Expansion redshift VS gravitational redshift?

[proteus13]

Wow, this thread gave me a headache. Some say that's an indication I've learned something but who knows.


The first answer to the question of anya was in regard only to the space between the observer and the observed object. My question is "What about the rest?"

What I mean is, if the universe is much bigger compared to the observable universe, and keeping in mind gravitational waves never fully stop, they are endless and just decay over distance, isn't the pull from OUTSIDE to observable universe, that can potentially be HUGE, isn't it's pull going to always EXCEED the gravitational lensing that occurs inside the bubble of observable universe, that could be tiny compared to the whole universe? In other words, it might not be the universe that's expanding, but gravity pulling light back outside the observable universe, creating similar effect to the proposed expansion?

And just to illustrate my point:
http://img503.imageshack.us/img503/4650/univ.png

If the universe is infinite, then it's pull will always exceed the pull of the finite, visible universe. So, in the center, our point of observation gravity from the whole universe is equally pulling light, neutralizing it's effect. But if we observe distant objects like A and B, the center of gravity will shift relative to our POV and the further the observed object, the more it's light will be pulled in direction, opposite of our observation. B will appear more redshifted than A not because space between has expanded, but because it is more affected from the pull in the direction, opposite to our POV. Now if the universe is endless, every point in it can be seen as it's center, so the effect exhibited is only present relative to our position and the position of the observed object. In other words - no expansion redshifts, only gravitational.

 

[Chalnoth]

People have looked into the effect that super-horizon anisotropies would have on our local universe. These were presented a few years ago as a potential explanation for the recently-detected acceleration of the expansion. But it turned out that when you work out the mathematics in detail, their effects simply cancel.

But I think you have this erroneous idea that gravity waves can somehow outrun light waves. This is not true. Both travel at the same speed, and neither can outrun the other. So, for instance, gravity waves from within a black hole can never reach the outside of the black hole.

 

[proteus13]

Yes, that's what I said, the effect will cancel out relative to the observed object, but relative to the point of observation a shift in light distribution may occur. Not to mention we know nothing about the region beyond our observation - the universe might not be that uniform as we think, and there might be a different portion of it that has everything blueshifted, the opposite of what we have. It is only natural, like it's summer in the northern hemisphere but it is winter in the southern.

I don't say gravity waves can "outrun" light waves in speed, they are supposed to be the same speed. However a flashlight cannot pass through the floor of the room, while gravity does pass, if it didn't we had to flow in the air. Also gravity affects light obviously. So gravity can extend further than light, effectively outrunning it.

Noone knows what a black hole really is, if it is a pinch in space, a form of very compressed space, where both light and gravity from our point of view will travel slower, infact they won't, they will just travel more space concentrated into a smaller volume in our perception. You say nothing leaves the black hole, but Steven Hawking says otherwise. I don't generally agree with most of his ideas, but for this one I think he is right.

The difference between a standard model mathematical black hole and an actual black hole might be huge, do we really want to rule out possibilities, just because they are not consistent with a theory?

I have the feeling people sometimes get so caught up with things, the forget it is MERELY A THEORY, and there is a high possibility for the theory to be wrong, historically every theory has been working for a given period of time, after that it gets obsolete and replaced with a better one. Do you really think we got the read deal now? It is just another version, and not a very consistent one. The idea that the world might be suspended on elephants and tortoises does not sound any more crazy than the big bang theory, and all its hypothetical fundamentals like dark matter and energy, which conveniently enough cannot be observed, measured or proved to exist in any way.

I know this words will be taken as bad as Galileo's when he said Earth revolves around the sun, because then the "mainstream" was that the whole universe revolves around us, but like every other concept it worked for a while but got old. I hope you do know history and you can extract some wisdom from it - looking back at a few thousands years of human history I'd say chances that the current model is "the real deal" are equal to 2012 being the end of time. In other words pretty slim. Cosmology is whatever me make out of it, and it is kind of sad to see science got so narrow minded and directed 100% in the current, pretty flawed, 100% hypothetical and theoretical model.

 

[Ich]

I count 85 points, not bad.

 

[Chalnoth]

Yes, that's what I said, the effect will cancel out relative to the observed object, but relative to the point of observation a shift in light distribution may occur.

Nope, there is no effect on the redshift.

Not to mention we know nothing about the region beyond our observation - the universe might not be that uniform as we think, and there might be a different portion of it that has everything blueshifted, the opposite of what we have. It is only natural, like it's summer in the northern hemisphere but it is winter in the southern.

There might well be regions of the universe that are collapsing. But it doesn't matter: they don't affect us. At all.

I don't say gravity waves can "outrun" light waves in speed, they are supposed to be the same speed. However a flashlight cannot pass through the floor of the room, while gravity does pass, if it didn't we had to flow in the air.

That's a different issue, though, one of how these waves interact with matter. Light interacts very strongly with matter, and so some sorts of matter are opaque (at least in certain wavelengths). Gravity doesn't have this issue: it interacts only extremely weakly with matter (around 10^40 times weaker than electromagnetism).

So gravity can extend further than light, effectively outrunning it.

No, it can't. To take a simple example, a black hole can also have electric charge, and if it does, will have an electromagnetic field extending out of it in the same way it has a gravitational field extending out of it. The two forces have the exact same properties where their range is concerned. They only differ in a couple of respects (gravity is weaker, always attractive, and couples to stress-energy instead of the electromagnetic charge).

Noone knows what a black hole really is, if it is a pinch in space, a form of very compressed space, where both light and gravity from our point of view will travel slower, infact they won't, they will just travel more space concentrated into a smaller volume in our perception. You say nothing leaves the black hole, but Steven Hawking says otherwise. I don't generally agree with most of his ideas, but for this one I think he is right.

Hawking radiation doesn't affect my statement in the least. For any astrophysical black hole, that is, one that came from the collapse of a star (or any more massive black hole), the Hawking temperature is dramatically smaller than the CMB temperature, and so might as well not be there at all (at least for the time being). So what I was attempting to show still holds: the gravitational field of the black hole extends outside the black hole without gravity waves from the interior reaching the exterior.

If you still think your statement here has any merit, consider this: as the black hole evaporates, the Hawking radiation increases in intensity, while the gravitational field gets weaker. How could one be the source of the other and still hold to this inverse relationship?

I have the feeling people sometimes get so caught up with things, the forget it is MERELY A THEORY, and there is a high possibility for the theory to be wrong, historically every theory has been working for a given period of time, after that it gets obsolete and replaced with a better one.

The thing is, proteus, everybody in science is aware of this. It's a worthless argument to make. You might as well be arguing vociferously that the sky is blue, as if people had forgotten the fact.

If you really want to attempt to claim that theory X might not be entirely accurate, then you only have two options before you:
1. Present an argument for why theory X breaks down at a certain point. Provide experimental support if possible, or some mathematical argument if that isn't available (finding a contradiction in the theory is usually a great way to do this).
2. Present a different theory. Preferably show how it fits experiment better than existing theory. Alternatively present an argument for why this theory is simpler and more elegant than existing theories.

But the thing is, you haven't even bothered to do either one. You haven't even presented a different theory: you've just thrown up a physical model of how the universe might be, and how things outside our horizon might affect stuff inside.

Any and all such models are correctly and properly analyzed by examining their effects based upon current theory. By current theory, stuff outside of our cosmological horizon can have no effect upon us. That means that it can't even have an effect upon anything we observe, by the way, such as redshifts.

Now, if you want to present some new theory where matter beyond our cosmological horizon can have an effect on our observations, by all means work carefully through its implications and present it. But merely attempting to claim that some theory might support your assertions is just being intellectually dishonest.

 

[Ich]

Ok, it seems unlikely right now, but maybe some day I'll discuss these issues again.

In case there is an interested reader somewhere, I'll follow https://www.physicsforums.com/showpost.php?p=2531175&postcount=30" and explain my position once more, for the record, so to speak. I'm still working on the "remain relaxed" trick, however.

1)

But how are we sure that is the case, and redshifts are not due to the gravitational pull of all those objects that lie between us and the observed objects?

Because gravitation effects of stuff in between the source and the observer cancel out.

Gravity works both ways, matter on the far side counters gravitational effects from the near side. A net zero effect.

Pull from the whole universe

One answer to all: If I draw a sphere around observer and emitter, the gravitational effects of everything outside that sphere cancel out.
This leaves the gravitational effect of all matter inside the sphere.
This effect causes deceleration or even collapse, and an additional blueshift to the doppler redshift (in regions where the distinction makes sense).

2)

Another way of arriving at this result can be found in arXiv:0911.1205

OMG. Topology.
Just let me stipulate that the topology is R³ on the scales we (Bunn, Wallace, and I) are talking about, and forget about that paper. If someday they find out that Andromeda is the Milky Way, I'll be the first to send the author my apologies.

 

[marcus]

The original poster used the idea "expansion redshift" in the thread title. Here is the original post:

expansion redshift VS gravitational redshift?

While objects closer to us tend to shift both in direction red or blue, depending on their movement in relation to us, distant objects such as galaxies tend to only shift to the red.

As I understand this is the base of the idea that the universe is expanding. But how are we sure that is the case, and redshifts are not due to the gravitational pull of all those objects that lie between us and the observed objects?

It seems appropriate here to give the basic equation for expansion redshift, what in cosmology discussions is often simply called redshift. And to confirm the correlation between the increase in distances and the increase in wavelength (i.e. how scalefactor ratio relates to wavelength ratio 1+z).

... In co-moving coordinates, the only source of redshift is the overall expansion, and so the redshift is simply:

$$z + 1 = \frac{a_{obs}}{a_{emit}}$$

However apparently the discussion became "convoluted"...

:sigh: I should know better than to continue, but "Once more unto the breach"...

The only reason any of this got at all convoluted is because you were incorrectly applying Newtonian physics, and thus I had to explain in gory detail how your re-collapsing dust ball example was perfectly consistant with a Newtonian description. Remember that you introduced the re-collapsing idea, and when doing so failed to correctly apply Newtonian physics.

If we just return to the original question, how to understand how gravity and motion play a role in the redshifts we see in the Universe, we see that there is one single unique way to describe this using Newtonian physics (we don't have general covariance in Newtonian physics, so it all becomes much simpler). Now, even given the gauge freedoms in GR, all co-ordinate systems will converge for small distances to satisfy the equivalence principle. Hence, the Newtonian description tells you clearly how the underlying physical mechanism are at play, in a way free from co-ordinate transformations. You don't even need to define any co-ordinates, you can just use words, but when you do so you are using words that have a direct physical meaning; gravity, motion etc.

On the other hand, when you wrap everything into a(t) and repeat the phrase 'expansion of space' to explain anything without any context you aren't learning, teaching or understanding anything more than the properties of one arbitrary foliation of the FRW space-time.

I'm not against the use of the phrase 'expansion of space' or the analogies that go along with it, I just object to their mis-use in contexts where it is not appropriate. When someone has asked how gravity and motion play a role in redshift, then this is clearly not the time to be invoking this concept.

I was never even touching on Newtonian physics in the discussion.

It seems that the original poster raised the expansion redshift concept up front. We then had what I think was a simple and helpful response, by Sylas and Chalnoth. But somehow, what was said (for example by Chalnoth) was misunderstood, or misinterpreted.
Also, when people (anya, Chalnoth, Sylas in post #8, etc...) referred to expansion they did not AFAICS mention any "analogies that go with it". Nobody spoke of space as a material. Noone talked about space as some kind of rubber which, by stretching, causes wavelengths to stretch. The fact is, such analogies do not immediately "go with" the mention of comoving distance, the type of distance used in stating the Hubble law v = Hd.

So why is this not the time to "invoke the concept" of expansion of distances, or expansion of universe, or whatever? It was already "invoked" by the OP. Shouldn't we be able to use the word "expansion" without this being admonished, sermonized, treated as a bugaboo?

 

[Chalnoth]

One answer to all: If I draw a sphere around observer and emitter, the gravitational effects of everything outside that sphere cancel out.

Well, not quite. This is true if the matter on the outside of that sphere is perfectly isotropic: the same in every direction.

In this situation, that requirement is not needed, as the effects of super-horizon matter that is different in different directions actually cancel out. I'd have to look up the exact physical effect at work here, but it shouldn't really be a surprise: if there was a measurable effect, then we'd be able to obtain information about the structure of the universe beyond our visible universe, which would be an indication of information traveling faster than light, which can't happen.

OMG. Topology.
Just let me stipulate that the topology is R³ on the scales we (Bunn, Wallace, and I) are talking about, and forget about that paper. If someday they find out that Andromeda is the Milky Way, I'll be the first to send the author my apologies.

The paper made no assumptions about the scale of the overall topology. It could well be orders of magnitude greater than our visible universe that it wraps back on itself.

Now, granted, this inconsistency doesn't mean that you can't go to the Newtonian approximation to try to get a feel for the physical interpretation of these variables. But it does mean that the physical description of expanding space is more correct than the Newtonian approximation (because when you insert expanding space into the Newtonian approximation, the contradiction disappears).

 

[Ich]

So the question was

But how are we sure that is the case, and redshifts are not due to the gravitational pull of all those objects that lie between us and the observed objects?

And the answer should be

It seems appropriate here to give the basic equation for expansion redshift, what in cosmology discussions is often simply called redshift. And to confirm the correlation between the increase in distances and the increase in wavelength (i.e. how scalefactor ratio relates to wavelength ratio 1+z).

??
Seems like missing the point.

We then had what I think was a simple and helpful response, by Sylas

Yeah. But it was wrong. (Just to make sure: I enjoy sylas's posts. I'm glad to have him here, this is nothing personal.)

But somehow, what was said (for example by Chalnoth) was misunderstood, or misinterpreted.

...or wrong (#5, 17, 19, 20, 33).

The fact is, such analogies do not immediately "go with" the mention of comoving distance, the type of distance used in stating the Hubble law v = Hd.

They do. Or how do you explain the fact that, when asked to explain Brooklyn's stability, people start handwaving, mentioning everything from quantum mechanics to binding forces stronger than expansion, instead of simply stating that there is no reason whatsoever for Brooklyn to expand.

Shouldn't we be able to use the word "expansion" without this being admonished, sermonized, treated as a bugaboo?

I see. Sorry, I didn't mean to persecute you with my agenda.
I think we will get along just fine as long as you're not telling newbies that galaxies aren't moving. Inform me when I get annoying.

Well, not quite. This is true if the matter on the outside of that sphere is perfectly isotropic: the same in every direction.

Isotropy given, do you agree then?

Now, granted, this inconsistency doesn't mean that you can't go to the Newtonian approximation to try to get a feel for the physical interpretation of these variables.

So the local Newtonian approximation is also declared valid?

But it does mean that the physical description of expanding space is more correct than the Newtonian approximation (because when you insert expanding space into the Newtonian approximation, the contradiction disappears).

Well, the description "maybe there's something going on" is even more correct, as it is consistent with a vast bigger number of cosmologies than the other two.
You see, if you have to describe quasistatic phenomena, static coordinates are most useful. If you have to describe global expansion, you'll choose cosmological coordinates.
Applying the former when topology becomes important can bring you in deep trouble.
Applying the latter to solar system physics is also a catastrophe.
The difference is that I know of no paper spreading misconceptions about the former case, while the latter still is deeply ingrained.

I don't want to discredit the "expansion" idea or cosmological coordinates. I just want to make sure that coordinates are not confused with physics.

 

[Chalnoth]

You see, if you have to describe quasistatic phenomena, static coordinates are most useful. If you have to describe global expansion, you'll choose cosmological coordinates.
Applying the former when topology becomes important can bring you in deep trouble.
Applying the latter to solar system physics is also a catastrophe.
The difference is that I know of no paper spreading misconceptions about the former case, while the latter still is deeply ingrained.

Why on Earth would anyone make use of coordinates that assume a homogeneous and isotropic universe for a system that is obviously very very very far from homogeneous or isotropic? It makes no sense. Who makes this mistake anyway?

 

[marcus]

... Sorry, I didn't mean to persecute you with my agenda.
I think we will get along just fine as long as you're not telling newbies that galaxies aren't moving.

Are you threatening to be obnoxious, then? As far as I know distant galaxies are not moving (except trivially) relative to CMB. You can always choose coordinates so that they move in those coordinates, of course.

But we have no evidence that typical galaxies are moving more than a few hundred km/'s, relative to CMB, and this is an interesting fact. I think it helps newcomers to hear it.

It also helps to explain what the basic equation v = Hd means. This is based on the type of distance which would be measured by timing a lightpulse with expansion frozen. It refers to that distance measured now, and to its rate of increase now.

The idea of now, and comoving distance, is built into the Hubble law and is natural to the standard model. Of course we can define alternative coordinates etc etc. But these are central, and it helps to communicate them up front.

Well, the description "maybe there's something going on" is even more correct, as it is consistent with a vast bigger number of cosmologies than the other two.
You see, if you have to describe quasistatic phenomena, static coordinates are most useful. If you have to describe global expansion, you'll choose cosmological coordinates.
Applying the former when topology becomes important can bring you in deep trouble.
Applying the latter to solar system physics is also a catastrophe.
The difference is that I know of no paper spreading misconceptions about the former case, while the latter still is deeply ingrained.

OK you have given us a little lecture or sermon about something or other. I'm not sure it's relevant to the basic problem of answering a newcomer's question. Or says anything new.

I don't want to discredit the "expansion" idea or cosmological coordinates. I just want to make sure that coordinates are not confused with physics.

Fine, I was not confusing them. How about keeping a police whistle in your mouth ready to blow whenever you see someone talking about coordinates and talking about physics and explicitly equating them. That could be useful, as long as you don't impute thoughts to other people that they didn't express.

 

[Pierre007080]

Hi Guys,
I have read and am impressed with the complexity of yoyr quoted models and opinions ... enough to scare novices like me off entirely. May I humbly ask two questions regarding red shift and get an answer that behoves the level at which it was asked?

Question 1. If galaxies are moving due to space itself expanding, how is it possible that a "Doppler" redshift can occur? Surely a "Doppler" effect requires the galaxy to be moving through space and not with it.

Question 2. If a light ray passes a large body (e.g. galaxy) tangentially, will the time dilation not cause a redshift? (I would like to separate this effect from the classical gravitational redshift and the difference in gravitational potential between source and observer. Also accept the maintenance of frequency of the wave dipping in and out of various densities or gravitational potentials.)

 

[Pierre007080]

"But we have no evidence that typical galaxies are moving more than a few hundred km/'s, relative to CMB, and this is an interesting fact. I think it helps newcomers to hear it."

Hi Marcus,
If this CBM means Cosmic Background Microwave, then this is knowledge new to me(no surprise) Where can I read more on this topic of galaxies relative movement to the CMB? This relates to my previous question about the Doppler effect only "being visible" if the galaxies are moving through space.

 

[Chalnoth]

Question 1. If galaxies are moving due to space itself expanding, how is it possible that a "Doppler" redshift can occur? Surely a "Doppler" effect requires the galaxy to be moving through space and not with it.

Well, this boils down to sort of the essence of what we've been talking about: there is no well-defined way to talk about something moving with respect to space. Basically, without looking outside, there is no possible way to tell whether or not you are moving. There is no experiment you could possibly perform to determine this (this fact is the very essence of relativity).

What this means is that if you want to know how much of the redshift comes from the Doppler effect, and how much comes from the expansion of space, well, the answer to that depends entirely upon what numbers you use to describe space. So in cosmology, we usually just avoid such questions altogether, and instead simply focus on things that are directly observable.

Question 2. If a light ray passes a large body (e.g. galaxy) tangentially, will the time dilation not cause a redshift? (I would like to separate this effect from the classical gravitational redshift and the difference in gravitational potential between source and observer. Also accept the maintenance of frequency of the wave dipping in and out of various densities or gravitational potentials.)

Well, as the light ray falls into the galaxy's potential well, it gains a blue shift due to the added energy. When it climbs back out, it loses that energy. As long as the galaxy's gravitational potential stays the same during the light ray's passage, then the two effects perfectly cancel and there is no change in the light ray's energy (i.e. no redshift or blueshift).

Time dilation really doesn't impact this picture at all. There also isn't anything related to the "maintenance" you're talking about here. It's just a matter of it entering and leaving the potential well. Or bumping into matter, but that's another story entirely.

 

[marcus]

Pierre,
I agree with Chalnoth's response, and also want to add a comment or two on your question #1

...
Question 1. If galaxies are moving due to space itself expanding, how is it possible that a "Doppler" redshift can occur? Surely a "Doppler" effect requires the galaxy to be moving through space and not with it.

BTW since I hadn't met you before, I looked at a few of your past posts in other threads. I appreciate that they give an impression of depth. I would also guess that you have some mathematical sophistication (terms like differential manifold, diffeomorphism, etc. might not be unfamiliar to you) but I can't point to any concrete reason why i have that impression.

Personally I never talk about "space itself" expanding. I don't like to perpetuate the idea of space as a material. Indeed it may eventually turn out to be some kind of aetherial substance! Fundamentally we humans are not sure we know what space is or what time is. Let's admit it. We don't know. The researchers pursuing various approaches to quantum gravity may provide some insight, but that is work in progress.

But cosmology is done in the context of General Rel, so that makes our life very simple. We talk about space as per GR. I remain skeptical. GR may be wrong or superficial, indeed there probably is something more, and deeper, to say. But let's just accept the situation as per GR.

Then space and spacetime have no objective physical reality---Einstein said this clearly in 1916. "The principle of general covariance [= diffeomorphism invariance] deprives space and time of the last remnant of objective reality."
In GR you make temporary use of a manifold to describe the distribution of matter and the metric, and then you pass to the equivalence class, under diffeos, of all manifolds and metrics which are equivalent to the given one under smooth mapping. So one throws away the manifold, in the end.

All that GR allows to exist is a web of geometric relations among events.

Pierre, please warn me if this sounds useless or strange to you, from your posts I get the impression that you have some philosophy/math sophistication which enables you to be comfortable with this. You seem not to be a newcomer to this kind of thinking. But I could be wrong. If so please let me know.

=========================
There is a lot more to say. Let me mention a couple of papers first, in case you want to read a little background.
Valerio Faroni is reputable (he has co-authored with worldclass people like Stefano Liberati and Thomas Sotiriou). He has a recent paper reviewing the issue of how best to interpret redshift. Because it is recent and (in my judgment) balanced, it has references to different advocacy-papers, and it weighs different sides. It may help with perspective, and pointers to earlier work.

Then there is a paper by Emory Bunn and David Hogg, which I like although I do not agree with their conclusion. They show that it is mathematically possible to set up an infinite chain of observers along the path of a photon and correctly analyze the redshift as the cumulative effect of an infinite series of infinitesimal dopplershifts (as one passes from one observer to the next.) Well this is certainly possible!

However they advocate this interpretation as the "natural" way of looking at the redshift, and in that respect I thing they go too far. Faraoni cites them, but tries to present a more balanced view.

I'll get the links later.
==================
I have to do something else for the moment. Hope to get back soon to this reply.

 

[Ich]

Are you threatening to be obnoxious, then?

I already am.

As far as I know distant galaxies are not moving (except trivially) relative to CMB.

Yes. Relative to the CMB.

You can always choose coordinates so that they move in those coordinates, of course.

Yes, the normal ones, for example. People don't think this is possible when they're being told that expansion is not motion. This leads to confusion.

Fine, I was not confusing them.

I didn't say you were.

Hi Chalnoth,

would you mind updating me on the status of our discussion first?

Isotropy given, do you agree then?
So the local Newtonian approximation is also declared valid?

If I presume you're d'accord with me, and you aren't, that's bad.
If I discuss the point further, and we're agreed already, that's bad also.

As to your question

Who makes this mistake anyway?

Obviously quite a few people who try do calculate the effect of expansion on the solar system. See for example the references in the paper bcrowell https://www.physicsforums.com/showpost.php?p=2518307&postcount=43". Or see the whole discussion there - if everybody knew the Newtonian approximation is valid, then why is there a discussion at all (except maybe for my bad wording in one sentence)?

 

[marcus]

Pierre, having said the above, I might as well "cut to the chase"---give you my personal viewpoint directly, with my personal opinion of what's the best way to present the basic cosmology picture.

The universe we live in has more than mere General Relativity.
It has an idea of being at rest (approximately) with respect to the ancient matter which we see by the CMB light that emitted.

I suspect you understand this already---what it means to be at rest relative CMB (no dipole, no doppler hotspot ahead or coldspot behind). The CMB has an amazingly uniform temperature---the deviations are on the order of one part in 100,000. Microkelvin!

This gives us an idea of a widespread collection of stationary observers. And it also defines for us an (approximate) idea of now: all the stationary observers who see the same CMB temperature we do.

So the universe provides us with a pragmatic (not the only possible!) idea of universe time, simultaneity, and distance----the distance which would be radar-measured between stationary observers at a particular moment if one could freeze expansion at that moment or, alternatively, have the collaboration of a large number of observers extending between the two points so that the radar-ranging could be done quickly.

Happily enough, this idea of now, this universe time, was already in the Friedman model (which dates back to 1922) a simplification of GR that is still used. The idea of stationary observer was understood even before the CMB was observed, as being at rest with respect to the expansion process itself (no dipole of a slow-recession spot ahead or a fast-recession spot behind, uniform Hubble law recession in all directions.) The idea of distance I described turns out to be the natural one to use in stating the Hubble law, and (called "comoving") is so used. And the idea of distance arises naturally in connection with the Friedman metric.

So as soon as one is introduced to the idea of being at rest relative to CMB, and pictures the distance defined by freezing expansion, a lot of what cosmologists regularly talk about becomes accessible.

This freezeframe distance is variously called "Hubble-law distance" or "proper distance" or "comoving distance" depending on context and taste, but it is essentially just a matter of timing a flash of light or radar-signal with the complication of distance expansion removed.
Standard online calculators (google "Wright cosmo calculator", or Morgan's "cosmos calculator") readily convert redshift numbers to this type of distance.

Have to go. Back fairly soon, I expect.

 

[Chronos]

Actually, every other observer in the universe sees a slightly warmer CMB than do we - for the simple reason they are observing it in our past [barring instantaeneous communication]. It is a trifling amount save at extemely distances. See, for example:
VLT Observations Confirm that the Universe Was Hotter in the Past
http://www.eso.org/public/news/eso0043/

 

[Chalnoth]

Obviously quite a few people who try do calculate the effect of expansion on the solar system. See for example the references in the paper bcrowell https://www.physicsforums.com/showpost.php?p=2518307&postcount=43". Or see the whole discussion there - if everybody knew the Newtonian approximation is valid, then why is there a discussion at all (except maybe for my bad wording in one sentence)?

Well, it makes sense that there would be some discussion, because one doesn't necessarily expect the result to be actually zero. But attempting to determine this small effect (if any) is fundamentally different from somebody making the mistake that everything expands and dumbly applying the Hubble law to, say, our solar system.

 

[Pierre007080]

Hi Chaloth and Marcus, I really appreciate your answers and their attempt to keep it on my level (didn't quite succeed). Looking "from the outside" at the discussion around redshift it appears that the interpretation of causes are not that certain?? Correct me please if this is not the case, but enormous assumptions have been made based upon this redshift that have basically driven the whole of astrophysics and cosmology in the inflationary universe direction. My question here is: how sure are we of our interpretation of this redshift observation? Einstein seemed to accept it without much comment??

 

[Pierre007080]

Time dilation really doesn't impact this picture at all. There also isn't anything related to the "maintenance" you're talking about here. It's just a matter of it entering and leaving the potential well. Or bumping into matter, but that's another story entirely.

Hi Chalnoth,
Could you please explain why time dilation does not play a role in the picture of a light ray passing tangentially through the spherical gravitational potential layers around a large object (eg galaxy)?

 

[Chalnoth]

Hi Chaloth and Marcus, I really appreciate your answers and their attempt to keep it on my level (didn't quite succeed). Looking "from the outside" at the discussion around redshift it appears that the interpretation of causes are not that certain??

Well, it's less that they're not certain, and more that they don't have meaning. This is perhaps one of the more difficult things to understand about relativity: many questions that we take for granted as having real, definite answers in everyday life turn out to only have definite answers because we make a number of unstated assumptions.

To take a simple example, take the question, "Is sally moving?" When asked in the context of everyday life, this makes perfect sense, and we expect it to have a definite yes or no answer. But that's only because there are some definite unstated assumptions we make when we ask (and answer) this sort of question. What we really mean is, "Is sally moving with respect to the surface of the earth?"

As long as Sally is on the ground, that question has an absolute, definitive answer. It's perhaps a little bit clear why it doesn't have an absolute answer unless we specify what the motion is relative to, but why is it only well-defined if Sally is also on the Earth? That comes down to General Relativity: one of the consequences of General Relativity is that there is no unique way to take a difference of two velocities unless those velocities are at the same space-time point.

This fact, ultimately, comes down the the curvature of space-time. If space-time in a region is nearly flat, then vectors in one location are basically the same as vectors in another location, so you can subtract velocities in different places in the exact same way as if they were in the same place. But if the space-time is curved, that subtraction stops working the same way, and starts to depend intimately upon which coordinate system you use: it would be almost as if the answer to the question, "Is Sally moving?" changes between "yes" and "no" depending upon whether you're using the metric system or English units! Granted, it's not quite that bad, but the idea is the same.

And so we are forced to only have well-defined relative velocities when two objects pass by one another.

Correct me please if this is not the case, but enormous assumptions have been made based upon this redshift that have basically driven the whole of astrophysics and cosmology in the inflationary universe direction. My question here is: how sure are we of our interpretation of this redshift observation? Einstein seemed to accept it without much comment??

The thing is, General Relativity makes some definite, unambiguous predictions as to what we should observe. Some questions that we are used to having definite answers to turning out to be a bit ambiguous doesn't harm this: the predictions, such as the relationship between redshift and brightness, are still quite definitive.

 

[Chronos]

Time dilation is a natural consequence of expansion. Photons do not realize space is stretching as they travel through it. The same number of wave crests reach us as were emitted - without regard for expansion [conservation of energy thing]. The finite speed of light means it takes longer for all of them to get here.