[Halc]

I don't understand how a redshift which we are using for galaxies can't also be used for the CMB?

They don't use a different kind of redshift for different things.  Galaxies are closer, so they're less redshifted, as evan_au points out.  Galaxies are things and have defined distances from us.  The CMB is not a thing or an object and has no distance to us.  It is a single flash of light that occurred everywhere (even here).  It left only the light and no object from which it came.  So there is no meaningful distance to the CMB since there is no CMB to which one might travel.

A redshift is a redshift. It comes as an information in the radiation. It comes from any kind of matter. We can call this matter: dust, Gas cloud, stars, galaxies... (What about CMB radiation from dark matter or dark energy?)

CMB is radiation from normal plasma/hydrogen, not dark anything.  Yes, it is redshifted like anything else that was emitted at a significant distance.

So, if we claim that a radiation from a galaxy with a redshift of 1100 represents a distance of  46GLY, why a radiation which we call CMB with a redshift of 1100 doesn't represents a radiation which had been emitted from a distance of 46GLY?

Nothing that distant can be seen.  It is too far away.  Please read what I said in the prior posts about this.

Few more questions:
1. Do you mean that the whole mass of the Universe were at some point of time at a maximal distance of only 1.3 Million LY from each other?

1.3 MLY is the distance from here that the CMB we see now was emitted.  That was never stated to be the radius of the whole universe at that time.

2. If so, do you agree that there is no way to set an infinite Universe in only 13.8 BY?

No.

3. Do you agree that based on the BBT the Universe must be finite?

No.

4. If we could eliminate completely the impact of the BBT on the CMB
What is the expected CMB that we should get with regards to Amplitude, Redshift and isotroic?

The question makes no sense.  The CMB is the big bang itself we are seeing.  Without the BB, there would be no CMB.

5. Do we have any idea what is the estimated amplitude degradation in the CMB per one million year?

It isn't a fixed value per million years, but is very predictable. I don't know the current rate.  A million years is about a 14000th the age of the universe, so I imagine it will lower the CMB temperature somewhere around the 5th digit in or thereabouts.

6. If we could come back to our Universe in one Billion or 10 Billion years from now, then what kind of CMB we might find?

3000K if you go all the way back to when it was emitted.

7. Can we extract from the CMB the total mass of the whole Universe?

No.  We don't know the size and the CMB is only a temperature of a shell of material at a fixed radius, centered on us.

[Halc]

Here is a good image illustrating all the concepts we're talking about.


The upper graph shows proper distance vs time, and the 'visible universe' is seen to be where the particle horizon crosses the 'now' line (which is defined as the age of the universe for a comoving ('stationary') object).  A comoving object is one for which the universe appears to be isotropic.  The solar system motion varies in the long term, but we're currently about 0.0013c from being comoving.  It's more than twice that when we get to the other side of the galaxy.
The lower graph shows the same thing in comoving coordinates, and the black dotted worldlines are just vertical lines in that view.

The event horizon (orange) delimits events which can never have a causal effect on us here even given infinite time.  The event horizon would not be there if the expansion of the universe was not accelerating.

The Hubble Sphere (purple) is the line where comoving objects are increasing their proper distance from us at light speed.  It would be straight if the expansion of space were not accelerating.

The red line is the light cone and represents what we can see.  Every event (every star in the sky, the moon, the CMB and the beer you're holding) is on that red line.  It seems to go furthest to the right at about 3.5 BY and reaches the 5.5 BLY mark.  That's as far as we can see.  R1 is 5.5 BLY.  If the universe has an edge and looks different there, the red line needs to cross it for us to notice it.

The CMB is on that red line and so low on the chart that you cannot make out how close it is to here.  I figure it to be around 1.3 MLY away, but I did not actually read that number anywhere.  The best way to figure that is to use the lower graph.

The black dotted lines are worldlines for comoving objects, and those within the particle horizon touch the red line, and those outside it do not.  The red line is 'what we see', so this difference defines 'the observable universe'.

You will notice that they don't even bother drawing a line to represent events simultaneous with us in our inertial frame.  I challenge you to plot that.  The inability to do so illustrates why special relativity is special and does not apply to our universe.

[Dave Lev (OP)]

Thanks Halc & Evan_au

I do appreciate the answers.

Let me ask the following:
Is there any way, any evidence, any discovery any issue which could convince you that there is a problem with the BBT?

3000K if you go all the way back to when it was emitted.

If we could travel in time:
If we could verify that at the early days (about 13 Billion years ago) the CMB was exactly as it is today, while also 10 Billion years from now in the future, the CMB is also the same.
What can we learn from that?

The CMB is the big bang itself we are seeing.  Without the BB, there would be no CMB.

Why are you so sure that without the Big bang there is no CMB?
For example:
Thermal emission of dust in the Milky way:
https://irsa.ipac.caltech.edu/applications/DUST/docs/background.html
"The dust temperature varies from 17 K to 21 K, which is modest but does modify the estimate of the dust column by a factor of 5".
So, the Milky Way has a thermal emission.
Let's assume that we could set the whole Milky way in some sort of closed sphere or galactic Oven.
In this case, what would be the thermal radiation amplitude in that galactic oven or closed sphere?
Actually, Oven could be a perfect example:
When the door is closed and we operate the oven, we can get over than 220c in the oven. However, once we open the door, the temp goes immediately down. We can set our hand in the oven and we won't feel that supper high temp.
So, I wonder what might be the "CMB" of the Milky Way if we could set it in some sort of a galactic oven.

[Halc]

Let me ask the following:
Is there any way, any evidence, any discovery any issue which could convince you that there is a problem with the BBT?
...
If we could travel in time:
If we could verify that at the early days (about 13 Billion years ago) the CMB was exactly as it is today, while also 10 Billion years from now in the future, the CMB is also the same.
What can we learn from that?

That would be pretty strong evidence for some sort of steady state theory.  If all the galaxies were younger and closer together 13 BY ago, then the steady state theory would be wrong as well.

Why are you so sure that without the Big bang there is no CMB?.

There is a CMB.  It is an empirical fact.  So another theory that accounts for it (and everything else) is another contender.

For example:
Thermal emission of dust in the Milky way:
https://irsa.ipac.caltech.edu/applications/DUST/docs/background.html.
"The dust temperature varies from 17 K to 21 K, which is modest but does modify the estimate of the dust column by a factor of 5".
So, the Milky Way has a thermal emission..

Sure.  So does the sun.  It looks nothing like the CMB.  The picture at the top of that website looks nothing like the CMB picture.

Let's assume that we could set the whole Milky way in some sort of closed sphere or galactic Oven.
In this case, what would be the thermal radiation amplitude in that galactic oven or closed sphere?.

If you put part of it in a closed sphere like that it would get warm in there and there would be no thermal radiation to the outside because the enclosure would reflect it back in.  Removing the enclosure would be something like opening an oven door, yes.

So, I wonder what might be the "CMB" of the Milky Way if we could set it in some sort of a galactic oven.

It would be the Milky Way light background (MWLB).  It has no thermostat like an oven so it would just keep getting hotter as energy is converted to heat.  The galaxy would also be less massive because the enclosure would prevent any new fuel from getting in, so the temperature seems to depend partly on how much it has when you seal it off, and how long you let it cook.

Concerning the graph I posted:

So, if we claim that a radiation from a galaxy with a redshift of 1100 represents a distance of  46GLY, why a radiation which we call CMB with a redshift of 1100 doesn't represents a radiation which had been emitted from a distance of 46GLY?

Galaxies are objects, and almost all of them are nearly comoving.  Objects have vertical lines in the 2nd graph.  There is a comoving distance to each of them.
The CMB is not an object, and so does not have a vertical line.  It is a horizontal line (it happened everywhere) in either graph, at the T=379,000 mark, which is at the bottom. It doesn't exist now because now isn't year 379,000. What we see now is anything at (not within) the red line.  We can see a galaxy only if the vertical black worldline of the galaxy object intersects the red line.  We can see now the CMB at exactly the one point where the red line intersects the horizontal CMB line, which in the first picture is at about 1.3 million light years.

[Kryptid]

Why are you so sure that without the Big bang there is no CMB?

The CMB is extremely uniform (temperature fluctuations from one place to another amount to a mere + 0.00335 kelvins), which means that it isn't radiation emitted by localized sources like stars or galaxies. It must have been emitted by something that once evenly filled all of space.

[Dave Lev (OP)]

Black Body Radiation:

The CMB is extremely uniform (temperature fluctuations from one place to another amount to a mere + 0.00335 kelvins), which means that it isn't radiation emitted by localized sources like stars or galaxies. It must have been emitted by something that once evenly filled all of space.

Wow
That exactly the message which I was looking for.
You claim that:"It must have been emitted by something that once evenly filled all of space"
I would like to add that: It is emitted by something that filled all of space.
In order to understand that, let's look at the Black body radiation.
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Map%3A_Physical_Chemistry_(McQuarrie_and_Simon)/01%3A_The_Dawn_of_the_Quantum_Theory/1.1%3A_Blackbody_Radiation_Cannot_Be_Explained_Classically
It is stated clearly:
"A body emits radiation at a given temperature and frequency exactly as well as it absorbs the same radiation"
Therefore:
"Blackbody radiator is any object that is a perfect emitter and a perfect absorber of radiation."
"So how do we construct a perfect absorber in the laboratory? OK, nothing’s perfect, but in 1859 Kirchhoff had a good idea: a small hole in the side of a large box is an excellent absorber, since any radiation that goes through the hole bounces around inside, a lot getting absorbed on each bounce, and has little chance of ever getting out again. So, we can do this in reverse: have an oven with a tiny hole in the side, and presumably the radiation coming out the hole is as good a representation of a perfect emitter as we’re going to find (Figure  1.1.2 )."
They also have used the same idea of Oven as I did: "have an oven with a tiny hole in the side"
So, if I understand it correctly, in order to get from a black body radiation, all/most of the radiation must stay at the object.
However, once we take it out from the "oven", there is no black body radiation any more.
So, Let's go back to the following statement:

It must have been emitted by something that once evenly filled all of space.

Therefore, at the early time the Big bang took all the early available space. Therefore at the first moment the radiation of the Bang was clearly black body.
However, once we start the inflation and the expansion we actually kill any possibility for black body radiation.
As we contradict the basic idea of "perfect absorber of radiation":
"Blackbody radiator is any object that is a perfect emitter and a perfect absorber of radiation"
Therefore I have stated: "It is emitted by something that filled all of space."
So, in order to get the black boday radiation in the CMB, it must also filled all of space today
Therefore, the idea of the inflation and expansion actually contradicts the possibility for black body radiation.
So, the only way to get this black body radiation is by setting the object in an oven (As I have used in my example).
Therefore, let me go back to my example:

Let's assume that we could set the whole Milky way in some sort of closed sphere or galactic Oven.
In this case, what would be the thermal radiation amplitude in that galactic oven or closed sphere?.

If you put part of it in a closed sphere like that it would get warm in there and there would be no thermal radiation to the outside because the enclosure would reflect it back in.  Removing the enclosure would be something like opening an oven door, yes.

So, you agree that we should get some thermal radiation if we put the Milky way at a galactic oven.
I assume that we will also get a black body radiation - as we fulfill the requirement for:
"Blackbody radiator is any object that is a perfect emitter and a perfect absorber of radiation"
I do not claim that it is feasible to set the Milky way in an oven, but I would like you to look at the impact of this hypothetical activity.
Therefore, do you agree that as long as the Milky Way will be in a galactic oven it will create a "CMB" which carry a black body radiation?
Do you also agree that once we open the oven, the black body radiation will be gone forever?
If so, how could it be that we still get a black body radiation from a CMB while the universe expands?
It seems to me that only if there is some envelope to our universe, than this might keep the black body radiation.
However, if the matter in our universe expands to the open infinity space - I really don't see any possibility to keep the black body radiation in the CMB.
Do you agree with that?
Do you agree that (based on hypothetical idea) if the Milky Way is in a galactic oven it can generate black body radiation?

[Kryptid]

However, once we start the inflation and the expansion we actually kill any possibility for black body radiation.

The radiation was emitted well after the inflationary epoch ended.

So, in order to get the black boday radiation in the CMB, it must also filled all of space today

The CMB does fill all of space today.

[Halc]

It is stated clearly:
"A body emits radiation at a given temperature and frequency exactly as well as it absorbs the same radiation"
Therefore:
"Blackbody radiator is any object that is a perfect emitter and a perfect absorber of radiation."

So, if I understand it correctly, in order to get from a black body radiation, all/most of the radiation must stay at the object.

I know 'understanding correctly' is not your forte, but you just directly contradicted the quote you gave, which says the black body emits as well as it absorbs.  The radiation does not stay at the object.

It must have been emitted by something that once evenly filled all of space.

Therefore, at the early time the Big bang took all the early available space.

It is space.  It doesn't take available space.

Therefore at the first moment the radiation of the Bang was clearly black body.

It isn't an object, so isn't a body at all.  There is nowhere else to send the radiation.  There is no oven or barrier which has an inside and an outside.  That's what is meant by 'everywhere'.

However, once we start the inflation

The start of expansion was 379000 years before, assuming we're talking about the decoupling event that sets up the CMB as we see it.

"Blackbody radiator is any object that is a perfect emitter and a perfect absorber of radiation"
Therefore I have stated: "It is emitted by something that filled all of space."
So, in order to get the black boday radiation in the CMB, it must also filled all of space today

The CMB does fill all of space today. It isn't blackbody radiation because there isn't a body.  Only the light from the thing that happened everywhere remains.  All we detect is the light.  The plasma from which it was emitted is long gone.

If you put part of it in a closed sphere like that it would get warm in there and there would be no thermal radiation to the outside because the enclosure would reflect it back in.  Removing the enclosure would be something like opening an oven door, yes.

So, you agree that we should get some thermal radiation if we put the Milky way at a galactic oven.

See the part I bolded. No, I agreed that you would release pent up thermal radiation if you opened the oven.  Putting it in an oven would serve to block that radiation.  Yes, there would be radiation detected from the inside from the walls of the enclosure.  Perhaps this is what you're asking.

I do not claim that it is feasible to set the Milky way in an oven, but I would like you to look at the impact of this hypothetical activity.
Therefore, do you agree that as long as the Milky Way will be in a galactic oven it will create a "CMB" which carry a black body radiation?

The enclosure is the black body, as viewed from inside?  If it radiates to the outside, is isn't really an enclosure, just a diffuser like frosted glass.  If there was a shell like you describe, it would get hotter in places that receive more radiation than others and the radiation from it would not appear isotropic.  For that, I think the Milky Way would need to be more spherically symmetrical instead of just circularly.
It isn't a model of the CMB because you are modeling a persisting finite object, something that isn't everywhere. Your model has radiation confined to inside an oven, which isn't everywhere.

Do you also agree that once we open the oven, the black body radiation will be gone forever?

No.  Radiation from it would just keep on going unimpeded into space.  Light doesn't stop just because you turned off the source.  It keeps going until it hits something.

If so, how could it be that we still get a black body radiation from a CMB while the universe expands?

The CMB is light, not the stuff from which it came.  We see it because that light is just now finding something to hit.  It isn't black body radiation.  It sort of was before year 379000.

However, if the matter in our universe expands to the open infinity space

No model suggests this.  It is a naive model of the universe as being an object that 'started' at one location instead of everywhere.  Space and time are parts of the universe, not preexisting things in which the universe happened.

- I really don't see any possibility to keep the black body radiation in the CMB.
Do you agree with that?

I never claimed it was black body radiation.  There's no body.  That would be a property of an object, and the universe is not an object, nor is the CMB.

[Dave Lev (OP)]

The CMB does fill all of space today.

How could it be that the CMB fills all space?
If the Universe is finite - that statement might be incorrect as follow:
After any given moment the Universe is bigger. If it is bigger than the CMB covers more space over time. So how can you claim that it fills all space while we know that at any given moment it moves to new space?
As the CMB is radiation - do you agree that it must move at speed of light? Hence, it must move much faster than the expansion in space. While the real matter/galaxies are limited to approximately 70 km/s/Mpc, the CMB is moving at the speed of light.
So, do you agree that the CMB covers much more sphere than our real Universe?
Our scientists only focus on the hypothetical sphere of our real Universe, but what about the Hypothetical sphere of the CMB?
If they are identical - than do you agree that there must be an envelope to our Universe?
If there is envelope, I fully agree that the CMB can carry a black body radiation.
However, what kind of force keeps the CMB in our finite universe (without envelope)?
Why it isn't moving to the infinity in just few moments or years?
What is the possibility for any kind of envelope around our real Universe?
How the CMB could have a black body radiation without an envelope, while it moves to the infinity at the speed of light?

However, if the Universe is infinite - than I fully agree - "the CMB fills all space" to the infinity.
Actually, if the CMB is moving to the infinity at the speed of light, it is still in the infinity.
Therefore, it fills the whole infinite space.
In this case, it can keep its amplitude and it should have black body radiation
Hence, do you agree that the Idea that CMB must fill all of space today, proves that the Universe is infinite?

[Kryptid]

How could it be that the CMB fills all space?

Because it was emitted by a substance that once filled all of space (hydrogen plasma).

After any given moment the Universe is bigger. If it is bigger than the CMB covers more space over time. So how can you claim that it fills all space while we know that at any given moment it moves to new space?

The CMB expands with space.

As the CMB is radiation - do you agree that it must move at speed of light?

Yes.

So, do you agree that the CMB covers much more sphere than our real Universe?

That makes no sense. The Universe in total would represent all of the space there is. If there is a limited amount of space, then the CMB cannot move outside of that limited space (outside of space is an oxymoron).

Our scientists only focus on the hypothetical sphere of our real Universe, but what about the Hypothetical sphere of the CMB?

It's the same, because the CMB fills all of space.

If they are identical - than do you agree that there must be an envelope to our Universe?

That depends on what you mean by an "envelope". There doesn't have to be any kind of solid barrier around the Universe, if that is what you are saying. In a hypersphere universe, for example, space is closed and travel in any one direction will eventually bring you back to your starting point. That represents a limited amount of space but there are no physical barriers present at any point to stop your movement.

However, what kind of force keeps the CMB in our finite universe (without envelope)?

No force, just the fact that you can't move outside of a finite space because there is no space outside of space to move into (that should go without saying).

Why it isn't moving to the infinity in just few moments or years?

If there is no infinity to move to then obviously it can't move there.

What is the possibility for any kind of envelope around our real Universe?

I don't know, but it must be beyond what we can see if it is there.

How the CMB could have a black body radiation without an envelope, while it moves to the infinity at the speed of light?

I don't understand why you think these two points are contradictory.

Actually, if the CMB is moving to the infinity at the speed of light, it is still in the infinity.
Therefore, it fills the whole infinite space.

Unless there is no infinite space in the first place.

Hence, do you agree that the Idea that CMB must fill all of space today, proves that the Universe is infinite?

Absolutely not.

[Halc]

A few comments to add to Kryptid's replies, with which I agree.

So how can you claim that it fills all space while we know that at any given moment it moves to new space?

No viable model has anything moving to 'new space'.  The universe does not occupy a fraction of space.  It is the space.  All of it, be it finite or not.  The plasma once filled all of space, meaning there is nowhere where it wasn't.

So, do you agree that the CMB covers much more sphere than our real Universe?

I want to echo Kryptid that this makes no sense.  There is nowhere that isn't the universe. That would mean there was a place where the plasma wasn't, but the model says it was everywhere.

Our scientists only focus on the hypothetical sphere of our real Universe, but what about the Hypothetical sphere of the CMB?

You are attributing a naive strawman model to scientists. They focus on no such thing. Finite space is usually modeled as the surface of a hypersphere which has no space that is 'outside'

However, what kind of force keeps the CMB in our finite universe (without envelope)?

There is nowhere else to go to get out. That's what finite space means.
You keep picturing an enclosure (delimiting 'universe') with space inside and more outside, but the stuff inside is not all of space then.  It isn't a model of finite space at all.

What is the possibility for any kind of envelope around our real Universe?

I don't know, but it must be beyond what we can see if it is there.

I cannot see it being meaningful. If the 'real' universe is confined to space inside the envelope, then the 'real' universe covers a subset of space.  That isn't a universe then, just an object (an isolated system) in a larger space.

[Dave Lev (OP)]

The Universe in total would represent all of the space there is. If there is a limited amount of space, then the CMB cannot move outside of that limited space (outside of space is an oxymoron).

No viable model has anything moving to 'new space'.  The universe does not occupy a fraction of space.  It is the space.  All of it, be it finite or not.  The plasma once filled all of space, meaning there is nowhere where it wasn't.

Thanks for the explanation.
So, what is the difference between the Universe and space?
I had the impression that the space represents the infinity, as there is no limit in space, while the Universe represents the size of our universe. (Therefore the Universe can take the whole size of the open infinity space or just part of it).
Now I understand that the universe and the space are identical.
Thanks for the explanation.
.
So, let's assume that our Universe/space is finite and go back to year 400,000 - just after the inflation process while the CMB gets already its Black body radiation.
Let's assume that the radius of the Universe/space at that time was only R1 - which represents a compact universe/space.
All the matter of our current Universe had been there in this compact universe/space.
There was nothing outside of this radius.
Due to the expansion in space of 70 km/s/Mpc, the space of our universe had been increased and therefore after 13.4 Billion year from that moment, our current Universe/space get's to its maximal radius of R2.
(while there is still nothing outside of R2 radius).
However, as our Universe/space had been increased at a rate of 70 km/s/Mpc, the CMB was moving in all directions at the speed of light.
Sooner or later all the energy of the CMB must get to the edge of that compact universe while it increases its size at a relatively low speed of only 70 km/s/Mpc.
What should happen with the CMB when it gets to the edge of the Universe/space?

No force, just the fact that you can't move outside of a finite space because there is no space outside of space to move into (that should go without saying).

As there is no space outside the edge, it is clear that the CMB can't cross the edge.
So, is it going to be reflected back from the edge?
If it is reflected back, what kind of element in the edge of that increasing compact Universe force the CMB to do so?
You claim that there is no force and no element or solid barrier around the Universe.

There doesn't have to be any kind of solid barrier around the Universe, if that is what you are saying. In a hypersphere universe, for example, space is closed and travel in any one direction will eventually bring you back to your starting point. That represents a limited amount of space but there are no physical barriers present at any point to stop your movement.

But, the radiation must move in direct line to all directions.
So, how could it be that a radiation/light which starts at point A moves in a direct line to point B, cross that point while it moves in the same direction and eventually gets back to point A without meeting the edge of the Universe?
If that is correct, do you agree that when we look at the left side of the space/Universe, we could see the light/radiation which is coming from the right side of the finite space/universe?
Is it real?

There is nowhere else to go to get out. That's what finite space means.
You keep picturing an enclosure (delimiting 'universe') with space inside and more outside, but the stuff inside is not all of space then.  It isn't a model of finite space at all.

Yes, now I do understand the meaning of Finite space.
However, any finite space must have some sort of size. Therefore, I don't understand how a light can cross a limited space or Finite space without getting to the edge of the space.
If the Universe has a limited/finite size than somewhere there must be an edge, otherwise it should go to the infinity.
Therefore, I can't understand how a finite Universe with a radius R has no edge.
Once we set a sphere for the finite space, we actually also set the edge of that space/sphere.
So, how could it be that there is a finite space with a finite sphere without edge?
This is a real enigma for me.
Would you kindly explain it.
However, if the radius of the Universe is R and there is an edge, at some point of time the CMB must get to that edge.
So, if there is no solid barrier around the Universe how could it be that the CMB doesn't cross the edge?
Can we assume that it is reflected back?
Let's make a simple calculation:
The speed of light is 299 792.458 kilometers / second, while the speed of the expansion is only 70 km/s/Mpc
The ratio between the two is : 4285 : 1
Hence, it is clear that during the last 13.4 Billion years the CMB had been reflected back from the edge of the Universe/space at least million or billion times.
Even if we set at the edge a perfect mirror, there must be some minor energy reduction at every reflection.
So, after all of those reflections, don't you think that the CMB should be virtually zero by now?
How could it be that we still get any sort of energy in the CMB, while it must be reflected again and again from the edge of limited/finite Universe/Space size?

[Halc]

The Universe in total would represent all of the space there is. If there is a limited amount of space, then the CMB cannot move outside of that limited space (outside of space is an oxymoron).

No viable model has anything moving to 'new space'.  The universe does not occupy a fraction of space.  It is the space.  All of it, be it finite or not.

Thanks for the explanation.
So, what is the difference between the Universe and space?

For purposes of the discussion at hand, no difference. But more precisely, the universe is all there is in our spacetime.  Space is 3 dimensional, and spacetime is 4 dimensional.  One can model the universe as 3D with time external to it, but that seems to require the same naive sort of thinking that puts space external to the universe.

I had the impression that the space represents the infinity, as there is no limit in space

Then you are using a model of infinite space. You need to use a different model when considering a finite size universe. You can't use both models at once. Of course they will contradict each other. Try the surface of hypersphere model that is far more mainstream. There's no infinity in that geometry.

while the Universe represents the size of our universe. (Therefore the Universe can take the whole size of the open infinity space or just part of it).

If the universe can take just part of something, it is an object within something larger, and the larger thing is the universe and the thing you're thinking of is just some object. There is no valid model that looks like that.

So, let's assume that our Universe/space is finite and go back to year 400,000 - just after the inflation process while the CMB gets already its Black body radiation.

The CMB is the radiation. There isn't a body that is doing any radiating or absorbing.

Let's assume that the radius of the Universe/space at that time was only R1 - which represents a compact universe/space.
All the matter of our current Universe had been there in this compact universe/space.
There was nothing outside of this radius.

Nor inside either.  The model I am assuming is the surface of a hypersphere, so space at a given moment covers only the surface, and R1 is not measured in a spatial dimension, but rather the time dimension.  The past lies inside R1 and the future lies outside it.  If the past and future do not exist, then time is external to the universe (making it an object of sorts again) and there is nothing inside or outside of R1.

Due to the expansion in space of 70 km/s/Mpc, the space of our universe had been increased and therefore after 13.4 Billion year from that moment, our current Universe/space get's to its maximal radius of R2.

So you know, that's the current rate, which reflects a doubling of size in 12-13 billion years (the age of the universe).  It was much younger then and would double in another 400000 years, so the km/s/Mpc figure back then would be about 30000 times that rate.
I really hate the expression of 70 km/s/Mpc because that makes it sound like a constant thing, but it has no dimensions, just a ratio.  That ratio equals the number of seconds in the age of the universe.  The age of the universe was much less back then, so the (unchanged) expansion rate would be that much greater.

I'm not disagreeing with anything you said since you're not computing anything with that 70 figure, but expansion cannot actually have a rate.

However, as our Universe/space had been increased at a rate of 70 km/s/Mpc, the CMB was moving in all directions at the speed of light.

The CMB would be moving at lightspeed even in non-expanding universe.  The two have little to do with each other.

Sooner or later all the energy of the CMB must get to the edge

There is no edge.  A circle is not a line segment with endpoints.  You go far enough you just get back to where you started.  That makes it a finite size closed dimension.  A hypersphere is like that, except it has 3 spatial dimensions on its surface and looks exactly like what we see.

As there is no space outside the edge,

There is no edge.  An edge sort of implies unoccupied space beyond.  Wrong model.  Yes, I agree that any model with an edge is probably implausible.  I do not know of one that is proposed by scientists, so there is little point in talking about it.

But, the radiation must move in direct line to all directions.
So, how could it be that a radiation/light which starts at point A moves in a direct line to point B, cross that point while it moves in the same direction and eventually gets back to point A without meeting the edge of the Universe?

Yes.  Space on the surface of Earth is finite (else real estate would be less expensive).  Yet there is no direction I can go to get to the edge of it and fall off.  If I go far enough, I get back to where I started.  It isn't any more complicated than that, yet you persist in the flat model with an edge. Yes, such a geometry would be non-Euclidean.

If that is correct, do you agree that when we look at the left side of the space/Universe, we could see the light/radiation which is coming from the right side of the finite space/universe?

If the size was really small, light may have had time to go around, yes.  Maybe all the CMB radiation (the oldest light there is) has done that.  How would you tell?

Is it real?

The model works.  So it is possibly real.

Yes, now I do understand the meaning of Finite space.
However, any finite space must have some sort of size. Therefore, I don't understand how a light can cross a limited space or Finite space without getting to the edge of the space.

If you don't understand the lack of an edge, then you don't understand the meaning of Finite space.

Once we set a sphere for the finite space

A sphere has and edge.  The surface of a hypersphere does not.  Space is not a sphere.

The speed of light is 299 792.458 kilometers / second, while the speed of the expansion is only 70 km/s/Mpc
The ratio between the two is : 4285 : 1

You are relating a speed (units km/s) to an expansion rate (km/s/Mpc).  You cannot compare two numbers in different units.  The result is meaningless.
You also give one figure to 9 digits of accuracy and the other to only 2.  That means the 4285 figure is accurate to only 2 digits.

Edit: Not meaningless.  From that you can conclude that the Hubble sphere has a radius of ~4300 Mpc.  The actual figure is more like 4100.

[Kryptid]

But, the radiation must move in direct line to all directions.
So, how could it be that a radiation/light which starts at point A moves in a direct line to point B, cross that point while it moves in the same direction and eventually gets back to point A without meeting the edge of the Universe?

All "straight" lines in a hypersphere-shaped universe are actually curved. They eventually loop back around on themselves. It's like the lines of longitude and latitude on a globe. Walk far enough on the Earth, and you'll end up back where you started (ignoring the fact that the oceans are in the way, of course). It's a similar idea.

If that is correct, do you agree that when we look at the left side of the space/Universe, we could see the light/radiation which is coming from the right side of the finite space/universe?

If such a universe was small enough, old enough and expanding slowly enough, yes. If not, then the light either hasn't had time to make a complete circuit around the universe yet or the universe is expanding too quickly at those distances for it to ever complete the journey.

However, any finite space must have some sort of size. Therefore, I don't understand how a light can cross a limited space or Finite space without getting to the edge of the space.
If the Universe has a limited/finite size than somewhere there must be an edge, otherwise it should go to the infinity.
Therefore, I can't understand how a finite Universe with a radius R has no edge.

The Earth is finite in size. If you walk far enough, will you reach the edge of the Earth? No, you won't. There is no edge. It's a similar idea to a hypersphere universe.

[Dave Lev (OP)]

Nor inside either.  The model I am assuming is the surface of a hypersphere, so space at a given moment covers only the surface, and R1 is not measured in a spatial dimension, but rather the time dimension.  The past lies inside R1 and the future lies outside it.  If the past and future do not exist, then time is external to the universe (making it an object of sorts again) and there is nothing inside or outside of R1.

I'm willing to accept any model that you wish.
However, I can't understand how we can use one Universe modeling for the CMB and other modeling for the expansion.
If we wish to explain the expansion based on three dimensions x,y,z - and one more for -t, than we must also to use the same modeling also for the CMB.
If we wish to explain the CMB based on two diminutions x,y (the surface of a hypersphere) - and one external for -t, than we have to use the same modeling also for the expansion.
We are living in one Universe. This Universe is the home for the expansion and the CMB (and also for us).
Therefore, I assume that we can't use different modeling for each one.
Our scientists have stated that the expansion rate today is 70 km/s/Mpc
If I understand it correctly - The Mpc represents a cube with x = 1Mpc, y = 1Mpc = 1Mpc.
Therefore, they have actually set the Universe in three dimensions x,y z.
If that is correct - they must explain/prove the CMB based on those three dimensions.
If they insist to prove the CMB based on only two dimensions ( the surface of a hypersphere), then please set/prove the expansion based on the same "surface of a hypersphere" modeling.

If that is correct, do you agree that when we look at the left side of the space/Universe, we could see the light/radiation which is coming from the right side of the finite space/universe?

If the size was really small, light may have had time to go around, yes.  Maybe all the CMB radiation (the oldest light there is) has done that.  How would you tell?

I don't understand how can we force the radiation/light to move around the surface of a hypersphere when we know that it must move in a direct line?
In any case, if there is a size to our Universe/space, do you agree that it also must have a volume? If there is a volume, then there must be three dimensions to this volume.
So, don't you agree that we must explain the CMB radiation based on the real modeling for three dimensions?

But, the radiation must move in direct line to all directions.
So, how could it be that a radiation/light which starts at point A moves in a direct line to point B, cross that point while it moves in the same direction and eventually gets back to point A without meeting the edge of the Universe?

All "straight" lines in a hypersphere-shaped universe are actually curved. They eventually loop back around on themselves. It's like the lines of longitude and latitude on a globe. Walk far enough on the Earth, and you'll end up back where you started (ignoring the fact that the oceans are in the way, of course). It's a similar idea.

You actually contradicts the Fermat’s principle.
http://scipp.ucsc.edu/~haber/ph5B/fermat09.pdf
Fermat’s principle states that “light travels between two points along the path
that requires the least time, as compared to other nearby paths.

The Earth is finite in size. If you walk far enough, will you reach the edge of the Earth? No, you won't. There is no edge. It's a similar idea to a hypersphere universe.

There is no obligation how do we walk. We can walk in three dimensions, two dimensions or even one dimension.
However, light must move in a direct line and in the least time!
We can't band the light/radiation just because it meet our expectation.
So, if we shot a laser beam in one direction - it won't come back to us unless it meets the law of Reflection and Refraction!
Hence, we can't assume that light/radiation can "walk" on the hypersphere universe and come back without any real impact as Reflection and Refraction.

If such a universe was small enough, old enough and expanding slowly enough, yes.

OK
But, we have to find a way how to overcome on the  Fermat’s Principle and the Laws of Reflection and Refraction.

If not, then the light either hasn't had time to make a complete circuit around the universe yet or the universe is expanding too quickly at those distances for it to ever complete the journey.

How can you keep the energy in the CMB if the Universe expands at the speed of light and the radiation is moving away in all directions?
Don't you agree that in just few moments after the bang - any bang, the radiation should go to zero?
How can you get any sort of radiation after 13.8 Billion years from the bang, while nothing from the radiation is reflected back?

[Kryptid]

You actually contedicts the Fermat’s principle.
http://scipp.ucsc.edu/~haber/ph5B/fermat09.pdf
Fermat’s principle states that “light travels between two points along the path
that requires the least time, as compared to other nearby paths.

Exactly. The shortest path in a curved space is therefore a curved line.

There is no obligation how do we walk. We can waltk in three dimentions, two dimentions or even one dimention.

I'm obviously talking about walking on the ground. Any path you choose, when walking without veering to the left or right, would eventually put you back where you started.

However, light must move in a direct line and in the least time!

Least time? Yes. Straight line? No. Gravitational lensing bends light so that it doesn't travel in a straight line. The path that will take the least time to traverse in a curved (non-Euclidean) space is a curved line.

We can't band the light/radiation just becouse it meet our exectation.

I don't understand your use of the word "band" in this sentence.

So, if we shot a lazer beam in one direction - it woun't come back to us unless it meets the law of Reflection and Refraction!
Hence, we can't assume that light/radiation can "walk" on the hypersphere universe and come back without any real impact as Reflection and Refraction.

The laser beam is traveling through curved space, just like the curved space around massive objects that causes gravitational lensing. That is what would make the beam go all the way around the universe and come right back to its starting point.

Hence, we can't assume that light/radiation can "walk" on the hypersphere universe and come back without any real impact as Reflection and Refraction.

Yes we can assume that. If the space is curved, the light has no choice but to curve along with it.

OK
But, we have to find a way how to overcome on the  Fermat’s Principle and the Laws of Reflection and Refraction.

There is nothing to overcome. Neither of those things are violated.

How can you keep the energy in the CMB if the Universe expands at the speed of light and the radiation is moving away in all directions?

The first law of thermodynamics guarantees that such energy cannot go to zero. It can become more diffuse as the universe expands, but that's all.

Don't you agree that in just few moments after the bang - any bang, the radiation should go to zero?

No, because that would break the first law of thermodynamics.

How can you get any sort of radiation after 13.8 Billion years from the bang, while nothing from the radiation is reflected back?

Why is there any need for reflection? Absolutely all of space is filled with the CMB. In any given volume of space, CMB photons are leaving it, but other CMB photons are entering it.

[Halc]

I'm willing to accept any model that you wish.

Obviously not.

However, I can't understand how we can use one Universe modeling for the CMB and other modeling for the expansion.

The BBT (a theory of the time evolution of the universe) explains the expansion and the CMB.  That theory does not conflict with various models of the geometry of the universe.

If we wish to explain the expansion based on three dimensions x,y,z - and one more for -t, than we must also to use the same modeling also for the CMB.

The xyz-t thing has been around for centuries and was probably mainstreamed with Minkowski's and Einstein's work at the beginning of the 20th century.  The BBT theory came later when the expansion was noticed, and the discovery of the CMB later still.  So the 4-D spacetime model had nothing to do with explaining these yet unknown things.

If we wish to explain the CMB based on two diminutions x,y (the surface of a hypersphere)

A hypersphere is a 4 dimensional object with a 3 dimensional surface.  You're thinking of a surface of a sphere here.

and one external for -t, than we have to use the same modeling also for the expansion.

Yes, the expansion also would need to be covered by it.  You will note that the 4D model was not falsified by the subsequent discovery of the expansion.  The 4D model also does not assert a finite or infinite space.  Those are variants, and since both are valid, there seems to be no obvious evidence to answer the question of the universe being infinite or not.

We are living in one Universe. This Universe is the home for the expansion and the CMB (and also for us).
Therefore, I assume that we can't use different modeling for each one.

I use any combination of models that don't contradict each other.  I cannot use a finite and an infinite model at the same time for instance.  But each of these theories might make different assertions, but they don't contradict each other.  They can all be true at once.
BBT says time has a singularity at a known point in the past and has evolved to its current state today.
4D model says time is part of the universe just like space.The two are essentially the same thing and can be measured in common units.
3D model says time is external to the universe, and the universe exists within it.  That means there is a larger container, and the universe is reduced to being more like an object, not really the same definition of 'universe' at all.  3D and 4D models are mutually contradictory, but either works with BBT.

Our scientists have stated that the expansion rate today is 70 km/s/Mpc
If I understand it correctly - The Mpc represents a cube with x = 1Mpc, y = 1Mpc = 1Mpc.

No.  A Mpc is a linear distance, not a measure of volume.  A liter is a measure of volume, and I'm not sure if they have a unit for cosmological scale volume other than say a Hubble volume with is a function of time, so not really a unit.
The current expansion rate is measured as a percentage increase in linear separation of points in space over time.  Any such specification can be reduced to a statement of the age of universe.  If you halved or doubled the expansion rate, the rate today would still be 70 m/s/Kpc.  So it isn't really a measure of a rate, just a measure of time.

I don't understand how can we force the radiation/light to move around the surface of a hypersphere when we know that it must move in a direct line?

Please familiarize yourself with non-Euclidean geometry. Don't criticize the mathematics that you clearly don't understand. The lines are very much straight lines.
Kryptid attempts some basics in his reply.

In any case, if there is a size to our Universe/space, do you agree that it also must have a volume?

Yes.  A finite 3D space has a meaningful finite volume.  An infinite space does not.  Earth has finite radius and thus a finite surface area.
I think the finite spacetime model would have to have a radius computed a way comparable to the Schwarzschild radius where they take the distance required to get back to the staring point and then divide by 2π, which may or may not be the same value as the length in the radial direction.  In the case of Schwarzschild coordinates, the radial distance is not meaningful.
Translation:  The volume of a hyperspherical universe is best expressed as a function of its circumference, not of its radius.

If there is a volume, then there must be three dimensions to this volume.

Yes. The model would be falsified if it didn't have 3 macro spatial dimension like we see.

So, don't you agree that we must explain the CMB radiation based on the real modeling for three dimensions?

This has nothing to do with the CMB.  The model was showing a finite volume with no edge. Yes, the CMB needs to be present equally at every point in that volume (in any of the 3D/4D finite/infinite models) at a fixed moment in absolute time.  It is not equal at every point in time, so one can measure a crude age of the universe just by looking at the CMB.

You actually contradicts the Fermat’s principle.
http://scipp.ucsc.edu/~haber/ph5B/fermat09.pdf
Fermat’s principle states that “light travels between two points along the path
that requires the least time, as compared to other nearby paths.

Non Euclidean space does not violate this principle, as Kryptid points out.

There is no obligation how do we walk. We can walk in three dimensions, two dimensions or even one dimension.

Yes there is obligation.  You are confined to space, and cannot leave it.  Light is similarly confined.

How can you keep the energy in the CMB if the Universe expands at the speed of light and the radiation is moving away in all directions?

The energy density goes down as the volume expands, but the total CMB energy only goes down when the radiation is absorbed by objects it encounters.

How can you get any sort of radiation after 13.8 Billion years from the bang, while nothing from the radiation is reflected back?

How could you not? Suppose I do it in reverse, tracing back light today into a contracting universe in reverse time.  In almost 13.8 billion years (down to year 379000) that light will be somewhere in (much smaller) space, and since the plasma was everywhere in space back then, there will be plasma there.  That's where the CMB we see now comes from.

[Dave Lev (OP)]

Thanks Halc & Kryptid
I do appreciate your efforts to explain the issue.
So, yes, now I have better understanding about the difference in the modeling.

Exactly. The shortest path in a curved space is therefore a curved line.

Let's assume that we are located just at the center of the Space/Universe which its radius is 52 BLY.
We see that all the furthest galaxies in all directions are moving away from us at ultra high speed.
Let's draw a line (in one direction) to the infinity.
If I understand it correctly, this line should come back to us, although we have draw a direct line to the infinity.
Let me use the following scenario:
At some point in this line to the infinity, we should get to the furthest galaxy that we still can see - Let's assume that it is located 13 BLY away.
This galaxy (Galaxy A) is moving away from us at almost the speed of light.
Now, let's assume that we can jump and be there in less than one second.
Once we are at galaxy A, do you agree that we should see a similar view as we see from our galaxy?
So, we should see that all the furthest galaxies in all directions are moving away from us at ultra high speed.
If we will continue with the same line that we have set to the infinity, we should get to another furthest galaxy (galaxy B).
So, it is located 13 BLY away from Galaxy A and 26 BLY away from us.
If we continue to jump on the same direction from galaxy to galaxy (while each jump represents 13 BLY)  - after 4 jumps we should get to 52 BLY away from us.
Let's assume that this point represents the last furthest galaxy in this direction.
So, what kind of view shall we see from this last galaxy?
What will happen if we will try to jump in the same direction that we have set?
Based on the curvature, does it mean that we should make a U turn at the edge of the space (however - now I know that there is no edge) and come back to galaxy C then B, A and finely to our galaxy?
All of that while we think that we still move in the same direction to the infinity.
So, do you mean that this scenario sets the shortest path in a curved space?
Hence, even if we are actually always jumping in one direction, eventually we are coming back to the starting point.
If that is correct, I must say that it is very difficult for me to accept this idea.

[Halc]

Let's assume that we are located just at the center of the Space/Universe which its radius is 52 BLY.

That's a sphere.  Unrealistic.
If space is the surface of a hypersphere, you can't be at the center since that's not the surface.

If I understand it correctly, this line should come back to us

You are picturing a sphere with a radial line coming out.  No, that ends when it hits the edge.

At some point in this line to the infinity

I'm going to assume we're talking about a finite model without infinity.  That means if you go far enough, you get back to the start just like I do when I walk west.

we should get to the furthest galaxy that we still can see - Let's assume that it is located 13 BLY away.

We can't see that galaxy since we can see zero distance in the present.  But it's there, sure.  What we see is light from events in the past, and if you look beyond 13 billiion years into the past, the galaxies have just not yet had time to form.  That furthest object we see is incredibly young.

This galaxy (Galaxy A) is moving away from us at almost the speed of light.

Maybe more.  Depends how you measure that 13 BLY.

Now, let's assume that we can jump and be there in less than one second.
Once we are at galaxy A, do you agree that we should see a similar view as we see from our galaxy?

Yes.  Principle of mediocrity says there is a similar view from anywhere.  An isotropic CMB and uniform sprinkle of galaxies in every direction.

If we continue to jump on the same direction from galaxy to galaxy (while each jump represents 13 BLY)  - after 4 jumps we should get to 52 BLY away from us.

And more beyond that, yes.

Let's assume that this point represents the last furthest galaxy in this direction.

No, that violates the principle of mediocrity.  You persist in positing an edge.  There is none.  You posited a radius of 52 BLY, so the circumference would then be ~325 BLY.  If you go that far, you get back here.

So, what kind of view shall we see from this last galaxy?
What will happen if we will try to jump in the same direction that we have set?
Based on the curvature, does it mean that we should make a U turn at the edge of the space (however - now I know that there is no edge) and come back to galaxy C then B, A and finely to our galaxy?

No edge, last galaxy, nor reflections, u-turns, or any of this stuff.  It looks like it does here from anywhere.  How many times do we have to repeat this?

All of that while we think that we still move in the same direction to the infinity.

Yes, just like I can walk west forever and never find the end of it.

So, do you mean that this scenario sets the shortest path in a curved space?

It sets a straight path.  That path isn't always the shortest in non-euclidean space.  I can fly from London to Paris the wrong way around and the path won't be the shortest, but the airplane won't need to turn to do it.  There are only two such paths in this case.  Any other either needs to turn or doesn't end up in Paris.

Hence, even if we are actually always jumping in one direction, eventually we are coming back to the starting point.
If that is correct, I must say that it is very difficult for me to accept this idea.

As I said before, familiarize yourself with non-euclidean geometry. That or stop being incredulous about its properties. I cannot help it that your education falls short of this point. They teach it in high school.

[Kryptid]

Based on the curvature, does it mean that we should make a U turn at the edge of the space (however - now I know that there is no edge) and come back to galaxy C then B, A and finely to our galaxy?

No, you would not make a sudden U-turn at any point and there is no edge of space where you could even make such a U-turn. The curvature is not sudden like that. You are traveling through curved space at all times along the journey, just like you are walking over the curved surface of the Earth at all times if you tried to walk around the globe. At no point do you turn around in 3-dimensional space just like at no point do you turn around when you walk around the Earth. Nor do you encounter any edge of space anymore than you encounter an edge to the Earth.