The observable universe, the actual uinverse, and CMB

[curioushuman]

Hi all. I relish hearing from our great cosmological explainers like Neil DeGrasse Tyson and Brian Greene and watch whenever I find something new on youtube, but one thing that I don't understand and haven't heard anyone specifically address is that, if the part of the universe we are able to see is smaller than the actual universe (owing to space expanding faster than the speed of light over all or part of the last 13 billion years), then how is it possible for us to see the cosmic background radiation (CMB) in all directions? Doesn't the CMB radiation represent the furthest back (in time) that we can detect how our universe would appear? And if that's the case, if we are able to see THAT in all directions and if that is the oldest thing we could possibly see, how could there be something beyond it (more stars and galaxies) that we are not seeing?

 

[mfb]

What we currently see as CMB is light emitted ~13.7 billion years ago, and the particles which emitted them are now in galaxies similar to those we see close to us.
On big scales, space is uniform in all directions, so we see (nearly) the same spectrum everywhere.
Where is the problem?

Imagine a toy universe - like our current one, but 1 day ago no light existed. You would see earth, sun (8 light minutes away), the planets, but no stars. They would be there, but light from them had not enough time to reach us yet.
Expansion of space makes calculations more complicated, but the concept stays the same.

 

[Naty1]

Hi curioushuman:

Cosmology is even crazier than general relativity! Fun, but not entirely intuitive. One needs to develop some new ways of thinking.

...if the part of the universe we are able to see is smaller than the actual universe (owing to space expanding faster than the speed of light over all or part of the last 13 billion years), then how is it possible for us to see the cosmic background radiation (CMB) in all directions?

Actually, we can 'see' out about 45 billion light years, so maybe this makes your problem appear worse! ! That limit is growing rapidly as relic radiation comes in from more and more distant matter. But the entire universe still extends further!

It is called the particle horizon {PH} and it is the distance TODAY of the matter which emitted light or other radiation about 13.7 billion years ago. So the PH is the distance of farthest matter we could in principle receive signals from now...what we see
are not galaxies nor stars [as exist nearby and also at that distance] because this relic radiation reflects the ionized state of the universe as it was about 13.7 billion years ago.

If you have seen stuff like 'in the beginning the universe was the size of a very dense pea' what is probably being referred to is the then visible part of the universe, the small part we can now see out to the origin of the CMB. The entire universe at that time extended past that radius perhaps even infinitely.

In other words, the initial bang was not ever at a point in space...

Cosmological 'measures', more 'calculated results' really, took me a long time to begin to understand. If you want some further insights try checking out the illustration and explanations here:

http://en.wikipedia.org/wiki/Metric..._two_points_measured_if_space_is_expanding.3F

Here is a discussion you may find helpful explaining the 'balloon analogy' and some of its limitations... from a poster here which resulted from discussions in these forums.

www.phinds.com/balloonanalogy

 

[curioushuman]

Hi mfb,

Everything you say makes perfect sense to me. To cast my issue a different way, when I see CMB (with the help of a special viewing instrument), the electromagnetic radiation entering my eyes (or my instruments "eyes") departed from its source close to 13 billion years ago. Stars and galaxies did not yet exist. When I see stars and galaxies looking out at the night sky, the light entering my eyes departed from its source anywhere from 4 years (a nearby star) to 10 billion years (the furthest galaxy -- I'm not sure what the facts are regarding when stars first began to form). If there are stars that are more than 13 billion (or 45 billion, to incorporate Naty1's point) light-years away from me, there's no way I could see them. So why wouldn't that apply to the CMB as well? Electromagnetic radiation is electromagnetic radiation; there's nothing special about it's being in the microwave range (AFAIK). Both the CMB and the visible light from stars originated from physical material that is now a certain distance from us. So wouldn't the CMB have gotten stretched out of view with the expanding universe just as the furthermost galaxies may have?

 

[Chronos]

Distances in cosmology are confusing. The CMB photons we now detect were emitted at a proper distance of about 42 million light years. The source of those photons are now at a proper distance of about 46 billion light years. What is popularly reported, even in professional papers, is the light travel time distance as determined by the redshift of photons we currently observe. This actually has no physical meaning, other than to compare the 'age' of light received from distant objects to the age of the universe. See http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html for discussion. And yes, the size of distant galaxies [and the CMB] is distorted by the expansion of the universe. See http://spiff.rit.edu/classes/phys443/lectures/classic/classic.html for discussion.

 

[mfb]

If there are stars that are more than 13 billion (or 45 billion, to incorporate Naty1's point) light-years away from me, there's no way I could see them. So why wouldn't that apply to the CMB as well?

It does apply to the CMB as well - there are CMB photons which did not reach us yet (and even photons which will never reach us with an accelerated expansion), exactly like stars. In other words, you'll still see the CMB tomorrow.

The only special thing about the CMB is its age - it has the "oldest" photons still existing.

 

[Naty1]

curious,
your first question:

Doesn't the CMB radiation represent the furthest back (in time) that we can detect how our universe would appear?

your second question:

So wouldn't the CMB have gotten stretched out of view with the expanding universe just as the furthermost galaxies may have?

You know these are contradictory, right??

[Regarding your second question, you might read about the Hubble radius...or Hubble sphere...the distance at which our models and calculations show recession at the speed of light, but NOT the distance of observation in the future. Redshift does not go to infinity for objects on our Hubble sphere and for many cosmological models we can see beyond it. There ARE some stars and galaxies beyond that distance that we WILL be able to see...others are so far away we'll never see them. ]

The one line answer is to your original question is: The furthest most portions of the universe have always been beyond our ability to detect, from the moment of the bang...and continuing now and into the future.

 

[curioushuman]

Naty1,

About the contradiction, yes, that was intentional on my part. That's my conundrum. They both can't be true. That is, I don't understand how it's possible for us to see the CMB radiation IF (and everyone seems to accept it as a reasonable suggestion) we can see only a portion of the universe. As you say in your one line answer, "The furthest most portions have always been beyond our ability to detect...". That's OK, but isn't what caused the CMB radiation IN that furthest most portion? If it is, then we shouldn't be able to see it either, but we do, obviously. So I don't understand how people can logically say that there are portions of the universe we can't see when the CMB radiation --the oldest and furthest thing we could conceivably see-- can, in fact, be seen in all directions.

 

[Naty1]

As you say in your one line answer, "The furthest most portions have always been beyond our ability to detect...". That's OK, but isn't what caused the CMB radiation IN that furthest most portion?

you are repeating the same claim/assumption and I am repeating
the same answer...no.

 

[Drakkith]

Naty1,

About the contradiction, yes, that was intentional on my part. That's my conundrum. They both can't be true. That is, I don't understand how it's possible for us to see the CMB radiation IF (and everyone seems to accept it as a reasonable suggestion) we can see only a portion of the universe. As you say in your one line answer, "The furthest most portions have always been beyond our ability to detect...". That's OK, but isn't what caused the CMB radiation IN that furthest most portion? If it is, then we shouldn't be able to see it either, but we do, obviously. So I don't understand how people can logically say that there are portions of the universe we can't see when the CMB radiation --the oldest and furthest thing we could conceivably see-- can, in fact, be seen in all directions.

There is no "furthest most portion" unless you are talking about the OBSERVABLE universe. The universe is possibly infinite in size. The CMB that we see today was emitted 13.7 billion years or so ago and has traveled for 45 billion light years to reach us. When it was emitted it was a mere 42 million light years from us at the time. HOWEVER, there was still plenty of universe beyond 42 million light years that also emitted the CMB. We just can't see that part of the CMB yet. The photons emitted from 43 million light years away at the time have yet to reach us.

 

[mfb]

I don't see the problem. Try to imagine the universe going backwards in time, and keep track of the CMB radiation we currently see - in the reverse direction, it is like a light signal we emit today. After you went back 13.7 billion years, this light is somewhere - at a sphere with some fixed radius (I forgot the value again, I think it was something like 40 million light years). But the universe is larger than this radius.

Edit: Oh, Drakkith has the value.

 

[Naty1]

Drakkith...

There is no "furthest most portion" unless you are talking about the OBSERVABLE universe...

sure there is, no I am not talking observable but what is BEYOND observable...
Probably I should have referred to the cosmic event horizon ...but that seems a bit too complicated here.

...HOWEVER, there was still plenty of universe beyond 42 million light years that also emitted the CMB.

we agree...that is exactly the idea I already described in my first post [where I also indicated the universe might have been in finite already] and referred to as "...furthermost portions...'...beyond what has ever been or will ever be observable...

but the OP seems not to have bought that yet...

you can take it from here! [LOL]

 

[Drakkith]

sure there is, no I am not talking observable but what is BEYOND observable...
Probably I should have referred to the cosmic event horizon ...but that seems a bit too complicated here.

You've lost me Naty. I don't know what you're trying to say here.

 

[Tanelorn]

All the CMB radation that we are viewing today here on Earth was emitted from a sphere of particles with a radius of 42 million light years around 13.7 Billion years ago.
Every particle in the whole universe emitted this radiation but this is the only CMB radiation that can be viewed today.
Radiation from closer particles has already passed us by and radiation from much more distant particles can never reach us.
This original sphere of particles is today expanded in size to a radius of 45 Billion light years, far beyond what can ever be viewed again.
I thought that this video of the various scales and sizes was useful for newcomers.
I am not sure if it is fully accurate but it is a good visualization. Use the mouse wheel.http://htwins.net/scale2/

 

[curioushuman]

No one here seems to understand the question I have regarding this matter. Let me try putting it in a more formal form:
The agreed upon "fact" part:
1) The CMB shows how our universe was configured at its earliest moment in time, before stars began to form. Just as visible light is emitted by stars, microwaves were emitted by our universe at its earliest stage. This is the CMB.
2) Evidence suggests that our universe experienced a period of inflationary expansion that far exceeded the speed of light. Expansion is still under way, and this expansion even now may be exceeding the speed of light.
3) As a direct result of #2, there must be regions of space --full of stars and galaxies-- that we will never see. And someday, if the expansion keeps up, we won't be able to see anything beyond our own galaxy.

What I can't reconcile:
4) The fact that we can see the CMB in all directions implies that we are able to see the earliest universe in all directions because the CMB is the electromagnetic radiation output from our universe in its earliest form.
5) Anything of an age less the age of our universe at the time the CMB was produced (which would include every star or gas cloud ever created in the history of our universe) would have to be physically interposed between us and the CMB. (I think this is what's being overlooked here. I think people treat the CMB as something special and not subject to the vicissitudes of space expansion --eg, getting so far removed as to not be visible.)
6) If we can see A, and if B is between us and A, then we can also see B.
7) All the stars EVER created would, therefore, have to be between us and the CMB.
8) If a star is between us and the CMB, then, in principle, we should be able to see it.
9) From 7 and 8, There there cannot be regions of our universe containing stars that are too remote to see.

 

[marcus]

No one here seems to understand the question I have regarding this matter. Let me try putting it in a more formal form:

==quote==
The agreed upon "fact" part:
1) The CMB shows how our universe was configured at its earliest moment in time, before stars began to form. Just as visible light is emitted by stars, microwaves were emitted by our universe at its earliest stage. This is the CMB.
==endquote==
No not at "earliest moment in time" it was emitted at estimated 380,000 years after start of expansion.
No, not true that "microwaves were emitted", the hot gas at that time emitted an orangish glow that was a mix of visible and infrared. Like the light from a star somewhat cooler than the sun.
The wavelengths got stretched out (along with distances) so that light eventually became the microwaves that we see.

==quote==
2) Evidence suggests that our universe experienced a period of inflationary expansion that far exceeded the speed of light. Expansion is still under way, and this expansion even now may be exceeding the speed of light.
==endquote==

That is CORRECT! In fact most of the galaxies which we see today were receding faster than light at the time they emitted the light which we are now receiving. Your first job, if you want to learn real cosmology rather than "popular explainer" version, could be to understand how that happens to grasp why it is so common for us to today be getting light from objects whose distance from us was, and still is today, increasing faster than light.

This is explained in many threads here at cosmo forum and also in the "charley" link in my signature. Or you can ask about it.

==quote==
3) As a direct result of #2, there must be regions of space --full of stars and galaxies-- that we will never see. And someday, if the expansion keeps up, we won't be able to see anything beyond our own galaxy.
==endquote==
The reasoning is INCORRECT because the conclusion (while partly true) is NOT "as a direct result of #2". The existence of a cosmic event horizon depends on the acceleration discovered in 1998.

==quote==
What I can't reconcile:
4) The fact that we can see the CMB in all directions implies that we are able to see the earliest universe in all directions because the CMB is the electromagnetic radiation output from our universe in its earliest form.
5) Anything of an age less the age of our universe at the time the CMB was produced (which would include every star or gas cloud ever created in the history of our universe) would have to be physically interposed between us and the CMB. (I think this is what's being overlooked here. I think people treat the CMB as something special and not subject to the vicissitudes of space expansion --eg, getting so far removed as to not be visible.)
6) If we can see A, and if B is between us and A, then we can also see B.
7) All the stars EVER created would, therefore, have to be between us and the CMB.
8) If a star is between us and the CMB, then, in principle, we should be able to see it.
9) From 7 and 8, There there cannot be regions of our universe containing stars that are too remote to see.
==endquote==

Every day the source material of the CMB is a little different, a little farther away on average. Right now the CMB we are getting comes from material which WAS 41 MILLION lightyears from our matter when it emitted the light and which IS NOW 45 BILLION lightyears from us. The distance to the source matter has increased by a factor of about 1100. The wavelengths of the light have increased by the same factor while they have been traveling.

To repeat, each day it is different matter, on average slightly more distant matter, that we see when we look at the CMB.

The location of the matter that emitted the CMB we currently receive is called the SURFACE OF LAST SCATTERING. It has a finite thickness but it is kind of a spherical shell around us with a radius of 45 billion lightyears. this is essentially or effectively the BOUNDARY OF OUR CURRENTLY OBSERVABLE UNIVERSE.

the evidence suggests that there is lots and lots and lots of universe outside that spherical shell.
Of course the shell is growing outwards. In a thousand years our currently observable universe will be bigger. But percentagewise it doesn't amount to much. This radius figure of 45 billion lightyears is only increasing at about 4 times the speed of light---3c of that being due to expansion. So a thousand years wouldn't make much difference.

 

[Naty1]

Marcus:

The location of the matter that emitted the CMB we currently receive is called the SURFACE OF LAST SCATTERING. ... this is essentially or effectively the BOUNDARY OF OUR CURRENTLY OBSERVABLE UNIVERSE...the evidence suggests that there is lots and lots and lots of universe outside that spherical shell.

yes,

and my prior point has been: There has ALWAYS been 'lots and lots' beyond our ability to observe...[the 'furthest most points' is the term I used] and always will be...

of course at 380,000 years after the bang, there were not the stars and galaxies that have now formed over billions of years, but there WERE the gravitational perturbations, plasma, or 'hot gas' to use Marcus' term.

Back then, the boundary of the observable universe would have been about 42 m ly...a tiny portion of what likely existed then; today that 'observable size' has expanded to 45B ly...and the CMBR long with it. [That's the approximate distance/size increase Marcus referred to: 1100 or so]

 

[curioushuman]

Thanks for going through that, Marcus. We are in agreement on my points #1-3. By "earliest" I just meant "way near the beginning" and 380K is fine. I didn't know that the CMB got turned into microwaves due to being stretched so much (I thought they just came out of the "soup" that way), but that's fine. And I didn't know that the cosmic event horizon was entirely the result of the recent uptick in our universe's expansion (I though the inflationary period could have caused a lot of that), but that's fine.

And what you have to say about the "thin wall" of the CMB 45 billion light years out there sounds perfectly reasonable to me. And the "lots and lots" of evidence for there being whole lots of space out there that, in principle, we can never see completely chimes in with what I've learned about cosmology. So we're on the same page there too.

I posted because I see a logical problem in these ideas, and I'm very curious if it has occurred to others (and how they then resolved it) or if there's an error in my analysis or some assumption I'm making (the most likely explanation!) that's made this seem to be a problem when it's really not. That's why I listed #4-9 carefully. Statement #9 and #3 contradict one another and cannot both be true. That's the issue here for me. We both agree #3 is true. So where am I going wrong in #4 to #9?

 

[mfb]

#5 and #7 are wrong, if you mean "the CMB radiation we currently see" with "the CMB". Note that every point in the universe emitted the radiation now known as CMB at that time.
If you mean something else with "the CMB" as position, I don't understand what (and it is probably wrong).

 

[marcus]

I want to amplify what Mfb said about #5 being wrong. At the end I will insert some extra words so as to make #5 a correct statement.

==quote==
What I can't reconcile:
...
5) Anything of an age less the age of our universe at the time the CMB was produced (which would include every star or gas cloud ever created in the history of our universe) would have to be physically interposed between us and the CMB. (I think this is what's being overlooked here. I think people treat the CMB as something special and not subject to the vicissitudes of space expansion --eg, getting so far removed as to not be visible.)
...
==endquote==

Every day the source material of the CMB is a little different, a little farther away on average. Right now the CMB we are getting comes from material which WAS 41 MILLION lightyears from our matter when it emitted the light and which IS NOW 45 BILLION lightyears from us. The distance to the source matter has increased by a factor of about 1100. The wavelengths of the light have increased by the same factor while they have been traveling.

To repeat, each day it is different matter, on average slightly more distant matter, that we see when we look at the CMB.

The location of the matter that emitted the CMB we currently receive is called the SURFACE OF LAST SCATTERING. It has a finite thickness but it is kind of a spherical shell around us with a radius of 45 billion lightyears. this is essentially or effectively the BOUNDARY OF OUR CURRENTLY OBSERVABLE UNIVERSE.

the evidence suggests that there is lots and lots and lots of universe outside that spherical shell.
Of course the shell is growing outwards. In a thousand years our currently observable universe will be bigger. But percentagewise it doesn't amount to much. This radius figure of 45 billion lightyears is only increasing at about 4 times the speed of light---3c of that being due to expansion. So a thousand years wouldn't make much difference.

we are currently getting the CMB radiation from matter that was 41 million ly from our matter, when it emitted the light. In time the Earth will be receiving CMB from matter that was 50 million ly from us when it emitted the light. As I explained, the "surface" where the current source matter is is constantly shifting outwards and including more and more matter within it

because the CMB light was only emitted by the hot gas during a brief episode after which the gas was cooled enough to stop glowing

so after we have gotten al the CMB light from stuff at a certain distance, we start getting light from stuff that was a little farther away at emission time, so the light from it has taken a little longer to get here
==corrected #5 quote==
Anything made of matter WHICH WAS NEARER THAN 41 MILLION LY at the time the CMB was produced (which would include every star or gas cloud ever created in the history of our universe if made of matter WHICH WAS NEARER THAN 41 MILLION LY) would have to be physically interposed between us and the CMB.
==endquote==
But there is no reason to imagine that there is not loads of stuff out beyond the current source material. The evidence is that the universe is much bigger than what we are currently getting light from and that there is tons and tons of stuff out beyond the current source shell that we will be getting CMB light from in future days.

 

[marcus]

I wanted a concrete example so I used Jorrie's calculator to do this post:
https://www.physicsforums.com/showthread.php?p=4236604#post4236604

Basically it says that the present is year 13.7 billion of the expansion and we are receiving CMB from hot matter that was 42.1 million ly from our matter ("us") at the time of emission
(and the wavelengths have been stretched by a factor of 1090)

But, it says, in year 17 billion we will be receiving CMB stretched by a factor of 1362
from matter that was 44.8 million ly from us at time of emission

And in year 19 billion we will be receiving CMB stretched by a factor of 1557
from matter that was 46.1 million ly from us at time of emission

the emission of the CMB all happened about the same time in a fairly narrow interval. It is just that the radius of the SOURCE SHELL pushes out farther and farther as time goes on.

BTW earlier I was saying 41 Mly for distance at time of emission, and the calculator says 42. That is OK the calculator uses more up to date cosmic parameters. they keep refining the numbers. I was recalling the answer 41 I got when I did the calculation some years back. You have to allow for a bit of fuzz in the numbers because of different sets of model parameters being used. And here I give the answers without much rounding off. More or less as I get them. You can round off as you see fit.

 

[Tanelorn]

==quote==
What I can't reconcile:
4) The fact that we can see the CMB in all directions implies that we are able to see the earliest universe in all directions because the CMB is the electromagnetic radiation output from our universe in its earliest form.
5) Anything of an age less the age of our universe at the time the CMB was produced (which would include every star or gas cloud ever created in the history of our universe) would have to be physically interposed between us and the CMB. (I think this is what's being overlooked here. I think people treat the CMB as something special and not subject to the vicissitudes of space expansion --eg, getting so far removed as to not be visible.)
6) If we can see A, and if B is between us and A, then we can also see B.
7) All the stars EVER created would, therefore, have to be between us and the CMB.
8) If a star is between us and the CMB, then, in principle, we should be able to see it.
9) From 7 and 8, There there cannot be regions of our universe containing stars that are too remote to see.
==endquote==

Marcus, I understand that the Universe back then was far far bigger than the 41Million light years CMBR shell radius, and so the vast majority of matter and energy in the Universe was well beyond this distance and so can never be observed. I do agree however that it should be theoretically possible to observe all matter or energy that was closer to us then than the CMBR radius was then.

I think I see the paradox, as time goes on, we get to see CMBR emitted at slightly greater distances, so why do we say that the observable universe is getting smaller, with galaxies disappearing forever beyond a shrinking observable Universe? Has someone made a mistake or do I need more coffee?

 

[marcus]

Marcus, I understand that the Universe back then was far far bigger than the 41Million light years CMBR shell radius, and so the vast majority of matter and energy in the Universe was well beyond this distance and so can never be observed. I do agree however that it should be theoretically possible to observe all matter or energy that was closer to us then than the CMBR radius was then.

I think I see the paradox, as time goes on, we get to see CMBR emitted at slightly greater distances, so why do we say that the observable universe is getting smaller, with galaxies disappearing forever beyond a shrinking observable Universe? Has someone made a mistake or do I need more coffee?

Jorrie calculator is the "A27" link in my signature. It's a convenient source for many of the numbers we are discussing. It has a cosmic event horizon column, which relates somewhat to your question.

You might also click on the "caltech" link in my signature. It isn't self explanatory. It is a figure with a lot of curves which wondering about what they mean can guide one's curiosity for a period of time. It's a figure to take in small doses. You get an article that the figure appeared in if you google "lineweaver expanding" or if you google "lineweaver cosmic".

Galaxies never leave our observable universe---what happens is their light becomes too redshifted to detect. They never stop being in principle observable because we continue getting light from them. But the light becomes so feeble and its wavelengths get so long that no practical device can detect it. Too big an antenna.

The EVENT HORIZON is something different. It answers the question can a galaxy send us a message TODAY which we will get sometime in the future.
If the galaxy is today farther than 15.6 Gly we will never get the message.

That doesn't mean the galaxy is outside our observable universe. Most of the galaxies we observe today are out beyond 15.6 Gly. They can't send us a message today that we would ever receive, but we still observe them (as they were sometime earlier of course.)

To find out some about the EVENT HORIZON click on Jorrie calculator and put in, for example, upper=1, lower=.01, step=-20
You will see the current distance to the EH is 15.6 and in the far future this distance will stabilize to around 16.3 Gly.

 

[Naty1]

Try 'cuppa'...they check this out:

https://www.physicsforums.com/showthread.php?p=4237146#post4237146Be sure to check the graph in Post #9 and Marcus' descriptions there...

edit: ...

I see Marcus posted while I was composing...

"so why do we say that the observable universe is getting smaller, with galaxies disappearing forever,,,,"

Those are two different concepts; the observable universe is not 'getting smaller', we just will be able to see less and less objects in it. You can get an idea from the above link...

Analogy: maybe think of a line of cars moving away from you where you are; fewer and fewer of them are observable as time passes, while in this case your horizon remains unchanged if you are stationary...

In the above linked graph the 'horizon' is the slowly INCREASING, towards the left side of the graph, sky blue line.

 

[Tanelorn]

I must have misunderstood something earlier then because I recall reading here that the observable universe is shrinking. Perhaps this is regarding light that is being emitted at the present day from distant galaxies which may no longer be able to reach us rather than light emitted back then in the CMBR era. You have to really concentrate to keep all this in your head!

 

[mfb]

I must have misunderstood something earlier then because I recall reading here that the observable universe is shrinking. Perhaps this is regarding light that is being emitted at the present day from distant galaxies which may no longer be able to reach us rather than light emitted back then in the CMBR era.

Right.
There are galaxies where we can see how they looked like in the past, but where we will never see them as they currently are.

 

[manojr]

What I can't reconcile:
4) The fact that we can see the CMB in all directions implies that we are able to see the earliest universe in all directions because the CMB is the electromagnetic radiation output from our universe in its earliest form.
...
7) All the stars EVER created would, therefore, have to be between us and the CMB.
8) If a star is between us and the CMB, then, in principle, we should be able to see it.
9) From 7 and 8, There there cannot be regions of our universe containing stars that are too remote to see.

About 4: We can see CMB in all directions because big bang happened everywhere.

About 9:
Around 380000 years after big bang, event called recombination occurred that allowed free flow of photons that we now see as CMB. At the time of recombination our current observable universe had radius of around 42 million light years. Space expanded with time and galaxies were formed everywhere in this expanding space. We can see photons from (oldest) galaxies which are now almost on edge of observable universe. Photons of CMB closer to us passed by and we continue to receive CMB photons from even further than most distant (oldest) galaxies. The point to understand is that, event of recombination happened everywhere before any galaxies were formed.

 

[curioushuman]

Okay, so it's #5) "Anything of an age less the age of our universe at the time the CMB was produced (which would include every star or gas cloud ever created in the history of our universe) would have to be physically interposed between us and the CMB."

Marcus, you added the qualification, "NEARER THAN 41 MILLION LY at the time the CMB was produced." That's fine, but wouldn't 41 million ly include everything in the universe at the time the CMB was produced (anyway)? So I wonder, why mention it, and then right after that I see when you bring up the reasonableness of there being loads of stuff "out beyond the current source material." By current source material, I take it you mean the primeaval gas that engendered the CMB. That "current source material" (I think it would be more accurate to refer to it as the place in space where the gas once was --since the thing itself is long gone) is no longer 41 million ly away (if that's the point you're making). It's much futher away. It's, as you later say, "the radius of the SOURCE SHELL (that) pushes out further and further as time goes on." Your combined reference to 41 Mly and "current" brings home to me that the nub of this may have to do with time. And maybe the following comments will resolve it. We can never see the universe as it is because light is so $#@! slow. So, in a way, it's trivial to say that there are parts of the universe we can never see. Of course we can't. But, we even though we can't see how the universe actually is, we can see how it "has been." When people talk about there being possible regions of space so vast that there is a high probability that, somewhere out there, there is an exact duplication of our solar system, I have to ask, "Where?" Beyond the radius of the CMB source shell? No way, that would be outside the universe itself. Inside the CMB source shell? OK, let's get some really good telescopes and see what we can find. Maybe we'll see it in some early form. To that, people say "No, you can't see it that way. It's, in principle, beyond what you can see." (Sigh).

On a different topic, later on you say, and this kind of shocked me, "Galaxies never leave our observable universe... they never stop in principle being observable." How do you square that with the claim that we all keep making that (my line #3) that there must be regions of space --full of stars and galaxies-- that we will never (in principle) see?

 

[Chronos]

There are 'tons' of stars and galaxies we will never see because they left our cosmological horizon before they ignited. We will never see how entities beyond our cosmic event horizon [currently ~z = 1.8] look now. So how do we see the CMB photons at z=1100? Simple, the CMB plasma was not beyond our cosmic event horizon at the time those photons were emitted - nor was any of the other stuff between z=1.8 and the CMB. We will, however, never receive any photons they emit today - only those emitted while still within our past cosmic event horizon. Similarly, distant galaxies emitting photons we can currently observe will never 'leave' the observable universe. They will, however, eventually redshift into non detectability.

 

[Naty1]

Sorry, this has become a useless repeat...
Suggest you reread the prior posts...

 

[curioushuman]

Chronos,

"There are 'tons' of stars and galaxies we will never see because they left our cosmological horizon before they ignited."

When you say "they" left before they were ignited, and I take it you mean the gas clouds from which those stars and galaxies ignited. If so, those pre-star gas clouds would have been in our cosmic horizon at some point, and we could get a telescope, in principle, and look at them. And if we can see the gas clouds, then we can see the space (albeit at a much older time and in a less-expanded state) in which those stars (now beyond our cosmic horizon) later formed. That's my point: the fact we can see the CMB seems to imply that there are not vast regions of our universe that are inaccessible to us since we can, at least, see those regions when they were younger.

Naty1,
Discussion on topics like this isn't "useless." Just ignore it if it doesn't appeal to you.

 

[George Jones]

There are 'tons' of stars and galaxies we will never see because they left our cosmological horizon before they ignited.

It is impossible to leave the cosmological horizon.

 

[Naty1]

Discussion on topics like this isn't "useless."

That's not what I posted.
It's repeatedly asking the same questions that is useless.
If you'd read posts carefully you'd see you have your answers.

 

[marcus]

Marcus, you added the qualification, "NEARER THAN 41 MILLION LY at the time the CMB was produced." That's fine, but wouldn't 41 million ly include everything in the universe at the time the CMB was produced (anyway)?

No. It would not include "everything in the universe". It would include everything in the currently observable portion of the universe.

==quote from same post by Curious==
On a different topic, later on you say, and this kind of shocked me, "Galaxies never leave our observable universe... they never stop in principle being observable." How do you square that with the claim that we all keep making that (my line #3) that there must be regions of space --full of stars and galaxies-- that we will never (in principle) see?
==endquote==

It is true that galaxies never LEAVE our observable universe. This is a simple consequence of the mathematical model of the universe which cosmologists use. In the longterm the light from them can be so red-shifted that no practical-size antenna or telescope can detect it. but we will be getting their light so they will still be (by definition) in our observable.

However the observable portion is NOT THE WHOLE MODEL. It is a slowly growing piece of the whole. The observable universe is always including more and more matter.

The current distance to the most distant matter that we are getting signals from is called the "particle horizon". It is around 45-46 Gly. It is INCREASING. The most distant matter we will ever be able to get signals from is NOW 63 Gly from us. As far as we know there is plenty of space and matter out beyond that.

But according to model, it is our destiny that the farthest matter we will ever see is matter which is now 63 billion lightyears from us. This is if you could stop the expansion process to let you measure.

You can see the 63 Gly (approximate) on this chart. It is the "Figure 1" that I keep in signature at the end of every post.
http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg
Look at the bottom figure, where the distance scale is socalled "comoving distance"
this is a permanent distance number attached to each bit of matter which is its distance from us NOW.

You can see from the Figure 1 that the most distant bits of matter we will ever get light or other signal from are the bits that are NOW at a distance of 63 Gly (if you could stop expansion so as to get a chance to measure).

 

[Chronos]

It is impossible to leave the cosmological horizon.

We are in disagreement, George. I contend objects routinely exit our cosmological event horizon. That doesn't mean they 'vanish', merely that they redshift into obscurity before any photons they emit 'now' can reach us.

 

[marcus]

There are 'tons' of stars and galaxies we will never see because they left our cosmological horizon before they ignited.

It is impossible to leave the cosmological horizon.

We are in disagreement, George. I contend objects routinely exit our cosmological event horizon. That doesn't mean they 'vanish', merely that they redshift into obscurity before any photons they emit 'now' can reach us.

Chronos, I find what you said about "tons" of stars and galaxies confusing. You clearly are talking about the cosmological event horizon (CEH). The CEH distance is now something like 15.6 Gly. But most of the galaxies we observe with the Hubble Space Telescope are farther than that.
So a galaxy being beyond the CEH does not imply it cannot be seen!
I can't make sense of your statement.

My hunch is there is some semantic inconsistency here. We need to sort out what is meant by the words.

One way to think about it is that the CEH is a limit of *reachabiity*, not a limit of *detectability*. If a galaxy is beyond 15.6 Gly then if you started off for it today you would never ever get there even traveling at the speed of light. But I suppose by "cosmological horizon" one might also mean the edge of the observable---now something like 46 Gly. That is very different from the CEH (a mere 15.6 Gly). It is usually called the particle horizon and at first i thought that might be what you meant when you said cosmological horizon. But then what you said would be wrong since I think nothing ever exits over the particle horizon. In comoving distance terms it keeps on growing and including more and more matter.

Figure 1 from Lineweaver Davis "Expanding Confusion" illustrates the particle horizon growth in comoving distance terms. Readers new to this can check page 3 of http://arxiv.org/pdf/astro-ph/0310808.pdf
or go directly to http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg
The vertical lines are the paths of matter particles or galaxies, in the two diagrams using comoving distance (because matter's comoving distance essentially does not change.)

So we may have a bit of a terminology vagueness here that could be sorted out

 

[curioushuman]

Go away Naty1!

 

[Chronos]

Agreed, of course we see stars and galaxies beyond our cosmological event horizon. Its just that we will never receive any photons they emitted after they crossed our CEH. In the paper Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe, http://arxiv.org/abs/astro-ph/0310808, on page 4 it is stated "Most observationally viable cosmological models have event horizons and in the CDM model of Fig. 1, galaxies with redshift z ~ 1.8 are currently crossing our event horizon. These are the most distant objects from which we will ever be able to receive information about the present day." I take this to mean galaxies at z~1.8 will cross our CEH at some point in the finite future and any photons emitted thereafter will never reach us. If you extend this to a star birthing region at z~1.8, the light emitted by any stars birthed after it crosses our CEH will never be observable by us. Hence, I deem it logical to assert there are vast numbers of stars that 'currently' exist in the universe whose light will never reach us.

 

[Tanelorn]

Chronos, I find what you said about "tons" of stars and galaxies confusing. You clearly are talking about the cosmological event horizon (CEH). The CEH distance is now something like 15.6 Gly. But most of the galaxies we observe with the Hubble Space Telescope are farther than that.
So a galaxy being beyond the CEH does not imply it cannot be seen!
I can't make sense of your statement.

I think it depends on whether we are talking about light being emitted today that we may view billions of years in our future or light being emitted in the distant past that we are just viewing now.