the observable universe, the actual uinverse, and CMB


by curioushuman
Tags: actual, observable, uinverse, universe
curioushuman
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#1
Jan17-13, 12:22 PM
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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?
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mfb
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#2
Jan17-13, 01:25 PM
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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
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#3
Jan17-13, 02:12 PM
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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_...s_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
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#4
Jan17-13, 04:04 PM
P: 8

the observable universe, the actual uinverse, and CMB


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
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#5
Jan17-13, 04:12 PM
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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...c/classic.html for discussion.
mfb
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#6
Jan18-13, 09:14 AM
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Quote Quote by curioushuman View Post
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
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#7
Jan18-13, 09:49 AM
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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
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#8
Jan18-13, 12:11 PM
P: 8
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
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#9
Jan18-13, 12:16 PM
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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
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#10
Jan18-13, 12:25 PM
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Quote Quote by curioushuman View Post
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
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#11
Jan18-13, 12:28 PM
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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
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#12
Jan18-13, 01:23 PM
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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
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#13
Jan18-13, 01:34 PM
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Quote Quote by Naty1 View Post
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
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#14
Jan19-13, 11:38 AM
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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
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#15
Jan19-13, 03:42 PM
P: 8
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
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#16
Jan19-13, 04:01 PM
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Quote Quote by curioushuman View Post
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
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#17
Jan20-13, 08:00 AM
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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
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#18
Jan20-13, 04:17 PM
P: 8
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?


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