How does the expansion of the universe stretch light?

In summary, the conversation discusses the relationship between the expansion of space and the movement of galaxies, particularly in regards to the effect on light. It is noted that the expansion of space does not put pressure on objects, but may stretch the wavelength of light. Doppler shift is mentioned as a potential factor, but it is acknowledged that there is no unique way to define the velocity of a far-away object. Therefore, the exact cause of redshift in faraway objects due to the expansion of space remains unclear.
  • #1
Peter Watkins
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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?
 
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  • #2
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.
 
  • #3
Peter Watkins said:
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.
 
  • #4
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.
 
  • #5
Drakkith said:
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.

Doppler_hattyu.jpg
 
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  • #6
Galron said:
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.
 
  • #7
Drakkith said:
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. :tongue2:
 
  • #8
Galron said:
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. :tongue2:

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!
 
  • #9
Drakkith said:
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!

:wink:
 
  • #10
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.
 
  • #11
Chalnoth said:
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.]
 
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  • #12
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?
 
  • #13
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.
 
  • #14
Peter Watkins said:
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.
 
  • #15
drollere said:
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.

Peter Watkins said:
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.

Chalnoth said:
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.
 
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  • #16
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".
 
  • #17
drollere said:
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.

drollere said:
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.
 
  • #18
drollere said:
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.

PAllen said:
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.
 
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  • #19
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).
 
  • #20
Peter Watkins said:
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.
 
  • #21
Peter Watkins said:
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 :biggrin:
 
  • #22
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.
 
  • #23
Peter Watkins said:
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).
 
  • #24
Chalnoth said:
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.
 
  • #25
drollere said:
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.
 
  • #26
Peter Watkins said:
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?
 
  • #27
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?
 
  • #28
Peter Watkins said:
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.
 
  • #29
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.
 
  • #30
TrickyDicky said:
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.
 
  • #31
Chalnoth said:
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.
 
  • #32
Peter Watkins said:
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:
Jocko Homo said:
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:
Code:
|----------|    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:

Code:
|--------------------|    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?
 
  • #33
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.
 
  • #34
Peter Watkins said:
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?
 
  • #35
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.
Peter Watkins said:
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.

Code:
|--------------------|    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.
 
<h2>1. How does the expansion of the universe stretch light?</h2><p>The expansion of the universe affects the wavelength of light as it travels through space. As the universe expands, the space between objects also expands, causing the wavelength of light to stretch. This phenomenon is known as cosmological redshift.</p><h2>2. Does the expansion of the universe affect all types of light?</h2><p>Yes, the expansion of the universe affects all types of light, including visible light, radio waves, and X-rays. However, the amount of stretching depends on the initial wavelength of the light. Longer wavelengths, such as radio waves, are stretched more than shorter wavelengths, like visible light.</p><h2>3. How does the stretching of light impact our observations of distant objects?</h2><p>The stretching of light can impact our observations of distant objects by making them appear redder than they actually are. This is because the wavelength of light is stretched towards the red end of the spectrum. This effect is taken into account by astronomers when studying distant objects.</p><h2>4. Can the expansion of the universe cause light to disappear?</h2><p>No, the expansion of the universe does not cause light to disappear. Instead, it changes the wavelength of the light. As the universe continues to expand, the wavelength of light will continue to stretch, but the light itself will not disappear.</p><h2>5. How does the expansion of the universe affect the speed of light?</h2><p>The expansion of the universe does not affect the speed of light. The speed of light is a constant, and it travels at the same speed regardless of the expansion of the universe. However, the wavelength of light may change, which can impact how we perceive the speed of light.</p>

1. How does the expansion of the universe stretch light?

The expansion of the universe affects the wavelength of light as it travels through space. As the universe expands, the space between objects also expands, causing the wavelength of light to stretch. This phenomenon is known as cosmological redshift.

2. Does the expansion of the universe affect all types of light?

Yes, the expansion of the universe affects all types of light, including visible light, radio waves, and X-rays. However, the amount of stretching depends on the initial wavelength of the light. Longer wavelengths, such as radio waves, are stretched more than shorter wavelengths, like visible light.

3. How does the stretching of light impact our observations of distant objects?

The stretching of light can impact our observations of distant objects by making them appear redder than they actually are. This is because the wavelength of light is stretched towards the red end of the spectrum. This effect is taken into account by astronomers when studying distant objects.

4. Can the expansion of the universe cause light to disappear?

No, the expansion of the universe does not cause light to disappear. Instead, it changes the wavelength of the light. As the universe continues to expand, the wavelength of light will continue to stretch, but the light itself will not disappear.

5. How does the expansion of the universe affect the speed of light?

The expansion of the universe does not affect the speed of light. The speed of light is a constant, and it travels at the same speed regardless of the expansion of the universe. However, the wavelength of light may change, which can impact how we perceive the speed of light.

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