# The observable universe, the actual uinverse, and CMB

by curioushuman
Tags: actual, observable, uinverse, universe
 Mentor P: 11,819 #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).
Astronomy
PF Gold
P: 23,204
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 by marcus ==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.
 Astronomy Sci Advisor PF Gold P: 23,204 I wanted a concrete example so I used Jorrie's calculator to do this post: http://www.physicsforums.com/showthr...04#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.
 P: 721 ==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?
Astronomy
PF Gold
P: 23,204
 Quote by Tanelorn 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.
 P: 5,632 Try 'cuppa'...they check this out: http://www.physicsforums.com/showthr...46#post4237146 Be 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.
 P: 721 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!
Mentor
P: 11,819
 Quote by 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.
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.
P: 62
 Quote by curioushuman 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.

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.
 P: 8 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 curent 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?
 Sci Advisor PF Gold P: 9,432 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.
 P: 5,632 Sorry, this has become a useless repeat... Suggest you reread the prior posts....
 P: 8 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.
Mentor
P: 6,242
 Quote by Chronos 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.
P: 5,632
 Discussion on topics like this isn't "useless."
That's not what I posted.
It's repeatedly asking the same questions that is useless.
Astronomy
PF Gold
P: 23,204
 Quote by curioushuman 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/M...es/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).
PF Gold
P: 9,432
 Quote by George Jones It is impossible to leave the cosmological horizon.
We are in disagreement, George. I contend objects routinely exit our cosmological event horizon. That doesnt mean they 'vanish', merely that they redshift into obscurity before any photons they emit 'now' can reach us.
Astronomy
PF Gold
P: 23,204
Quote by George Jones
 Quote by Chronos 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.
 Quote by Chronos We are in disagreement, George. I contend objects routinely exit our cosmological event horizon. That doesnt 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/M...es/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

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