Expanding space and the wavelength of light

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Expanding space and the wavelength of light

#1  Postby jamest » Mar 24, 2017 3:53 pm

I watched a BBC documentary this week about the expansion of the universe. It stated (I think) that the expansion of spacetime accounts for the expansion of the universe between galaxies and that light which has travelled immense distances has had its wavelength 'stretched' [by the expanding spacetime] so that old light is red-shifted, enabling us to date distant galaxies.

... So basically, I'm puzzled by why/how the wavelength of the light is red-shifted. Can somebody explain that to me because at the moment it reads [to me] as though light and spacetime are essentially the same thing? :scratch:

... Also, given that the universe hasn't expanded at a constant rate and given (I assume?) that we don't even know the rate at which the universe is expanding right now, how is it possible to accurately measure the age of distant galaxies? I may have asked this particular question before, I can't remember. :think:

Thanks in advance for enlightening me.
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Re: Expanding space and the wavelength of light

#2  Postby newolder » Mar 24, 2017 4:06 pm

This APOD gives various measures that are pertinent to your query.

The associated paper that defines the terms is in the arxiv here.
The paper-and-pencil calculator is a cosmological nomogram which allows to find relations between redshift, distance, age of the Universe, physical and angular sizes, luminosity and apparent magnitude for the standard cosmological model with parameters from the Planck mission.
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Re: Expanding space and the wavelength of light

#3  Postby jamest » Mar 24, 2017 4:22 pm

newolder wrote:This APOD gives various measures that are pertinent to your query.

The associated paper that defines the terms is in the arxiv here.
The paper-and-pencil calculator is a cosmological nomogram which allows to find relations between redshift, distance, age of the Universe, physical and angular sizes, luminosity and apparent magnitude for the standard cosmological model with parameters from the Planck mission.

Thanks, though that only relates to my second question and I can't see how it answers that.
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Re: Expanding space and the wavelength of light

#4  Postby newolder » Mar 24, 2017 4:28 pm

jamest wrote:
newolder wrote:This APOD gives various measures that are pertinent to your query.

The associated paper that defines the terms is in the arxiv here.
The paper-and-pencil calculator is a cosmological nomogram which allows to find relations between redshift, distance, age of the Universe, physical and angular sizes, luminosity and apparent magnitude for the standard cosmological model with parameters from the Planck mission.

Thanks, though that only relates to my second question and I can't see how it answers that.

I'm easily confused. Could you ask the question again, please?

If it's the one about measuring distances to galaxies then that concerns finding 'standard candles' that shine equally in all galaxies. Saul Perlmutter used Type 1-a supernovae as standard candles and his Nobel Prize lecture is available to watch here:
https://www.nobelprize.org/nobel_prizes ... cture.html
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Re: Expanding space and the wavelength of light

#5  Postby newolder » Mar 24, 2017 5:52 pm

Obviously, I wasn’t paying much attention when I first read your post - sorry. I’ll try to concentrate…

You seem to be puzzled as to how light is red shifted and how light is different to spacetime.

In reverse order: consider spacetime to be the arena in which events unfold. It’s a 4 dimensional arena that increases can change in size as time passes. The passage of light through arena spacetime is one of the possible events.

Light is red shifted when the arena spacetime grows in size as a light beam passes by. For example: Let blue light enter, stage left when spacetime is, say, 50 metres wide; the arena spacetime grows in extent to, say, 100m wide and the distance between any two points (including consecutive wave crests in a light beam) is doubled; red shifted light, therefore, exits stage left.
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Re: Expanding space and the wavelength of light

#6  Postby jamest » Mar 24, 2017 7:05 pm

newolder wrote:
I'm easily confused. Could you ask the question again, please?

If it's the one about measuring distances to galaxies then that concerns finding 'standard candles' that shine equally in all galaxies.

This leads me to ask another question (sorry squire!) : apparently red-shifted light has less energy, so why would these standard candles not vary in brightness according to distance - assuming brightness correlates with light-energy?
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Re: Expanding space and the wavelength of light

#7  Postby crank » Mar 24, 2017 7:10 pm

"Brightness" is a photons per some area thing, isn't it? You'd have the same brightness, but at longer wavelength.
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Re: Expanding space and the wavelength of light

#8  Postby newolder » Mar 24, 2017 7:16 pm

jamest wrote:
newolder wrote:
I'm easily confused. Could you ask the question again, please?

If it's the one about measuring distances to galaxies then that concerns finding 'standard candles' that shine equally in all galaxies.

This leads me to ask another question (sorry squire!) : apparently red-shifted light has less energy, so why would these standard candles not vary in brightness according to distance - assuming brightness correlates with light-energy?

No promble and yes, these effects are accounted for. The measured spectrum has a red shift. Simply remove the red shift (shift the spectrum back to the blue end by the specified amount) and integrate to get the total energy in the spectrum. This is the standard candle. There are other effects due to absorption by interstellar and intergalactic media that are also inferred/accounted for, but I don't have the details to hand...
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Re: Expanding space and the wavelength of light

#9  Postby jamest » Mar 24, 2017 7:17 pm

newolder wrote:Obviously, I wasn’t paying much attention when I first read your post - sorry. I’ll try to concentrate…

No problem.

You seem to be puzzled as to how light is red shifted and how light is different to spacetime.

Yes.

Light is red shifted when the arena spacetime grows in size as a light beam passes by. For example: Let blue light enter, stage left when spacetime is, say, 50 metres wide; the arena spacetime grows in extent to, say, 100m wide and the distance between any two points (including consecutive wave crests in a light beam) is doubled; red shifted light, therefore, exits stage left.

That's a good explanation which makes sense, thank you.
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Re: Expanding space and the wavelength of light

#10  Postby newolder » Mar 24, 2017 7:21 pm

crank wrote:"Brightness" is a photons per some area thing, isn't it? You'd have the same brightness, but at longer wavelength.

Line-of-sight absorption affects received brightness too. There are models from other measures to help. Ooh - and gravitational lensing may affect some measurements and other stuff that I haven't thought about yet, and...

So it goes.
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Re: Expanding space and the wavelength of light

#11  Postby crank » Mar 24, 2017 8:48 pm

newolder wrote:
crank wrote:"Brightness" is a photons per some area thing, isn't it? You'd have the same brightness, but at longer wavelength.

Line-of-sight absorption affects received brightness too. There are models from other measures to help. Ooh - and gravitational lensing may affect some measurements and other stuff that I haven't thought about yet, and...

So it goes.

I understand a lot of that, I was only addressing the brightness/energy question. The more interesting question is how that violates conservation of energy or does it? I saw something that said it did, from a good source, and forgot what , if any, was the explanation.
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Re: Expanding space and the wavelength of light

#12  Postby newolder » Mar 24, 2017 9:06 pm

I haven't heard of any problems with energy conservation before. The stretching of photon wavelengths seems (to me at least) would cost energy. Perhaps that supplies the apparent energy loss. :dunno: I'll have a rootle about and/or wait for a more informed reply.

ETA Physics Stack Exchange has this: http://physics.stackexchange.com/questi ... -conserved

The linked paper starts:
A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon's path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.

arxiv link
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Re: Expanding space and the wavelength of light

#13  Postby crank » Mar 25, 2017 12:43 am

newolder wrote:I haven't heard of any problems with energy conservation before. The stretching of photon wavelengths seems (to me at least) would cost energy. Perhaps that supplies the apparent energy loss. :dunno: I'll have a rootle about and/or wait for a more informed reply.

ETA Physics Stack Exchange has this: http://physics.stackexchange.com/questi ... -conserved

The linked paper starts:
A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon's path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.

arxiv link
So my explanation is duff. Heigh ho. Learned something new today. :thumbup:

The paper may be a great explanation, but it doesn't change a basic fact, a red shift is about the relative velocities of the source at emittance and receiver at reception, the separation/travel time is irrelevant. The cosmic red shift doesn't much care about relative velocities, they will generally be fairly insignificant, the shift is actually determined by travel time/separation. That seems kinda fundamental to me, even if the actual physics involved allows for a different view point. In other words, there's a categorical difference that the physics is blind to. Or maybe I'n saying it wrong, or I'm full of shit. :scratch:
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Re: Expanding space and the wavelength of light

#14  Postby jamest » Mar 25, 2017 1:26 am

crank wrote:[qIn other words, there's a categorical difference that the physics is blind to.

Please 'expand'.
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Re: Expanding space and the wavelength of light

#15  Postby newolder » Mar 25, 2017 9:23 am

crank wrote:
newolder wrote:I haven't heard of any problems with energy conservation before. The stretching of photon wavelengths seems (to me at least) would cost energy. Perhaps that supplies the apparent energy loss. :dunno: I'll have a rootle about and/or wait for a more informed reply.

ETA Physics Stack Exchange has this: http://physics.stackexchange.com/questi ... -conserved

The linked paper starts:
A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon's path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.

arxiv link
So my explanation is duff. Heigh ho. Learned something new today. :thumbup:

The paper may be a great explanation, but it doesn't change a basic fact, a red shift is about the relative velocities of the source at emittance and receiver at reception, the separation/travel time is irrelevant. The cosmic red shift doesn't much care about relative velocities, they will generally be fairly insignificant, the shift is actually determined by travel time/separation. That seems kinda fundamental to me, even if the actual physics involved allows for a different view point. In other words, there's a categorical difference that the physics is blind to. Or maybe I'n saying it wrong, or I'm full of shit. :scratch:

Yes, it's obviously more complicated than my simple brain can cope with and I may easily misremember stuff and get ideas conflated. There's the Integrated Sachs-Wolfe effect and Sunyaev-Veldovitch effect that may have something to say on these things. I recall (vaguely) an explanation by Martin Rees (Astronomer Royal) about how a CMBR photon gains energy as it falls into a void but, because the void grows during photon passage, it loses more energy on the way out (it has to climb further out than it fell in). This leads to cold spots on the CMBR. Of course, this could all be irrelevant. :dunno:
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Re: Expanding space and the wavelength of light

#16  Postby crank » Mar 25, 2017 2:34 pm

jamest wrote:
crank wrote:[qIn other words, there's a categorical difference that the physics is blind to.

Please 'expand'.

Are you blind, the explanation is clearly indicated, right where I said "...or I'm full of shit" :dance: Also, the previous phrase "Or maybe I'm saying it wrong". In other words, I don't understand this enough to explain what I'm thinking, and what I'm thinking is likely to be wrong, I have read about 1/3 [3 pages] of the paper right after that post, then had other stuff to do, am going to get back to soon. I can tell I'm not going to understand a lot of what is said, but there is one bit that says a lot that I did not have a clue about:
In the curved spacetime of general relativity, there is no unique way to compare vectors at widely separated spacetime points, and hence the notion of the relative velocity of a distant galaxy is almost meaningless.
Also, some concepts I have little to no understanding of are rife, e.g., the very crucial to this whole issue, "comoving", which I vaguely understand, I think it is basically the relative velocities of objects if you 'take out' the expansion, and 'Minkowski space', locally Minkowskian', etc which I have little understanding of. And don't really know what they mean by a "locally comoving family of observers".

Another bit I'm struggling with, which doesn't bode well for me understanding the paper at all, is this:
An expanding universe with density Ω = 0 (often known as the Milne model ) is merely the flat Minkowski spacetime of special relativity expressed in nonstandard coordinates. In an Ω = 0 universe there are no gravitational effects at all, so any observed redshift, even of a very distant galaxy, must be a Doppler shift.


This implies that the expansion is not a gravitational effect, but I don't understand why, in the absence of gravitational effects, the conclusion is that the redshift must then be doppler. I'm missing the crucial severing of expansion and wavelength and how gravity is needed to get the photon to shift frequency, or stretch, or whatever it's doing. There's another thing that brought me up short as to what I had understood [should say 'thought I understood'], it's this:
A student presented with the stretching-of-space description of the redshift cannot be faulted for concluding,
incorrectly, that hydrogen atoms, the Solar System, and the Milky Way Galaxy must all constantly “resist the
temptation” to expand along with the universe. One way to see that this belief is in error is to consider the problem
sometimes known as the “tethered galaxy problem,” in which a galaxy is tethered to the Milky Way, forcing
the distance between the two to remain constant. When the tether is cut, does the galaxy join up with the Hubble
flow and start to recede due to the expansion of the universe? The intuition that says that objects suffer from a
temptation to be swept up in the expansion of the universe will lead to an affirmative answer, but the truth is the
reverse: unless there is a large cosmological constant and the galaxy’s distance is comparable to the Hubble length,
the galaxy falls toward us. Similarly, it is commonly believed that the Solar System has a very slight tendency to
expand due to the Hubble expansion (although this tendency is generally thought to be negligible in practice). Again,
explicit calculation shows this belief not to be correct. The tendency to expand due to the stretching of space is
nonexistent, not merely negligible.


If I could grasp this, then I think i'd understand the issue. I don't get how you show '"The tendency to expand due to the stretching of space is nonexistent, not merely negligible." Obviously, it's in the maths, but I'm not into the maths enough, so it's looking like this will for now be opaque to me. One of the 'if I won the lottery' type fantasies I've entertained frequently is to find a physics grad student I could hire, like on a retainer, so he would be on call when I needed something like this explained. He'd have to be quite patient, and tolerant to idiocy, but since so many of them tend to having little money, it might not be too hard to find one.

I had to reread the 3 pages to make this post, got small blip of an increased understanding, not significant, too much time spent on this topic.

Now, to actually address your question :oops: , my remarks come from a too simple understanding of the issue, it relies on the naive view that the first quoted text above demolishes. The 'categorical difference' relied on there being something real in the question of relative velocity of two widely separated observers. I thought if you had two observers who were close enough to determine they had zero relative velocity wrt each other 13 billion years ago, and that neither accelerated for 13 billion years, then after 13 billion years of expansion, they should still have zero relative velocity, but this seems to not make sense in GR terms. 13 proves unlucky, and I hope I can stop the blood oozing out of both ears now. Unless someone steps in with a virtuoso explanation that is simple enough to grasp by my obtuse brain but complete enough to convey the physics involved, I'm afraid the issue will remain fairley opaque to me. I'll finish the paper, maybe something will spark off some comprehension, but I doubt it. It doesn't help that I've lost the focus I had way back in my college days and forgotten too much of the math.
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Re: Expanding space and the wavelength of light

#17  Postby crank » Mar 25, 2017 2:43 pm

newolder wrote:
crank wrote:
newolder wrote:I haven't heard of any problems with energy conservation before. The stretching of photon wavelengths seems (to me at least) would cost energy. Perhaps that supplies the apparent energy loss. :dunno: I'll have a rootle about and/or wait for a more informed reply.

ETA Physics Stack Exchange has this: http://physics.stackexchange.com/questi ... -conserved

The linked paper starts:
A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon's path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.

arxiv link
So my explanation is duff. Heigh ho. Learned something new today. :thumbup:

The paper may be a great explanation, but it doesn't change a basic fact, a red shift is about the relative velocities of the source at emittance and receiver at reception, the separation/travel time is irrelevant. The cosmic red shift doesn't much care about relative velocities, they will generally be fairly insignificant, the shift is actually determined by travel time/separation. That seems kinda fundamental to me, even if the actual physics involved allows for a different view point. In other words, there's a categorical difference that the physics is blind to. Or maybe I'n saying it wrong, or I'm full of shit. :scratch:

Yes, it's obviously more complicated than my simple brain can cope with and I may easily misremember stuff and get ideas conflated. There's the Integrated Sachs-Wolfe effect and Sunyaev-Veldovitch effect that may have something to say on these things. I recall (vaguely) an explanation by Martin Rees (Astronomer Royal) about how a CMBR photon gains energy as it falls into a void but, because the void grows during photon passage, it loses more energy on the way out (it has to climb further out than it fell in). This leads to cold spots on the CMBR. Of course, this could all be irrelevant. :dunno:

It's definitely irrelevant, because I don't have a clue about any of what you said. But I do really appreciate your efforts to inform. :cheers: Please see my prior post^^. I'm going to finish reading the paper you linked, and I've hardly looked at any of the stackflow stuff, but I will go look at some more, thanks for the links. I'm fascinated by this stuff, but without the maths, it will mostly remain opaque to me, and that's the word of the day, 'opaque', I hate having to say that about so many things. Oh well, i hear they're doing a head transplant, hmmmm, I really need to have a 2nd brain, an auxiliary brain, plugged into mine, that's what it would take for me to grasp a lot of this.
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Re: Expanding space and the wavelength of light

#18  Postby newolder » Mar 25, 2017 3:13 pm

and, of course, I've mis-remembered the void thing. It's gravitationally uphill into a void so the description I posted is wrong, again. :doh: I'm going for a lie down...
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