# Is Krauss looking at this right?

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If the speed of light were constant compared to an expanding metric like you say then it would be variable compared to observers that don't expand in galaxies that don't expand like pennies glued on the rubber sheet.

The speed of light is constant (constant mean that it don't change over time) with respect to the pennies [and you can't imagine how confirmed such a basic fact is because not the expanding metric. The exact thing you said about relativity and about your expectation of a misinterpreted constant c were exactly wrong. Cant you just move on?

You clearly can't make,

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The speed of light is constant (constant mean that it don't change over time) with respect to the pennies [and you can't imagine how confirmed such a basic fact is because not the expanding metric. The exact thing you said about relativity and about your expectation of a misinterpreted constant c were exactly wrong. Cant you just move on?

You clearly can't make,

I can accept that i am wrong but I must understand my mistake first.

when c=m/s in one system,

then C=M/S in the second system

If space only is scaled by two then we have M=2m and s=S (because time is not scaled)

Is this wrong?

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I can accept that i am wrong but I must understand my mistake first.

You are right to say so. I've seen you accept an error when it is validly pointed out to you, so I do sincerely apologize for saying what I said. My last post was sloppily (very sloppily) submitted in a rush basically while walking in the door from an overindulged party.

Is this wrong?

when c=m/s in one system,

then C=M/S in the second system

If space only is scaled by two then we have M=2m and s=S (because time is not scaled)

That is correct. [edit, I mean to say: "the thing you quoted is not wrong"]

Edited by Iggy
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In this video, Krauss, having the desire to be the first to know how the universe ends, describes what the universe will look like to a Milky Way civilization many billions of years in the future. He says that since the universe will be expanding at an accelerated rate, eventually everything outside the Milkyway will be out of sight. I disagree on general princple. It will just look a whole lot redder. Like the radio waves we view as the cosmic background radiation.

>>>he also seems to have a desire to prove absence of deity by proving that at first there was nothing.

And although this might mean they are not "visible", being of such long wavelength as to be out of a humans visible light spectrum, the waves will certainly

not be invisible to science, as he projects.

Regards, TAR2

>>>I do like how points out invisible masses.

Plus he ignores the records that all the previous civilizations might have left, for those "hundreds of billion of years in the future" scientists.

For instance, what if his video was passed down from parent to child, for a zillion generations?

P.S. I grabbed Inows post off the Philosophy thread, "How did everything really begin?".

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You are right to say so. I've seen you accept an error when it is validly pointed out to you, so I do sincerely apologize for saying what I said. My last post was sloppily (very sloppily) submitted in a rush basically while walking in the door from an overindulged party.

That is correct. [edit, I mean to say: "the thing you quoted is not wrong"]

Wonderful.

And if the metric of space is expanding, it means that we are currently in the large system C=M/S observing the small system c=m/s. (and a lot of other smaller systems as much as we look far away)

Because c=m/s lies in the past.

I hope that is correct too.

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Wonderful.

And if the metric of space is expanding, it means that we are currently in the large system C=M/S observing the small system c=m/s. (and a lot of other smaller systems as much as we look far away)

Because c=m/s lies in the past.

I hope that is correct too.

I don't follow. The speed of light is locally c, but is not necessarily c in an expanding metric.

Or:

if the road is 10 inches long, one inch travel is 10% of the road.

When suddenly the road extends to 20 inches long (by scaling), then one "scaled-inch" still is 10% of the length, or 2 inches. The important thing is that for the same time, the distance as measured in the new space system (the scaled-space) remains constant: that's the main property of C.

That is not the main property (or any property) of c. c is not constant in an expanding metric.

...The speed of a photon must remain constant. and for that to be true the speed must be relative to the size of the grid.

No, I'll try to restate. C is constant. See below.

c is not constant in relativity. It is not constant relative to a nearby observer whom is accelerating, a metric that is expanding, or any observer in a gravitational field.

The best way to understand this is to glue pennies on a rubber sheet. The distance between pennies increases as you expand the sheet, but the photon always moves as if to cross a local penny in one second. The speed of a photon relative to one penny is not the same as the speed relative to another penny. Not constant, in other words.

Constant is the opposite of relative. If the distance traveled per time relative to one thing is different from the distance traveled per time relative to another thing then it is not constant.

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I don't follow. The speed of light is locally c, but is not necessarily c in an expanding metric.

That is not the main property (or any property) of c. c is not constant in an expanding metric.

c is not constant in relativity. It is not constant relative to a nearby observer whom is accelerating, a metric that is expanding, or any observer in a gravitational field.

The best way to understand this is to glue pennies on a rubber sheet. The distance between pennies increases as you expand the sheet, but the photon always moves as if to cross a local penny in one second. The speed of a photon relative to one penny is not the same as the speed relative to another penny. Not constant, in other words.

Constant is the opposite of relative. If the distance traveled per time relative to one thing is different from the distance traveled per time relative to another thing then it is not constant.

Sorry I don't get it.

If "the photon always moves as if to cross a local penny in one second", then it is constant.

But you just said it is not.

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Sorry I don't get it.

Either we're both talking past one another or you're mimicking me. Either way, I'll try harder to successfully communicate.

...and for that to be true the speed must be relative to the size of the grid.

No. C can be (and is) variable with respect to the the global expanding coordinate system while superluminal sources are observed due to deceleration.

The important thing is that for the same time, the distance as measured in the new space system (the scaled-space) remains constant: that's the main property of C.

How do you maintain saying this when you must have looked it up by now and found out it is false? Look at the formula for distance as a function of scale factor if you have to. It isn't constant... and of course it isn't constant.

If the speed of light were constant relative to an expanding coordinate system then it wouldn't be constant in flat static pace lie Minkowski spaceime -- but it is. Not only does everyone maintain that c has no universal consistency in relativity, your own logic tells you as much. Why can't you accept it and accept your mistake ?

God knows we've all had t0 d0 it

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Sorry I don't get it.

If "the photon always moves as if to cross a local penny in one second", then it is constant.

But you just said it is not.

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I can accept that i am wrong but I must understand my mistake first.

Well Michel, you are still wrong, Iggy is perfectly correct and your inability to understand doesn't change that.

If you really want to learn, I suggest you start putting a lot more effort and thoughts into your questions.

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Well Michel, you are still wrong, Iggy is perfectly correct and your inability to understand doesn't change that.

Well Spy, my thoughts are that a "local penny" in Iggy's analogy represents an object, at most a cluster of galaxies which is a "gravitationally bound" system and I understand that an observer inhabitant of the "local penny" is inside a gravitational field and thus should not observe C as a constant at all, following Iggy's explanation.

My thoughts are that in Iggy's explanation, from an expanding metric POV, there is no possibility to observe anything faster than c, only slowest. That is because the system of reference expands while the distance traveled by light in 1 sec diminishes relatively to the expanding metric. My thoughts are that in Iggy's explanation there is no way to observe anything receding faster than c (see below).

My thoughts were that from a "local penny" point of vue, in order to observe something receding faster than c, the receding object must be "carried" by space and its displacement mus be proportional to the grid. And if this is correct, then I'd like to understand how space can "carry" a material object like a galaxy and cannot "carry" a photon.

But I wanted to say that step by step because I'd like to understand where exactly I am wrong.

--------------

(edit) I don't want simply to "learn" , I want to understand.

If you really want to learn, I suggest you start putting a lot more effort and thoughts into your questions.
That is still not an answer to my question. Edited by michel123456
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Sorry I don't get it.

If "the photon always moves as if to cross a local penny in one second", then it is constant.

But you just said it is not.

I have no idea why you think it would imply that so it is difficult to answer. "constant velocity" means constant relative to any observer. It doesn't mean constant relative to a local penny.

Pennies, like galaxies and the people in them, don't expand.

The grid on the rubber sheet, like intergalactic distance, does expand.

Light acts like a bug that is capable of crossing a penny in one second. That is how fast its feet will carry it. Set the bug on the rubber sheet and let it go. Stretch the sheet. The bug will cross its local penny in one second, but the distance between the bug and a distant penny will change by much more than the width of a penny in one second.

My thoughts are that in Iggy's explanation, from an expanding metric POV, there is no possibility to observe anything faster than c, only slowest. That is because the system of reference expands while the distance traveled by light in 1 sec diminishes relatively to the expanding metric. My thoughts are that in Iggy's explanation there is no way to observe anything receding faster than c (see below).

You can if the model decelerated. I'll quote wikipedia for all the good it'll do:

Objects at the Hubble limit have an average comoving speed of c relative to an observer on the Earth so that, in a universe with constant Hubble parameter, light emitted at the present time by objects outside the Hubble limit would never be seen by an observer on Earth. That is, Hubble limit would coincide with a cosmological event horizon (a boundary separating events visible at some time and those that are never visible[6]).

However, the Hubble parameter is not constant in various cosmological models[3] so that the Hubble limit does not, in general, coincide with a cosmological event horizon. For example in a decelerating Friedmann universe the Hubble sphere expands faster than the Universe and its boundary overtakes light emitted by receding galaxies so that light emitted at earlier times by objects outside the Hubble sphere still may eventually arrive inside the sphere and be seen by us.[3] Conversely, in an accelerating universe, the Hubble sphere expands more slowly than the Universe, and bodies move out of the Hubble sphere.[1]

http://en.wikipedia.org/wiki/Hubble_volume#Hubble_limit_as_an_Event_Horizon

For the first few billion years the universe had decelerating expansion. This allowed events on galaxies with superluminal recession at the time of emission to eventually be seen by us. We are seeing such events now.

The base of knowledge necessary to understand this, and / or the level of communication needed to successfully communicate it are prohibitive. I don't think, in other words, it does us any good to keep discussing it.

The simpler point is that you asserted that c is constant in relativity. That is easily refuted and itself shows a profound lack of knowledge.

Edited by Iggy
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I have no idea why you think it would imply that so it is difficult to answer. "constant velocity" means constant relative to any observer. It doesn't mean constant relative to a local penny.

Pennies, like galaxies and the people in them, don't expand.

The grid on the rubber sheet, like intergalactic distance, does expand.

Light acts like a bug that is capable of crossing a penny in one second. That is how fast its feet will carry it. Set the bug on the rubber sheet and let it go. Stretch the sheet. The bug will cross its local penny in one second, but the distance between the bug and a distant penny will change by much more than the width of a penny in one second.

You can if the model decelerated. I'll quote wikipedia for all the good it'll do:

For the first few billion years the universe had decelerating expansion. This allowed events on galaxies with superluminal recession at the time of emission to eventually be seen by us. We are seeing such events now.

The base of knowledge necessary to understand this, and / or the level of communication needed to successfully communicate it are prohibitive. I don't think, in other words, it does us any good to keep discussing it.

The simpler point is that you asserted that c is constant in relativity. That is easily refuted and itself shows a profound lack of knowledge.

from http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html © 1992—2009 by Scott Chase, Michael Weiss, Philip Gibbs, Chris Hillman, and Nathan Urban

General RelativityEinstein went on to discover a more general theory of relativity which explained gravity in terms of curved spacetime, and he talked about the speed of light changing in this new theory. In the 1920 book "Relativity: the special and general theory" he wrote: . . . according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [. . .] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Since Einstein talks of velocity (a vector quantity: speed with direction) rather than speed alone, it is not clear that he meant the speed will change, but the reference to special relativity suggests that he did mean so. This interpretation is perfectly valid and makes good physical sense, but a more modern interpretation is that the speed of light is constant in general relativity.

The problem here comes from the fact that speed is a coordinate-dependent quantity, and is therefore somewhat ambiguous. To determine speed (distance moved/time taken) you must first choose some standards of distance and time, and different choices can give different answers. This is already true in special relativity: if you measure the speed of light in an accelerating reference frame, the answer will, in general, differ from c.

In special relativity, the speed of light is constant when measured in any inertial frame. In general relativity, the appropriate generalisation is that the speed of light is constant in any freely falling reference frame (in a region small enough that tidal effects can be neglected). In this passage, Einstein is not talking about a freely falling frame, but rather about a frame at rest relative to a source of gravity. In such a frame, the speed of light can differ from c, basically because of the effect of gravity (spacetime curvature) on clocks and rulers.

If general relativity is correct, then the constancy of the speed of light in inertial frames is a tautology from the geometry of spacetime. The causal structure of the universe is determined by the geometry of "null vectors". Travelling at the speed c means following world-lines tangent to these null vectors. The use of c as a conversion between units of metres and seconds, as in the SI definition of the metre, is fully justified on theoretical grounds as well as practical terms, because c is not merely the speed of light, it is a fundamental feature of the geometry of spacetime.

Like special relativity, some of the predictions of general relativity have been confirmed in many different observations. The book listed below by Clifford Will is an excellent reference for further details.

Finally, we come to the conclusion that the speed of light is not only observed to be constant; in the light of well tested theories of physics, it does not even make any sense to say that it varies.

I know, it says "in inertial frames".

I have no idea why you think it would imply that so it is difficult to answer. "constant velocity" means constant relative to any observer. It doesn't mean constant relative to a local penny.

Pennies, like galaxies and the people in them, don't expand.

The grid on the rubber sheet, like intergalactic distance, does expand.

O.K. for the quote above.

Light acts like a bug that is capable of crossing a penny in one second. That is how fast its feet will carry it. Set the bug on the rubber sheet and let it go. Stretch the sheet. The bug will cross its local penny in one second, but the distance between the bug and a distant penny will change by much more than the width of a penny in one second.

So are you saying that a bug that walks for 1 sec upon a stretched rubber sheet will physically travel the same distance than a parallel bug walking on the ground ?

Edited by michel123456
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from http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html © 1992—2009 by Scott Chase, Michael Weiss, Philip Gibbs, Chris Hillman, and Nathan Urban

I know, it says "in inertial frames".

and it simply does not apply to the invariant sense in which you used it in post 119 that started us off. What you quote agrees that general relativity is not constant in that way. Your statement that the speed of light scales with the cosmological scale factor needs corrected because it is demonstrably false. They are precisely not constant in that way. It needs to be admitted.

So are you saying that a bug that walks for 1 sec upon a stretched rubber sheet will physically travel the same distance than a parallel bug walking on the ground ?

You missed entirely what I was saying which reinforces my fears that we have no common ground on which to talk

Edited by Iggy
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and it simply does not apply to the invariant sense in which you used it in post 119 that started us off. What you quote agrees that general relativity is not constant in that way. Your statement that the speed of light scales with the cosmological scale factor needs corrected because it is demonstrably false. They are precisely not constant in that way. It needs to be admitted.

You missed entirely what I was saying which reinforces my fears that we have no common ground on which to talk

Yes obviously I miss something.

Isn't it correct that light "stretches" when space expands?

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Iggy,

In general relativity such and such is true.

In special relativity such and such is true.

Is there not, that is should there not be that which is true any way you look at it?

Perhaps I am critically devoid of the knowledge of which you speak. I do not know when a theory is being applied to reality and when reality is being utilized to form the theory.

If we observe the universe from here and now, then what we see is the universe. I am at a loss, from the beginning of this, as to what the rules are, when we use the term "currently" and apply it to a galaxy moving away from us, at superluminal speeds. Such a galaxy is by definition "out of sight" currently. But if we were to be looking at such a galaxy, which is currently receeding at such speed, it obviously is neither invisible, nor moving away in an unnoticable fashion, NOW. That it is an "old" image, from which we have derived it's "current" location, demands a shift, a mental shift between the same galaxy as what it was, and what it is. This is confusing, because one of us, does not know "which" galaxy is being referred to, by the other, in a given consideration. And since by common sense, there must only be one instance of said galaxy, we should either go by what we see of it now, or go by what we will never see of it, but not consider what we will see of it later, by considering it in a state that we will never see.

We should, in my estimation, consider only what we see of it now. Measure how what we see of it now is changing over time, and project that out to whatever time we would like to place a future Milky Way scientist, in.

We need not be concerned about ever seeing what is currently going on, in a distant galaxy.

They are too far away, for that.

Regards, TAR2

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Iggy,

In general relativity such and such is true.

In special relativity such and such is true.

Is there not, that is should there not be that which is true any way you look at it?

Perhaps I am critically devoid of the knowledge of which you speak. I do not know when a theory is being applied to reality and when reality is being utilized to form the theory.

If we observe the universe from here and now, then what we see is the universe. I am at a loss, from the beginning of this, as to what the rules are, when we use the term "currently" and apply it to a galaxy moving away from us, at superluminal speeds. Such a galaxy is by definition "out of sight" currently. But if we were to be looking at such a galaxy, which is currently receeding at such speed, it obviously is neither invisible, nor moving away in an unnoticable fashion, NOW. That it is an "old" image, from which we have derived it's "current" location, demands a shift, a mental shift between the same galaxy as what it was, and what it is. This is confusing, because one of us, does not know "which" galaxy is being referred to, by the other, in a given consideration. And since by common sense, there must only be one instance of said galaxy, we should either go by what we see of it now, or go by what we will never see of it, but not consider what we will see of it later, by considering it in a state that we will never see.

We should, in my estimation, consider only what we see of it now. Measure how what we see of it now is changing over time, and project that out to whatever time we would like to place a future Milky Way scientist, in.

We need not be concerned about ever seeing what is currently going on, in a distant galaxy.

They are too far away, for that.

Regards, TAR2

I like your post and the quandary I believe it identifies. Looking at galaxies as they were billions of years ago does make me stop and think what's really going on now? Additionally, I wonder about theories that involve things we can't see. Some scientist think the have seen what is probably the black hole at the center of the Milky Way. Maybe so, but the theories poping up about what can or cannot escape the black hole, what really happens at the event horizon? We can't see it really, nor can we see dark matter/force/energy, and now some are talking about Boson field/energy. Sure you can measure around the black hole and conclude that must be the black hole causing that wobble, but still, how many light years ago did that wobble happen?

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Isn't it correct that light "stretches" when space expands?

the wavelength, yes

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the wavelength, yes

Right.

Edited by michel123456
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Right.

You can measure a photon's redshift and determine the amount of cosmic expansion to which the photon has been subjected. Measuring its velocity can't tell you that.

Let's make this perfectly clear. Bob sends a photon off into the cosmic distance. He notes when it is sent that it has a certain wavelength. Alice, on the other side of the visible universe, receives the photon. She notes its new wavelength. The difference in wavelength is directly related to the change in scale factor of the universe between emission and observation. It depends on that and nothing else.

Bob sends a photon off at velocity c. It crosses the universe and Alice receives it measuring its velocity at c. The difference in velocity (there isn't one) has absolutely nothing to do with the scale factor, or how much the scale factor has changed, or how quickly it changed, or any combination of the above.

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You can measure a photon's redshift and determine the amount of cosmic expansion to which the photon has been subjected. Measuring its velocity can't tell you that.

Let's make this perfectly clear. Bob sends a photon off into the cosmic distance. He notes when it is sent that it has a certain wavelength. Alice, on the other side of the visible universe, receives the photon. She notes its new wavelength. The difference in wavelength is directly related to the change in scale factor of the universe between emission and observation. It depends on that and nothing else.

Bob sends a photon off at velocity c. It crosses the universe and Alice receives it measuring its velocity at c. The difference in velocity (there isn't one) has absolutely nothing to do with the scale factor, or how much the scale factor has changed, or how quickly it changed, or any combination of the above.

(bolded mine)

You mean velocity (of a photon) is constant.

Edited by michel123456
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I am operating on the TAR model, and that is the only one I am capable of considering. It can and has been modified over the years, by evidence and logic provided by others. I can focus on, and evaluate various aspects of it to consider if something is or is not the case. When my conclusions differ from the conclusions of others, it is difficult to drop my notions, in favor of theirs. However I generally give particular others, or individual others, the benefit of the doubt and demand of myself that I either accept as true, that which they are telling me, and modify my model appropriately, or find an objective view from which both my model and theirs are modeling the same reality.

In this pursuit, I am assuming there is a consistent reality, that is similtaneously being modeled by both me and the other.

Ok, can you give this simple river analogy some thought and then tell us how the TAR model differs and why?

Simple river analogy

Earth is our harbour in a river, space is the water and photons are boats from other harbours along the river.

(All other harbours are stationary relative the local water they float in and not relative the riverside.)

When the river is not flowing we will recieve a boat from a new harbour further away for everyday that passes. This is simply because after two days a boat from two days travel away will reach us and after three days a boat from three days away will make it here and so on.

(not flowing river = static space without expansion)

If the water in the river would suddenly start to grow with a constant percentage each day, then the river would start to flow away from our harbour and the speed of its flow would increase with the distance from us since twice as much water grows twice as much and so forth. At some fixed distance very far from us the river would be flowing away faster than the boat speeds. Boats from harbours beyond this border can not reach us because the river will carry them backwards away from us even though they race forward very fast against the water beneath them.

(constant growth = expansion of constant rate)

If the growth of the water would start to drop, that is for each days that passes the growth would be a little lesser than the previous day, then the fixed distance where boats are carried away faster than they can manage to go forward would change equally, moving this border further away from us. Boats that yesterday was slightly outside of this border and unable to reach us will get passed by this outward moving border and find themselves in water flowing slower than the boat speed, such that they can start to make progress towards us again.

(decreasing growth = decelerating expansion)

If the growth of the water would enlarge, that is for every day that passes the growth would be a little higher than the previous day, then the fixed distance where boats are carried away faster than they can manage to go forward would change equally, moving this border closer towards us. Boats that yesterday was slightly inside of this border and making slow progress towards us will get overtaken by this border and find themselves in water flowing faster than the boat speed, such that they now are receding away from us.

(enlarged growth = accelerated expansion)

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Photons (not boats) always reach us at C, no matter what.

When a light source comes towards us at 99,99% of C, the photons from this source reach us at C.

When a light source goes away from us at 99,99% of C, the photons from this source reach us at C.

And I guess that from a hypothetical source that goes away from us at 5000000000000000 times C (for any reason), the photons from this source reach us at C.

No matter what.

That's what "no matter what" means.

And the concept itself of a photon receding from us because the source is getting away from us transported by space expansion, this idea is very peculiar. It looks like a photon going away from me, me being the source and not the receptor.

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It is only an analogy and far from perfect, however for describing the effects of accelerated expansion of space to tar I think it is good enough.

Photons are not able to reach us when space between them and us is expanding faster than they can propagate, no matter what.

There is a very important difference if photons are emitted from objects traveling through space or receding from us due to expansion of space.

When for ever lightyear they travel in our direction there will be two new lightyears between us and them, then they will never reach us.

No matter what.

That's what "no matter what" means.

And while the concept might seem very peculiar to you, it is still common standard cosmology.

Edited by Spyman
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It is only an analogy and far from perfect, however for describing the effects of accelerated expansion of space to tar I think it is good enough.

Photons are not able to reach us when space between them and us is expanding faster than they can propagate, no matter what.

There is a very important difference if photons are emitted from objects traveling through space or receding from us due to expansion of space.

When for ever lightyear they travel in our direction there will be two new lightyears between us and them, then they will never reach us.

No matter what.

That's what "no matter what" means.

And while the concept might seem very peculiar to you, it is still common standard cosmology.

(Bolded mine)

But we can observe such objects, don't we?

--------------------

edit

And this sentence of yours:

Photons are not able to reach us when space between them and us is expanding faster than they can propagate, no matter what.

placed next to Iggy's

Bob sends a photon off at velocity c. It crosses the universe and Alice receives it measuring its velocity at c. The difference in velocity (there isn't one) has absolutely nothing to do with the scale factor, or how much the scale factor has changed, or how quickly it changed, or any combination of the above.

If velocity "has absolutely nothing to do with the scale factor", then it has nothing to do with the scale factor. Point.

Velocity cannot in the same model be influenced by the scale factor in such a way that "Photons are not able to reach us when space between them and us is expanding faster than they can propagate".

Edited by michel123456

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