that receding galaxy, and the others all around it, above, below, to the side, in front and so on, have upon the departed photons? Stretched by a high recession rate at the moment of leaving I can understand, but how the moving apart of galaxies elsewhere can cause light waves to stretch seems to make no sense.

Wait a second... You do understand that galaxies are receding from us because space itself, between it and us, is stretching, right? You don't understand how a light wave in this region of stretching space will have it's wavelength stretched longer?

 

[Peter Watkins]

So what is the mechanism that is stretching space, and how can it be that a galaxy of sufficient distance to have it's light stretched by 10% would see a 200 metre radio wave extended by 20 metres, whilst gamma rays are stretched, (by the same space), by only one tenth of the width of an atom? As John Brummer once wrote, "What mad universe is this?

 

[Chalnoth]

So what is the mechanism that is stretching space, and how can it be that a galaxy of sufficient distance to have it's light stretched by 10% would see a 200 metre radio wave extended by 20 metres, whilst gamma rays are stretched, (by the same space), by only one tenth of the width of an atom? As John Brummer once wrote, "What mad universe is this?

1. The expansion is multiplicative. That is, every photon, of whatever wavelength, has its wavelength multiplied by the same number. The same is true of the distances between galaxies: the distance between galaxies is multiplied by the same number, so if two galaxies a billion light years away from one another move 10% further apart, or 100 million light years further, then two galaxies two billion light years apart will separate by 200 million light years.

2. The mechanism is gravity. When you have a bunch of matter flying apart, as the matter in our universe is, General Relativity, our current theory of gravity, says that photons will be stretched right along with the matter that is moving apart.

 

[TrickyDicky]

Maybe what confuses the matter is that while it is argued that atoms and their particles are not stretched by expansion because expansion doesn't act at those scales (or at much greater scales) or because EM forces counteract it, or because of gravity binding, apparently photons don't have any of these problems to be stretched by expansion.
It would seem the "multiplicative" effect of expansion were affecting only photons but not fermionic matter.

This is not the case, however. Because there are many orders of magnitude of difference between the number of fermions in a given long enough path and the stream of photons coming from a source at that given distance.

 

[Chalnoth]

Maybe what confuses the matter is that while it is argued that atoms and their particles are not stretched by expansion because expansion doesn't act at those scales (or at much greater scales) or because EM forces counteract it, or because of gravity binding, apparently photons don't have any of these problems to be stretched by expansion.
It would seem the "multiplicative" effect of expansion were affecting only photons but not fermionic matter.

This is not the case, however. Because there are many orders of magnitude of difference between the number of fermions in a given long enough path and the stream of photons coming from a source at that given distance.

I wouldn't say that.

The issue here is that the description of the expansion itself stems from a universe which is the same everywhere (homogeneous) and in every direction (isotropic). This is true of our universe on large scales, so we can talk about the expansion at large scales. But it is absolutely not true on smaller scales, such as within a galaxy cluster or within a galaxy. When you examine the way gravity interacts in a universe that isn't actually smooth, you not only find that there is expansion on average at large scales, but you also find that there isn't expansion on small scales.

So the answer to the fact that, say, the Earth doesn't experience expansion has nothing (directly) to do with the fact that the Earth is made of normal matter: it has to do with the fact that the Earth is a massive overdensity which, according to General Relativity, will not expand even in an expanding universe.

 

[TrickyDicky]

So the answer to the fact that, say, the Earth doesn't experience expansion has nothing (directly) to do with the fact that the Earth is made of normal matter: it has to do with the fact that the Earth is a massive overdensity which, according to General Relativity, will not expand even in an expanding universe.

First of all I was not answering why the Earth doesn't experience expansion but the related question why it is confusing for some to hear that photons stretch while atoms do not.
So please do not conflate things at your convenience. If you don't understand something you read just ask.
Second , in fact the clarification I offered is in consonance with what you just wrote about homogeneity and fermionic overdensities, so you clearly didn't follow or didn't bother to think about it or just like to disagree for the sake of it.

 

[Jocko Homo]

So what is the mechanism that is stretching space, and how can it be that a galaxy of sufficient distance to have it's light stretched by 10% would see a 200 metre radio wave extended by 20 metres, whilst gamma rays are stretched, (by the same space), by only one tenth of the width of an atom? As John Brummer once wrote, "What mad universe is this?

First of all, since this appears to be a response to this question here:

Wait a second... You do understand that galaxies are receding from us because space itself, between it and us, is stretching, right? You don't understand how a light wave in this region of stretching space will have it's wavelength stretched longer?

...it would be nice if you actually answered the questions posed. However, I will interpret your post as a tacit admission that you do now understand how universal expansion will stretch light waves and try my best to answer your questions...

While I am told that the mechanism of this expansion is a consquence of General Relativity, I lack the specific expertise to confirm this for myself...

In response to your second question, let me draw a primitive diagram of a region of space with two light waves in it:

|----------|    space that's 10 units long
|-----|         light wave whose wavelength is 5 units long
|-|             light wave whose wavelength is 1 unit long

I've drawn this diagram within code quotes merely to use the fixed-width font.

How do you suppose that the length of the light waves change if I were to stretch the space they were in? If I stretched the space to say twice its original length, do you suppose it's reasonable to say that the first light wave will still be half the length of the space? ...and the second light wave still one tenth the length? Let's draw this:

|--------------------|    now 20 units long
|----------|              now 10 units long
|--|                      now 2 unit long

As you can see, the first light wave has been stretched by 5 units. Judging by your second question, it sound like you expected the second (shorter) wave of light to also stretch by 5 units. Do you still believe that this should be so?

 

[Peter Watkins]

Light beams are made up of photons. It is these photons that describe the wave length. It is these photons that are moved apart by the supposed expansion of space. If we have two photons, side by side, one from visible light and one from gamma rays, why would one be pulled along at a faster rate than the other? Also, if the light rays really are stretched, by say, 30%, wouldn't this mean that the light is traveling at faster than light speed? Additionally, if the light-waves are stretched, doesn't it make more sense to ascribe this to our rate of recession? Our high rate of separation from the most distant galaxies would produce the effect of stretched light.

 

[Chalnoth]

Light beams are made up of photons. It is these photons that describe the wave length. It is these photons that are moved apart by the supposed expansion of space. If we have two photons, side by side, one from visible light and one from gamma rays, why would one be pulled along at a faster rate than the other? Also, if the light rays really are stretched, by say, 30%, wouldn't this mean that the light is traveling at faster than light speed? Additionally, if the light-waves are stretched, doesn't it make more sense to ascribe this to our rate of recession? Our high rate of separation from the most distant galaxies would produce the effect of stretched light.

No, it wouldn't mean they're traveling faster than the speed of light. Why would you think that?

 

[cephron]

Disclaimer: I am no expert on this matter; I'm here to assist with metaphors and intuitive ways of thinking about stuff. Dear PF guys who know way more than me, please correct me on any important matters on which I misstep.

Light beams are made up of photons. It is these photons that describe the wave length. It is these photons that are moved apart by the supposed expansion of space. If we have two photons, side by side, one from visible light and one from gamma rays, why would one be pulled along at a faster rate than the other?

I think "pulled along" is not a good way to think about it - I would think instead of the photons as being inflated, like a long balloon.

Now, photons don't have a proper rest frame for various reasons. And I don't know if one would talk about photons actually having length. But for illustration, let's pretend we're somehow looking at the rest frame of your side-by-side gamma ray and visible light photons, and we'll do a new version of Jocko Homo's illustration.

|--------------------|    space that's 20 units long
       |------|           photon of visible light
         |--|             photon of gamma ray

Later, after some time has passed and space has expanded...

|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|    space that's 40 units long
              |-+-+-+-+-+-+|                  photon of visible light, redshifted
                  |-+-+|                      photon of gamma ray, redshifted

Here, the plus signs show the "new space" that has "spawned" out of nowhere.
(Of course, empty space doesn't spawn in units like this,
but again, for illustration...)

So, as space expands, distances grow proportionally. The visible light isn't pulled along any faster than the gamma ray, it's just inflated more because it was bigger to start with. And if you're concerned that it's front has gotten further ahead than that of the gamma ray, notice that the back has gotten equally further behind the back of the gammy ray. They're still side by side, one will never be "pulled ahead" of the other.

 

[Peter Watkins]

Thank you for your patience gentlemen, but consider Jock Homo's example, if the photons left their source at the same time then they are all together. Put these on the rubber sheet, stretch it, and they will remain together but their wave forms will be stretched, by factors according to their original length. The unequal starting points produce an incorrect picture.

 

[Chalnoth]

Thank you for your patience gentlemen, but consider Jock Homo's example, if the photons left their source at the same time then they are all together. Put these on the rubber sheet, stretch it, and they will remain together but their wave forms will be stretched, by factors according to their original length. The unequal starting points produce an incorrect picture.

What do you mean, an incorrect picture?

 

[Jocko Homo]

Light beams are made up of photons. It is these photons that describe the wave length.

What? How so?

In the future, instead of just saying things hoping that we'll just agree with you, you should try to explain your conclusions (like how is it that photons describe wavelength?). Doing so will allow you to either see for yourself why your arguments don't make sense and avoid posting them in the first place or allow us to follow your correct chain of thought. This greatly helps communication...

It is these photons that are moved apart by the supposed expansion of space. If we have two photons, side by side, one from visible light and one from gamma rays, why would one be pulled along at a faster rate than the other? Also, if the light rays really are stretched, by say, 30%, wouldn't this mean that the light is traveling at faster than light speed? Additionally, if the light-waves are stretched, doesn't it make more sense to ascribe this to our rate of recession? Our high rate of separation from the most distant galaxies would produce the effect of stretched light.

Light exhibits wave-particle duality. This means that, in come contexts, it acts like a stream of particles and, in other contexts, it acts like a wave. When you're talking about light red-shifting, you're treating it like a wave so that's how you should probably be visualizing it...

So, if you go back to my ASCII diagram:

|----------|    space that's 10 units long
|-----|         light wave whose wavelength is 5 units long
|-|             light wave whose wavelength is 1 unit long

You'll see that, not surprisingly, the light waves exist in space. Now let's stretch this region of space to twice the original length and look at that diagram again:

|--------------------|    now 20 units long
|----------|              now 10 units long
|--|                      now 2 unit long

You can imagine that each point of the light wave is stuck to each point of space that the light wave exists in. Because we've stretched space, the light wave also stretches. Because space itself is morphing, the very notion of "speed" will be a little funny. However, note how all proportions are the same. The two light waves still coincide with each other with exactly the same proportions so, in this sense, they are still traveling at the same speed...

There is a red (and blue) shift due strictly to movement through space, called the Doppler effect. However, there's also a strictly red shifting on everything we look at and this is due to universal expansion...

 

[Peter Watkins]

I don't want to run the risk of another infraction, but interpretation is in the mind of the reader. The example you show above sees the leading edge of the upper light unit traveling at 5 times the rate of the lower unit. Carried to it's logical conclusion, if the longer unit took 200 years to arrive here, then the shorter would take a thousand years!
You say that the red and blue shift are due strictly to movement through space. Haven't we been told, endlessly, that we are not moving through space, but rather, that expanding space is carrying the galaxies apart, and due to this they are exempt from the light speed limit. If this is not the case then there will have to be some serious recalculation of various recession rate tables.
Some while back I posed a very specific question which was; "is the red shift referred to by cosmologist the shift of the absorbtion lines, seen on the spectra of stars and galaxies, toward the red end of the spectrum?" The reply by Marcus was an unequivocal and unqualified "Yes". It is obviously convenient to refer to this as the Doppler effect as it saves explanation. For much of the time that the universe has been viewed through telescopes there has been no colour change and this shift was not a Doppler effect. I realize that the advent of ever better telescopes has probably now seen the true Doppler effect. At what recession rate can this first be seen?
Chalnoth, When a full spectrograph of a distant galaxy is taken, did all the light rays that produced this spectrum leave their source at the same time?

 

[Chalnoth]

I don't want to run the risk of another infraction, but interpretation is in the mind of the reader. The example you show above sees the leading edge of the upper light unit traveling at 5 times the rate of the lower unit.

Ahhh, perhaps that's where your confusion lies.

This point of view that you are espousing just isn't accurate. Every piece of every photon is traveling at the speed of light. It's just that the space is expanding, so that even traveling at the same speed, they end up further apart in the end.

One way to sort of see this is to imagine an expanding universe where we're just looking at two little particles of matter that are separated by some distance, and which aren't moving with respect to the expansion and have no significant gravitational attraction between them (either their masses are too low or they are too far from one another, take your pick).

I hope you can see that since they are stationary with respect to the expansion, they will simply grow further apart as the universe expands. If the universe expands by a factor of two, they will be twice as far apart.

Now, what happens if we take the same situation, but give each particle the same additional velocity in the same direction? Well, as it turns out, none of the logic changes: after the universe has expanded by a factor of two, they will be precisely a factor of two further apart from one another. They will have moved with respect to the expansion, but compared to each other they'll still be further apart.

All you have to do now is imagine these "particles" as moving at the speed of light and the initial separation being the wavelength, and you've described how a photon expands.

 

[Jocko Homo]

I don't want to run the risk of another infraction, but interpretation is in the mind of the reader. The example you show above sees the leading edge of the upper light unit traveling at 5 times the rate of the lower unit. Carried to it's logical conclusion, if the longer unit took 200 years to arrive here, then the shorter would take a thousand years!

What? How so? Please let me quote myself:

In the future, instead of just saying things hoping that we'll just agree with you, you should try to explain your conclusions (like how is it that photons describe wavelength?). Doing so will allow you to either see for yourself why your arguments don't make sense and avoid posting them in the first place or allow us to follow your correct chain of thought. This greatly helps communication...

It's good advice. Please heed it...

One light wave is five times as long as the other. How does that mean that it's traveling five times as fast? Indeed, I didn't mean to imply that they were traveling at all since that's irrelevant. They'd behave the same way whether they were moving or standing still, which makes sense because how could you tell the difference? Standing still is moving...

You say that the red and blue shift are due strictly to movement through space. Haven't we been told, endlessly, that we are not moving through space, but rather, that expanding space is carrying the galaxies apart, and due to this they are exempt from the light speed limit. If this is not the case then there will have to be some serious recalculation of various recession rate tables.

Is English your first language? If it isn't, I will make my posts more redundant in the future. Either way, you should make an effort to read my posts (indeed, all posts) more carefully. I'll quote myself again:

There is a red (and blue) shift due strictly to movement through space, called the Doppler effect. However, there's also a strictly red shifting on everything we look at and this is due to universal expansion...

The bold word was previously emphasized using italics. However, because all quoted text is italicized, I've chosen to bold it in the quote...

I didn't say "the red shift" was strictly due to movement through space. I said there is a red shift that is strictly due to movement. Everything we may look at will have a red or blue shift, caused by its motion through space (the aforementioned Doppler effect). There will also be a strict red shift on everything caused by the expansion of space itself. These effects are cumulative. For those objects that are moving away from us, they will look even redder due to expansion. For those objects that are moving towards us, if they are moving fast enough relative to their distance from us, their Doppler shift may be blue enough to overcome the red shift of expansion. If they're not fast enough or are too far away, they will still look red due to expansion but they won't look quite as red due to their blue shift. Get it?

Some while back I posed a very specific question which was; "is the red shift referred to by cosmologist the shift of the absorbtion lines, seen on the spectra of stars and galaxies, toward the red end of the spectrum?" The reply by Marcus was an unequivocal and unqualified "Yes". It is obviously convenient to refer to this as the Doppler effect as it saves explanation. For much of the time that the universe has been viewed through telescopes there has been no colour change and this shift was not a Doppler effect. I realize that the advent of ever better telescopes has probably now seen the true Doppler effect. At what recession rate can this first be seen?

By your description, it doesn't sound like Marcus disagrees with me...

Doppler shift cannot explain the proportionality of the red shift with distance...

 

[PAllen]

Some while back I posed a very specific question which was; "is the red shift referred to by cosmologist the shift of the absorbtion lines, seen on the spectra of stars and galaxies, toward the red end of the spectrum?" The reply by Marcus was an unequivocal and unqualified "Yes". It is obviously convenient to refer to this as the Doppler effect as it saves explanation. For much of the time that the universe has been viewed through telescopes there has been no colour change and this shift was not a Doppler effect. I realize that the advent of ever better telescopes has probably now seen the true Doppler effect. At what recession rate can this first be seen?
Chalnoth, When a full spectrograph of a distant galaxy is taken, did all the light rays that produced this spectrum leave their source at the same time?

Automatically calling redshift doppler is not convenient, it is wrong. Redshift is a measured shift in received frequency versus emitted frequency. Doppler refers to one of two formulas (pre-relativistic; relativistic) for relating redshift to velocity. It is a particular explanation of redshift, with a particular formula. It is not a measure of redshift. Cosmological redshift is typically considered distinct from Doppler redshift because it is a relation between distance and redshift rather than speed and redshift, under the assumption that both source and target are motionless relative to center of mass of the local matter (here, local is quite large - galaxy or galaxy cluster).

 

[TrickyDicky]

It's just that the space is expanding, so that even traveling at the same speed, they end up further apart in the end.

This assertion is wrong according to these peer-reviewed references and authors (among many others):
Davis, Lineweaver and Webb (2003)
Whiting (2004)
Barnes et al. (2006)
Peacock (2007)
Chodorowski (2007)
Hogg and Bunn (2009)
See for instance:
http://arxiv.org/abs/0808.1081

Personally I'm actually agnostic wrt this concept of "expansion of space". I'll just mention that in order to contend it one logically must consider "space" as something material, an expandible object, a knd of medium or substance, and this is rejected by mainstream science along with all ether theories.

 

[ptalar]

I am but a simple minded engineer who enjoys cosmology as good mind stimulation. But let me see if I can describe Mr. Watkins initial concern and frame it in a different light, no pun intended. And offer up some logical approach to answering the question.

Basic Problem Statement: If the Universe is expanding at roughly 3 times the speed of light, and the visible Universe is approximately 46 billion light years in any direction. Why can we see 46 billion light years away? Remember the actual Universe is about 14 billion years old in absolute time. So in theory we should only be able see 14 billion light years in any direction, not 46 billion light years.

Observations in prep for a plausible explanation:

1) Dark energy or vacuum energy is expanding space at 3 times the speed of light because space has no mass. Space is nothing. Space does not have to follow the laws of physics if it is a massless nothing. Most of the limitations of physics are due to particles having mass.

2) Photons have a very small mass (<1×10−18 eV/c²) so they are subject to the laws of physics. Theoretically, they travel at the speed of light since they are considered essentially massless. But they do not travel faster than the speed of light.

3) There is essentially no gravity in deep space unless you are near a planet, or galaxy or star.

Explanation: I have no explanation because it would all be empirical and would be considered philosophy. Maybe somebody else has a better explanation.

 

[Chalnoth]

This assertion is wrong according to these peer-reviewed references and authors (among many others):
Davis, Lineweaver and Webb (2003)
Whiting (2004)
Barnes et al. (2006)
Peacock (2007)
Chodorowski (2007)
Hogg and Bunn (2009)
See for instance:
http://arxiv.org/abs/0808.1081

Personally I'm actually agnostic wrt this concept of "expansion of space". I'll just mention that in order to contend it one logically must consider "space" as something material, an expandible object, a knd of medium or substance, and this is rejected by mainstream science along with all ether theories.

Uh, what? Why do you think any of these articles contradict what I wrote?

 

[Chalnoth]

Basic Problem Statement: If the Universe is expanding at roughly 3 times the speed of light,

Let me just stop you right there. Expansion isn't a speed. It's a rate. The units are different. Saying that the universe is expanding at faster than the speed of light is like saying that your car's engine revolving at 3000rpm's is faster than 60mph. The statement doesn't even make sense.

Because expansion is a rate, there is no sense in which it is either faster or slower than the speed of light. And at any non-zero rate of expansion, there will be objects some distance away that have recession velocities faster than the speed of light. At faster rates of expansion, those objects will be closer.

Why can we see 46 billion light years away? Remember the actual Universe is about 14 billion years old in absolute time. So in theory we should only be able see 14 billion light years in any direction, not 46 billion light years.

Because the rate of expansion is not uniform but has changed over time. Back when the photons we see today were first emitted, our universe was expanding at a much, much higher rate, carrying away the photons moving in our direction at a high speed. Since then, the expansion has slowed, such that eventually those photons started being able to make headway against the expansion. It shouldn't be much of a surprise that the amount of time it took those photons to reach us (13.7 billion years) is between the distance from which those photons were originally emitted (42 million light years) and the distance that matter is away from us today (46 billion light years).

 

[ptalar]

Let me just stop you right there. Expansion isn't a speed. It's a rate. The units are different. Saying that the universe is expanding at faster than the speed of light is like saying that your car's engine revolving at 3000rpm's is faster than 60mph. The statement doesn't even make sense.

Because expansion is a rate, there is no sense in which it is either faster or slower than the speed of light. And at any non-zero rate of expansion, there will be objects some distance away that have recession velocities faster than the speed of light. At faster rates of expansion, those objects will be closer.


Because the rate of expansion is not uniform but has changed over time. Back when the photons we see today were first emitted, our universe was expanding at a much, much higher rate, carrying away the photons moving in our direction at a high speed. Since then, the expansion has slowed, such that eventually those photons started being able to make headway against the expansion. It shouldn't be much of a surprise that the amount of time it took those photons to reach us (13.7 billion years) is between the distance from which those photons were originally emitted (42 million light years) and the distance that matter is away from us today (46 billion light years).

In regard to your first assertion I just made a rough engineering approximation to get a physical feel of what was going on. If the size of the visible Universe is 46 billion light years in all directions and the absolute age of the Universe is 14 billion years then I assumed that the rate of expansion at the leading edge of the Universe(if there is one), as it expands into the bulk, is roughly 3 times the speed of light: 46/14 which is approximately 3. What I think you are really saying is that the expansion of the Universe and the speed of light are not mathematically related. The expansion of the Universe is its own unique algorithim based on dark energy or vacuum energy and the fact that nothing need not conform to any law of physics. I agree. I was just trying to find a physical rationale for the rate of expansion and get a determination of how fast it was expanding in engineering terms.

As far as your second assertion I think I see where you are going. As the Universe expands there is still a trail of light at any given point in the visible Universe. That light is not being stretched. Rather it is just the light at any given distance during the expansion time period of 13.7 billion years. The expansion rate and the speed of light are not related except the distance of a luminous object can be determined by its red shift. The red shifts of the most distant visible objects indicate a 46 billion light year distance in any direction. Am I on the correct thought trail?

 

[Chalnoth]

In regard to your first assertion I just made a rough engineering approximation to get a physical feel of what was going on. If the size of the visible Universe is 46 billion light years in all directions and the absolute age of the Universe is 14 billion years then I assumed that the rate of expansion at the leading edge of the Universe(if there is one), as it expands into the bulk, is roughly 3 times the speed of light: 46/14 which is approximately 3. What I think you are really saying is that the expansion of the Universe and the speed of light are not mathematically related. The expansion of the Universe is its own unique algorithim based on dark energy or vacuum energy and the fact that nothing need not conform to any law of physics. I agree. I was just trying to find a physical rationale for the rate of expansion and get a determination of how fast it was expanding in engineering terms.

Yes, I noticed that. I was trying to say that that reasoning doesn't work. Hopefully the car analogy I used was a sufficiently good engineering example for you to see it. If not, here's another, different analogy that is no less relevant: saying the expansion is faster than the speed of light is like saying that the height of a building is faster than 30m/s. The statement doesn't even make sense.

As far as your second assertion I think I see where you are going. As the Universe expands there is still a trail of light at any given point in the visible Universe. That light is not being stretched. Rather it is just the light at any given distance during the expansion time period of 13.7 billion years. The expansion rate and the speed of light are not related except the distance of a luminous object can be determined by its red shift. The red shifts of the most distant visible objects indicate a 46 billion light year distance in any direction. Am I on the correct thought trail?

Sort of. The issue here is that it is not only the redshift that matters, but also how our universe has expanded over time. The redshift only tells us the total amount of expansion since the light was emitted. How fast that expansion occurred over time determines both how far away the object was and how long ago it emitted that light.

One rough analogy I might use is that of a person driving between two locations. Imagine, if you will, that knowing the redshift is sort of kinda like knowing the origin and destination, e.g. knowing that the person drove from Denver to Chicago. But knowing the origin and destination is not sufficient to know how far the driver went or how long it took to do it: we also need to know how fast the driver was moving, and what specific route the driver took. With a light ray, the speed is, of course, given by the speed of light. But the path the light ray takes is given by how space expands in the intervening time.

To go back to the example of the furthest light we see, that from the Cosmic Microwave Background, that light was emitted a mere 42 million light years away, but because of the expansion of the universe it was forced to take a path towards us that made it take 13.7 billion years. That that stuff that originally emitted that light is now some 46 billion light years away is interesting but incidental.

 

[TrickyDicky]

Uh, what? Why do you think any of these articles contradict what I wrote?

Sorry, I had a copy-paste error with the references, actually I'm basically referring to the Chodorowski and Hogg and Bunn ones.

 

[Chalnoth]

Sorry, I had a copy-paste error with the references, actually I'm basically referring to the Chodorowski and Hogg and Bunn ones.

You didn't answer my question.