How does the expansion of the universe stretch light?

[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 absorption 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 absorption 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 absorption 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 algorithm 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 algorithm 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.

 

[ptalar]

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.


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.

Thanks Chalnoth. I believe I understand. What I forget is that me, the observer on earth, has also moved in space, due to expansion since the big bang. This would take what would of been a 42 milliion year journey (for CMBR to reach Earth (observer) from the origin), if both objects were fixed and static, take 13.7 billion years due to the change in locations as a result of expansion (of observer and origin) resutling in a much more circuitous and longer path for light to travel. The redshift of the light would then indicate the object to be 46 billion light years away since the CMBR was first emitted thus giving us a rough size of the visible Universe.

 

[mathal]

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.

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

When we look out into deep space we are looking back into time. We can only see that part of the event that occurred ~14 billion years ago with light reaching us now. The particles that produced this light are now 46 billion light years away, way out of sight. Every moment we are receiving light from this event from a larger sphere of more distant time away from us. What amount of the universe that exists 'now' can we see. Clearly the light hasn't reached us yet so we can't see anything it it's 'present' state.

mathal

 

[ptalar]

This might be a good article that can shed some light on the discussion in this thread.

http://adsabs.harvard.edu/abs/2004PASA...21...97D

 

[Naty1]

Regarding post 46 and the subsequent posts:

from MISCONCEPTIONS ABOUT THE BIG BANG by Lineweaver and Davis in the March 2005 issue Scientific American.

"If space were not expanding, the most distant object we could see would be about 14B light years away from us, the distance light could have traveled in the 14 billion (B) years since the big bang."... but because the universe is expanding as light travels towards us and light therefore takes longer to get here, we can see out about 46B light years. In other words, the oldest light is 46B years old.

Age of the Universe:
About 380,000 years after inflation concluded, what we now see as cosmic microwave background radiation (CMBR) could begin to get through early matter after all the initial high energy ionization subsided...

We only get the CMBR light from some particular batch of early matter once and it passes by. Tomorrow we will get light from matter that is farther away than that batch whose light we got yesterday because the universe continues to expand. The source of the CMBR is NOW about 46b light yrs away but the light we get from it was emitted 41 million light years ago. It has taken so long to get here because the universe has expanded...by a factor of about 1080 times..."

and a prior discussion in these forums explaining redshift:

https://www.physicsforums.com/showthread.php?t=368958

 

[Chalnoth]

but because the universe is expanding as light travels towards us and light therefore takes longer to get here, we can see out about 46B light years. In other words, the oldest light is 46B years old.

This part is just wrong, no matter how you slice it. The furthest we can see is the cosmic microwave background, the light from which was emitted about 43 million light years away, but due to the expansion took 13 billion years to get here. The matter that once emitted that light is currently about 46 billion light years away.

 

[Naty1]

Chalnoth: thanks...
I'll have to search exactly where I got that quote...if I were younger with better memory, I'd remember but alas!...anyway, the article I thought I referenced now seems to cost $7.95 from Scientific American...I know I found it free of charge previously...

When I posted the quotes I noticed that "46B years old.." and briefly wondered myself...but did not think about it

...the light from which was emitted about 43 million light years away, but due to the expansion took 13 billion years to get here

yes, of course the universe is about 13B years old.

 

[Naty1]

I reread the article by Lineweaver and Davis and sure enough they got it right here:

If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years.

Somehow I got an excerpted quote wrong, but it was copied not rewritten (not retyped) , so I'm stumped...anyway, I updated my notes with the correct excerpt.

 

[Chalnoth]

I reread the article by Lineweaver and Davis and sure enough they got it right here:



Somehow I got an excerpted quote wrong, but it was copied not rewritten (not retyped) , so I'm stumped...anyway, I updated my notes with the correct excerpt.

Well, if I'm reading your use of quotes in your post correctly, you quoted the first part of that statement, and paraphrased the second part.