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Staticman
2010-Jul-11, 05:53 PM
I guess it could given that it is filled with CMBR photons.

Cougar
2010-Jul-11, 06:01 PM
I guess it could given that it is filled with CMBR photons.

Except that the CMBR photons are apparently not interacting with the vacuum, and they're coming from the last scattering surface, which is the blackbody.

Staticman
2010-Jul-11, 06:21 PM
Except that the CMBR photons are apparently not interacting with the vacuum, and they're coming from the last scattering surface, which is the blackbody.

Ok, I was not talking about the origin of the radiation in time.
So I am stil not sure if you think interstellar space could or could not be considered (currently) as a blackbody radiator. I mean regardless of their procedence CMBR photons fill the space now, don't they?

tusenfem
2010-Jul-11, 07:31 PM
"Interstellar space" (better yet intergalactic) is just the container "holding" the photons. It is not absorbing and emitting the photons as a black body would.
Holding in quotation marks because in the laboratory there are real walls, which naturally in space are not there. So, you cannot compare the universe with a BB appliance in a lab, the famous heated room with a small hole in it,

Geo Kaplan
2010-Jul-11, 08:46 PM
Ok, I was not talking about the origin of the radiation in time.
So I am stil not sure if you think interstellar space could or could not be considered (currently) as a blackbody radiator. I mean regardless of their procedence CMBR photons fill the space now, don't they?

Tusenfem's point is exactly spot on. The mere presence of photons in some volume of space does not make that volume the radiator. Cougar pointed out that the source of the radiation (which is what would be the radiator) is the surface of last scattering.

astromark
2010-Jul-11, 09:17 PM
As empty space is full of transiting stuff... It can not be called a source...

Its just a medium for transmission through. Empty space is not so empty. As has been clearly said.

Thats not where those photons are from...

It would seem my understanding of a black body emission is as yet unclear...

could I ask here for more information of the definition of that black body ?

Could source as yet unknown suffice...

Staticman
2010-Jul-11, 10:02 PM
"Interstellar space" (better yet intergalactic) is just the container "holding" the photons. It is not absorbing and emitting the photons as a black body would.
Holding in quotation marks because in the laboratory there are real walls, which naturally in space are not there. So, you cannot compare the universe with a BB appliance in a lab, the famous heated room with a small hole in it,
That is taking it to absurd extremes, where are the walls and the small hole in the LSS plasma?
All I was saying is that we receive the CMBR from outer space right now, so in that sense, probably trivial way, is as if the vacuum is acting as a blackbody radiator now even if it is only transmitting those photons from the LSS. But if it is not the case, that is fine with me.

korjik
2010-Jul-11, 10:14 PM
That is taking it to absurd extremes, where are the walls and the small hole in the LSS plasma?
All I was saying is that we receive the CMBR from outer space right now, so in that sense, probably trivial way, is as if the vacuum is acting as a blackbody radiator now even if it is only transmitting those photons from the LSS. But if it is not the case, that is fine with me.

What you are being told is: No, space is not a radiator. It is just space.

Staticman
2010-Jul-11, 10:39 PM
What you are being told is: No, space is not a radiator. It is just space.

Great, as long as you understand that space is not "just" space(perhaps reading something on QM might help), and also understand something about blackbody radiation, approximated in laboratory by the radiation from a small hole entrance to a large cavity wich walls are heated, where the hole, then, is a close approximation of a theoretical black body. But You'll say that a hole is not a radiator, is just a hole.

Geo Kaplan
2010-Jul-11, 10:54 PM
Great, as long as you understand that space is not "just" space(perhaps reading something on QM might help), and also understand something about blackbody radiation, approximated in laboratory by the radiation from a small hole entrance to a large cavity wich walls are heated, where the hole, then, is a close approximation of a theoretical black body. But You'll say that a hole is not a radiator, is just a hole.

Trust me, korjik understands this better than you. You seem resistant to accepting the mainstream answer to your question. If you have an ATM position, then please articulate it and defend it in the appropriate forum. If you are merely confused about the mainstream answer, then by all means continue.

A radiator is an agent. Radiation is contained in a volume, but that does not imply that the volume itself is the agent. No one argues that the hole in a cavity is itself a blackbody. The radiation is the result of actions within the cavity, and interactions with the walls. The hole is incidental.

Nereid
2010-Jul-11, 11:11 PM
I guess it could given that it is filled with CMBR photons.
If it were a blackbody radiator, then the interstellar vacuum would have an optical depth >1 (at least in the range of electromagnetic frequencies ~2 orders of magnitude above and below the peak).

Yet, interstellar - and intergalactic - space is largely transparent, out to at least z ~7, and where it's not transparent, it's due to stuff that does not have the CMBR (blackbody) spectrum.

So the simple answer to your question is no, the same as several other BAUTians have already said.

Andrew D
2010-Jul-11, 11:45 PM
"Interstellar space" (better yet intergalactic) is just the container "holding" the photons. It is not absorbing and emitting the photons as a black body would.
Holding in quotation marks because in the laboratory there are real walls, which naturally in space are not there. So, you cannot compare the universe with a BB appliance in a lab, the famous heated room with a small hole in it,

The validity of your statement depends on the observer. An accelerating observer will observe black-body radiation where an inertial observer would observe a vacuum.

tusenfem
2010-Jul-12, 07:03 AM
The validity of your statement depends on the observer. An accelerating observer will observe black-body radiation where an inertial observer would observe a vacuum.

But now you are not talking about the CMBR.

Staticman
2010-Jul-12, 08:54 AM
If it were a blackbody radiator, then the interstellar vacuum would have an optical depth >1 (at least in the range of electromagnetic frequencies ~2 orders of magnitude above and below the peak).
Yet, interstellar - and intergalactic - space is largely transparent, out to at least z ~7, and where it's not transparent, it's due to stuff that does not have the CMBR (blackbody) spectrum.

Thanks for your answer.
Can you back that assertion about optical depth and blackbody radiation with some reference please?
Are you sure that is the case inside the hohlraum, is the optical depth there >1?

Andrew D
2010-Jul-12, 09:00 AM
But now you are not talking about the CMBR.

Would an accelerated observer be able to tell the difference?

Staticman
2010-Jul-12, 09:08 AM
The validity of your statement depends on the observer. An accelerating observer will observe black-body radiation where an inertial observer would observe a vacuum.
Yes that is an interesting angle, I guess you are referring to the Unruh radiation (totally Mainstream, no need to worry guys). ;-)
That raises an interesting question whose answer might be ATM so be careful(anyone dares? :D ) , should we as observers be considered as accelerated by the earth or as inertial observers?

Disclaimer: I don't intend to raise any ATM responses nor even ATM thoughts by formulating these questions, I am only confused about the answers as Geo Kaplan very aptly indicated.

Regards

Nereid
2010-Jul-12, 10:16 AM
Would an accelerated observer be able to tell the difference?
Accelerated? Or accelerating? You've used both terms.

An accelerating observer will not observed an isotropic blackbody; indeed, we do not observed an isotropic one ... there is a 'cosmic' dipole, and our acceleration, in our (or WMAP's and Planck's) orbit also leaves a clear signal (one which is removed early in the data reduction process).

Nereid
2010-Jul-12, 10:23 AM
Thanks for your answer.
Can you back that assertion about optical depth and blackbody radiation with some reference please?
Sure.

There are plenty of point sources at, or near, the peak of the CMB blackbody SED; the number observed matches that expected, to an frighteningly good extent.

And what are the point sources expected? Away from the galactic plane, various quasars, AGNs, radio-loud galaxies, etc.

See, for example, First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Foreground Emission (http://arxiv.org/abs/astro-ph/0302208) (link is to arXiv abstract); the point source catalogue, and analyses, was updated in each of the subsequent sets of papers (Three Year, Five Year, Seven Year).


Are you sure that is the case inside the hohlraum, is the optical depth there >1?
Of course not.

But then it isn't expected to be ... the BB SED is generated by the (interior) walls, not the hollow space within it. And the optical depth of the walls, at BB frequencies, is most certainly >> 1!

Staticman
2010-Jul-12, 05:51 PM
There are plenty of point sources at, or near, the peak of the CMB blackbody SED; the number observed matches that expected, to an frighteningly good extent.

And what are the point sources expected? Away from the galactic plane, various quasars, AGNs, radio-loud galaxies, etc.

See, for example, First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Foreground Emission (http://arxiv.org/abs/astro-ph/0302208) (link is to arXiv abstract); the point source catalogue, and analyses, was updated in each of the subsequent sets of papers (Three Year, Five Year, Seven Year).


I donít know what the point sources or the different masks used to clean the CMB map of anisotropies off foreground objects have to do with my question, but nevermind, it's an interesting subject too.
A question rises for the non-expert from the paper you linked, how can we be sure we have masked all the extragalactic point sources, is that easy to know? Or there could always be some point source in the anisotropies map that is not originated in the primordial epoch? Please keep technicalities to a minimum.

Regards

Nereid
2010-Jul-12, 06:14 PM
There are plenty of point sources at, or near, the peak of the CMB blackbody SED; the number observed matches that expected, to an frighteningly good extent.

And what are the point sources expected? Away from the galactic plane, various quasars, AGNs, radio-loud galaxies, etc.

See, for example, First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Foreground Emission (link is to arXiv abstract); the point source catalogue, and analyses, was updated in each of the subsequent sets of papers (Three Year, Five Year, Seven Year).I donít know what the point sources or the different masks used to clean the CMB map of anisotropies off foreground objects have to do with my question, but nevermind, it's an interesting subject too.
If you 'see' objects as distant as z ~5 (say), the intergalactic (and interstellar) space between us and those objects must have an optical depth considerably less than 1 (i.e. it's transparent).

That seems to be pretty good evidence that space is not, itself, a blackbody radiator.


A question rises for the non-expert from the paper you linked, how can we be sure we have masked all the extragalactic point sources, is that easy to know?
Yes ... and no.

Point sources are, by definition, unresolved sources. Crudely, that means their emission comes from a region on the sky that is smaller than the beam size. A very great deal of work - calibration if you will - has gone into determining beam size, resolution, interference, etc, etc, etc (the gory technical details are in other WMAP papers, both pre-launch and First Year one; there are also updates in the later Year papers).

And a point source is just like a star, to our eyes - a point of brightness greater than its immediate surroundings. The data reduction routines are quite robust wrt finding point sources.


Or there could always be some point source in the anisotropies map that is not originated in the primordial epoch? Please keep technicalities to a minimum.
A lot of follow-up work has been done on the point sources identified in the WMAP data; AFAIK all but a tiny handful have been successfully identified as various quasars, AGN, radio-loud galaxies, etc. Further, the number of non-identified sources is compatible that that expected, given the noise statistics (that's pretty darn impressive).

So none of the point sources originate further than z ~6 (IIRC; it might be 7); that's a very long way from z ~1100.

If this is too technical, holler; I'll be happy to try a simpler explanation.

Staticman
2010-Jul-12, 06:30 PM
Thanks for the explanation.

Ken G
2010-Jul-12, 06:37 PM
All I was saying is that we receive the CMBR from outer space right now, so in that sense, probably trivial way, is as if the vacuum is acting as a blackbody radiator now even if it is only transmitting those photons from the LSS.The answers you've been receiving are correct in what they are saying, but the plot can be thickened in interesting ways. Since those answers have left you unsatisfied, I'm going to assume you would benefit from some plot-thickening.

There is indeed some sense to which the vacuum is "acting as a blackbody radiator", but it is also true that the vacuum does not meet the standard definition of a blackbody. So there is a challenge here between teasing out the semantic issues, from the issues of actual physical interest.

The definition of a blackbody is an object that absorbs all light that is impingent on it. In most cases we are not as interested in the light impingent on it as we are in what it emits, but under the normal expectation of equilibrium and energy conservation, there is a close connection between the two. Since the vacuum doesn't absorb light, it isn't a blackbody, and that connects with the answers you have been given so far.

But those answers don't satisfy you, because you are wanting to treat the vacuum itself as a kind of source of the CMB. Is there any good reason to do that? Well, there kind of is, in fact-- because the CMB is the radiation from a 2.7 K blackbody, yet there are no actual 2.7 K blackbodies responsible for it! Remarkably, as the universe expands, a thermal radiation field coming from the 3000 K blackbody that was the LSS redshifts in such a way that it continues to be a thermal radiation field. It is just as if the temperature of the source was dropping, which is also a lot like the vacuum is having the effect of absorbing the hotter radiation, and emitting it at a lower temperature. Indeed, you will find language like "a 2.7 K vacuum" as a kind of shortcut for talking about the action of real blackbodies coupled with cosmological redshifting. So whether or not the vacuum is acting like a blackbody is very much a semantic issue, and connects closely with the homogeneity, isotropy, and thermal character of the CMB in the presence of cosmological expansion.

I would be content to say that in the context of the CMB (only), the vacuum is acting like a blackbody but isn't a blackbody, and picturing it as one can only go so far without possibly making errors.

Staticman
2010-Jul-12, 06:45 PM
Thanks Ken.
Further, this is from a known site about BB: http://www.talkorigins.org/faqs/astronomy/bigbang.html#CMBR that reinforces what you say and is close to what I implied in the OP.

"This makes the universe a perfect absorber; no photons could leave the universe, so they would put the whole universe (or at least that part that was causally connected) in thermal equilibrium. As such, we can actually describe the universe as having a unique temperature. In classical thermodynamics, photons emitted by a blackbody at a given temperature have a very specific distribution of energies."

Regards

Nereid
2010-Jul-12, 09:12 PM
Thanks Ken.
Further, this is from a known site about BB: http://www.talkorigins.org/faqs/astronomy/bigbang.html#CMBR that reinforces what you say and is close to what I implied in the OP.

"This makes the universe a perfect absorber; no photons could leave the universe, so they would put the whole universe (or at least that part that was causally connected) in thermal equilibrium. As such, we can actually describe the universe as having a unique temperature. In classical thermodynamics, photons emitted by a blackbody at a given temperature have a very specific distribution of energies."

Regards(bold added)

If I may, the OP carried no such implication for this particular reader: "interstellar space" is, to me, nowhere near equivalent to "the universe"! :p

Gosh, to me, the usual meaning of interstellar space is only that which is found within galaxies; most of the universe - volume-wise - is to be found between galaxies (YMMV).

Ken G
2010-Jul-12, 09:49 PM
Yes, "interstellar" was probably an unfortunate choice of words, because it runs afoul of a common jargon in astronomy that "interplanetary" means inside a star system, "interstellar" means inside a galaxy, and "intergalactic" means between galaxies.

Andrew D
2010-Jul-13, 05:15 AM
Accelerated? Or accelerating? You've used both terms.

An accelerating observer will not observed an isotropic blackbody; indeed, we do not observed an isotropic one ... there is a 'cosmic' dipole, and our acceleration, in our (or WMAP's and Planck's) orbit also leaves a clear signal (one which is removed early in the data reduction process).

Does an accelerating observer record the same sources of anisotropy as an inertial observer, or does the accelerating observer record a different temperature than the inertial observer but the same source?

Staticman
2010-Jul-13, 08:03 AM
(bold added)
If I may, the OP carried no such implication for this particular reader: "interstellar space" is, to me, nowhere near equivalent to "the universe"! :p

I think it was perfectly clear what I was referring too, even if I didn't use the exact astronomical term. Perhaps if you had left aside your "ATM paranoia" you would have understood.

Ken G
2010-Jul-13, 08:42 AM
I can see, Staticman, how you could have felt a bit put-upon by the initial reaction to your post, but please don't take it personally-- ATM paranoia comes about because there really are a lot of posters who have hidden ATM agendas, and a lot of them center on alternate explanations to the CMB. It's just unfortunate happenstance that you got caught in that particular crossfire. Also, you have to admit that although thinking of the vacuum as a blackbody does have some advantages in the overall picture of the CMB, it was a little hard to see that's what you were asking about, because if one uses that picture too literally one is led to all kinds of incorrect ideas about the vacuum (like it changes the direction that individual photons are moving as it cools their temperature with expansion). It's really only an effective picture, so the posters who responded to you initially were taking it as a literal picture. Sometimes we all have to step back a minute and get underneath the dogma that comes so quickly to mind, to find what is interesting about an idea that on the surface sounds counter to the standard canon, but that's not always easy so try to be forgiving. Communication is very hard, and a few errant terms in an OP can cause a whole thread to go sideways.

Staticman
2010-Jul-13, 09:09 AM
I can see, Staticman, how you could have felt a bit put-upon by the initial reaction to your post, but please don't take it personally-- ATM paranoia comes about because there really are a lot of posters who have hidden ATM agendas, and a lot of them center on alternate explanations to the CMB. It's just unfortunate happenstance that you got caught in that particular crossfire. Also, you have to admit that although thinking of the vacuum as a blackbody does have some advantages in the overall picture of the CMB, it was a little hard to see that's what you were asking about, because if one uses that picture too literally one is led to all kinds of incorrect ideas about the vacuum (like it changes the direction that individual photons are moving as it cools their temperature with expansion). It's really only an effective picture, so the posters who responded to you initially were taking it as a literal picture. Sometimes we all have to step back a minute and get underneath the dogma that comes so quickly to mind, to find what is interesting about an idea that on the surface sounds counter to the standard canon, but that's not always easy so try to be forgiving. Communication is very hard, and a few errant terms in an OP can cause a whole thread to go sideways.

Mostly agree with you, Ken.
But I'm sure you will understand that taking that precautions to the extreme as some posters do goes against free interchange of ideas, of course, if ATM ideas are clearly presented they should be redirected to their approriate place, but being in an excesively defensive attitude does not lead to fruitful discussions in my opinion. What's worse, this tendency is contagious, I have found myself warning others,which I shouldn't have. Let's leave that work to the moderators, they do it fine, that is all I'm saying.
But as you say communication is hard, and even harder in a forum.

Regards

Ken G
2010-Jul-13, 11:41 AM
But I'm sure you will understand that taking that precautions to the extreme as some posters do goes against free interchange of ideas, of course, if ATM ideas are clearly presented they should be redirected to their approriate place, but being in an excesively defensive attitude does not lead to fruitful discussions in my opinion.Yes, that was pretty much what I was thinking when I read the first few responses, but I know where they were coming from so I could see the disconnect playing out.


But as you say communication is hard, and even harder in a forum.
Exactly, where we only get a few typewritten words with no personality and no physical cues. But it would cost too much money to all get into the same conference room and have these threads as discussions off in the various corners of the room! (But I wonder how different they would be if they were done like that.)

Nereid
2010-Jul-13, 12:02 PM
The answers you've been receiving are correct in what they are saying, but the plot can be thickened in interesting ways. Since those answers have left you unsatisfied, I'm going to assume you would benefit from some plot-thickening.

There is indeed some sense to which the vacuum is "acting as a blackbody radiator", but it is also true that the vacuum does not meet the standard definition of a blackbody. So there is a challenge here between teasing out the semantic issues, from the issues of actual physical interest.

The definition of a blackbody is an object that absorbs all light that is impingent on it. In most cases we are not as interested in the light impingent on it as we are in what it emits, but under the normal expectation of equilibrium and energy conservation, there is a close connection between the two. Since the vacuum doesn't absorb light, it isn't a blackbody, and that connects with the answers you have been given so far.

But those answers don't satisfy you, because you are wanting to treat the vacuum itself as a kind of source of the CMB. Is there any good reason to do that? Well, there kind of is, in fact-- because the CMB is the radiation from a 2.7 K blackbody, yet there are no actual 2.7 K blackbodies responsible for it! Remarkably, as the universe expands, a thermal radiation field coming from the 3000 K blackbody that was the LSS redshifts in such a way that it continues to be a thermal radiation field. It is just as if the temperature of the source was dropping, which is also a lot like the vacuum is having the effect of absorbing the hotter radiation, and emitting it at a lower temperature. Indeed, you will find language like "a 2.7 K vacuum" as a kind of shortcut for talking about the action of real blackbodies coupled with cosmological redshifting. So whether or not the vacuum is acting like a blackbody is very much a semantic issue, and connects closely with the homogeneity, isotropy, and thermal character of the CMB in the presence of cosmological expansion.

I would be content to say that in the context of the CMB (only), the vacuum is acting like a blackbody but isn't a blackbody, and picturing it as one can only go so far without possibly making errors.
It's also important, I think, to point out that once you start making these kinds of re-interpretations, you will, certainly, sooner or later, run into contradictions created by those re-interpretations.

One example has already come up, in this thread, the concept of optical depth: emission from something means absorption too, at the same wavelengths, and a decrease in transparency. Further, this connection - emission means opacity - is built in to the definitions of the terms, so if you choose to re-interpret one term, you need to re-define the other terms too. Nothing wrong with that, of course, but it certainly mean a lot of work, and makes communication much harder.

Nereid
2010-Jul-13, 12:08 PM
If I may, the OP carried no such implication for this particular reader: "interstellar space" is, to me, nowhere near equivalent to "the universe"! I think it was perfectly clear what I was referring too, even if I didn't use the exact astronomical term. Perhaps if you had left aside your "ATM paranoia" you would have understood.
As Ken G said, communication is hard.

And in this case I clearly erred in thinking that you were using a standard term in its normal meaning (after all, from reading your other posts in BAUT, I got the impression that you are quite au fait with the standard terms). And, in communication, it's not what the speaker intends to say that counts, it's what the audience understands.

wrt to intention vs understanding, may I ask you to explain to me how you perceived "ATM paranoia"? I really would like to know, so I can do a better job, in future, or not creating unintended perceptions.

Nereid
2010-Jul-13, 12:17 PM
Does an accelerating observer record the same sources of anisotropy as an inertial observer, or does the accelerating observer record a different temperature than the inertial observer but the same source?
Neither.

An accelerating observer would not record a uniform, all-sky (isotropic) emission.

Suppose the sky were covered with point sources, all of which emit a blackbody SED, at a rest-frame temperature of T, all of which are at the same, very large distance, from our observer.

An observer would see, in an isotropic, homogeneous universe, the point sources all having the same blackbody temperature ... provided the observer were at rest wrt the sources.

If the observer were moving - including being accelerated - the point sources would be seen to have different temperatures, with the pattern of temperatures dependent on the direction of motion, etc.

Note: this is simplified; there are caveats to add, and details to explain ...

jlhredshift
2010-Jul-13, 12:37 PM
In this thread it has been mentioned that during data reduction, processing, that the dipole is removed and point sources are removed, are there other things that are removed from the data? Also, because I do not understand it, does Tollman dimming apply the the CMB light path?

Nereid
2010-Jul-13, 12:51 PM
In this thread it has been mentioned that during data reduction, processing, that the dipole is removed and point sources are removed, are there other things that are removed from the data?
There are various masks used, at various stages, to handle diffuse foreground emission (mostly within the Milky Way).

It gets very technical, very quickly, and the few words we've used so far to describe data reduction are, to some extent, somewhat misleading. One example: one of the key outputs is the angular power spectrum, which is not derived from a map of the non-dipole anisotropies!

If you'd like to discuss this in more detail, just holler.


Also, because I do not understand it, does Tollman dimming apply the the CMB light path?
Do you mean Tolman dimming, a cosmological surface brightness dimming that goes as (1 + z)^4?

If so, then not, AFAIK, to the various analyses of the CMB that I've seen published; the anisotropies that are studied are temperature (and, with Planck, polarisation) fluctuations, not surface brightness ones.

jlhredshift
2010-Jul-13, 01:04 PM
There are various masks used, at various stages, to handle diffuse foreground emission (mostly within the Milky Way).

It gets very technical, very quickly, and the few words we've used so far to describe data reduction are, to some extent, somewhat misleading. One example: one of the key outputs is the angular power spectrum, which is not derived from a map of the non-dipole anisotropies!

If you'd like to discuss this in more detail, just holler..

My bold

I do not know what that is, so, "HOLLER".



Do you mean Tolman dimming, a cosmological surface brightness dimming that goes as (1 + z)^4?

If so, then not, AFAIK, to the various analyses of the CMB that I've seen published; the anisotropies that are studied are temperature (and, with Planck, polarisation) fluctuations, not surface brightness ones.

Yep, that is what I meant. So, it would apply to a high z quasar but not the LSS of the CMB?

Nereid
2010-Jul-13, 01:32 PM
There are various masks used, at various stages, to handle diffuse foreground emission (mostly within the Milky Way).

It gets very technical, very quickly, and the few words we've used so far to describe data reduction are, to some extent, somewhat misleading. One example: one of the key outputs is the angular power spectrum, which is not derived from a map of the non-dipole anisotropies!

If you'd like to discuss this in more detail, just holler..My bold

I do not know what that is, so, "HOLLER".
I don't know how much you know, so what follows may be too easy, or too hard (or both!). I'd be happy to explain further, in either case.

If you analyse a pure musical tone - an 'A' tuning fork, for example - with a microphone and oscilloscope, you'll find it has but a single frequency; it looks like a pure sine wave on the 'scope.

The same 'note' plucked on a violin string, or from a wind instrument, does not look like a pure sine wave, on the 'scope; rather it is 'rougher' (or some other adjective). That's because the violin string, or sound chamber of the wind instrument, is producing 'notes' of frequencies other than just A; for example there are harmonics.

You can, with some neat electronics, decompose (or analyse) the sound of a violin's A into pure notes (or frequencies); you'll find that the A has the highest amplitude, the most power (energy per unit time) of all the frequencies.

The mathematical technique of frequency decomposition is called Fourier analysis*, after Joseph Fourier, the mathematician who first studied this topic.

Now Fourier analysis can be done in with things other than sound; a spectrograph which splits light into its component wavelengths is, in one sense a kind of Fourier analysis (the power, at any given frequency, in this case, is expressed as flux, or, sometimes, luminosity).

It can also be done in more than one dimension; in helioseismology, for example, Fourier analysis (or similar) teases out the various modes of oscillation of the Sun - how can a sphere shiver and quake? Let me count the ways ...

Which brings us to the CMB: the sky is the surface of a sphere, for our observer purposes, and the CMB is shivering and quaking. Fourier analysis breaks this shivering and quaking down into spherical harmonics (like harmonics on a string, only in 2D), and the CMB analyses reduce this to just a single dimension - angle (this corresponds to the spherical harmonic dimension l). So, the angular power spectrum tells you how much power (shivering and quaking) the CMB does, on scales of 90o, 60o, 30o, ... in fact, at all scales smaller than ~180o (that's the dipole) and larger than ~1o (from memory; it's almost certainly not quite right).




Do you mean Tolman dimming, a cosmological surface brightness dimming that goes as (1 + z)^4?

If so, then not, AFAIK, to the various analyses of the CMB that I've seen published; the anisotropies that are studied are temperature (and, with Planck, polarisation) fluctuations, not surface brightness ones.
Yep, that is what I meant. So, it would apply to a high z quasar but not the LSS of the CMB?
No, only to high-z galaxies.

Why? Because quasars are, almost by definition, point sources, and the Tolman dimming refers to surface brightness (i.e. the brightness (= integrated magnitude, say) of a patch of galaxy, or nebula, rather than a point). Surface brightness is somewhat tricky - there are several, quite lengthy, Q&A and Astronomy threads on it - especially when it comes to astronomical objects.

* well, there are others, but I'm keeping it simple

jlhredshift
2010-Jul-13, 01:54 PM
I don't know how much you know, so what follows may be too easy, or too hard (or both!). I'd be happy to explain further, in either case.

If you analyse a pure musical tone - an 'A' tuning fork, for example - with a microphone and oscilloscope, you'll find it has but a single frequency; it looks like a pure sine wave on the 'scope.

The same 'note' plucked on a violin string, or from a wind instrument, does not look like a pure sine wave, on the 'scope; rather it is 'rougher' (or some other adjective). That's because the violin string, or sound chamber of the wind instrument, is producing 'notes' of frequencies other than just A; for example there are harmonics.

You can, with some neat electronics, decompose (or analyse) the sound of a violin's A into pure notes (or frequencies); you'll find that the A has the highest amplitude, the most power (energy per unit time) of all the frequencies.

The mathematical technique of frequency decomposition is called Fourier analysis*, after Joseph Fourier, the mathematician who first studied this topic.

Now Fourier analysis can be done in with things other than sound; a spectrograph which splits light into its component wavelengths is, in one sense a kind of Fourier analysis (the power, at any given frequency, in this case, is expressed as flux, or, sometimes, luminosity).

It can also be done in more than one dimension; in helioseismology, for example, Fourier analysis (or similar) teases out the various modes of oscillation of the Sun - how can a sphere shiver and quake? Let me count the ways ...

Which brings us to the CMB: the sky is the surface of a sphere, for our observer purposes, and the CMB is shivering and quaking. Fourier analysis breaks this shivering and quaking down into spherical harmonics (like harmonics on a string, only in 2D), and the CMB analyses reduce this to just a single dimension - angle (this corresponds to the spherical harmonic dimension l). So, the angular power spectrum tells you how much power (shivering and quaking) the CMB does, on scales of 90o, 60o, 30o, ... in fact, at all scales smaller than ~180o (that's the dipole) and larger than ~1o (from memory; it's almost certainly not quite right).

I understand, I think. Years ago I read Smoot's book about COBE and the description of the antennae horns rotating so as to measure the difference of one direction versus another.

And, thank you for taking the time to respond to my simplistic questions.


No, only to high-z galaxies.

Why? Because quasars are, almost by definition, point sources, and the Tolman dimming refers to surface brightness (i.e. the brightness (= integrated magnitude, say) of a patch of galaxy, or nebula, rather than a point). Surface brightness is somewhat tricky - there are several, quite lengthy, Q&A and Astronomy threads on it - especially when it comes to astronomical objects.

* well, there are others, but I'm keeping it simple

Ok, but I thought that we had resolved some high z quasars and they are high z galaxies, but I get your point about a "patch" of brightness.

Nereid
2010-Jul-13, 02:01 PM
I understand, I think. Years ago I read Smoot's book about COBE and the description of the antennae horns rotating so as to measure the difference of one direction versus another.

And, thank you for taking the time to respond to my simplistic questions.
No worries, any time! :)


Ok, but I thought that we had resolved some high z quasars and they are high z galaxies, but I get your point about a "patch" of brightness.
Yes, they have ... but only a tiny, tiny handful. IIRC, there are now well over a million identified quasars, in the various astronomical databases, and only a handful (<100?) of resolved quasars, and most of those have low redshifts (for quasars!).

Ken G
2010-Jul-13, 02:42 PM
One example has already come up, in this thread, the concept of optical depth: emission from something means absorption too, at the same wavelengths, and a decrease in transparency. Further, this connection - emission means opacity - is built in to the definitions of the terms, so if you choose to re-interpret one term, you need to re-define the other terms too. Or you can just recognize it as an effective treatment that lends some kind of pedagogical simplification without being taken too literally. This kind of thing happens all the time-- for example, one way to explain the cooling of the CMB is to let the light bounce around inside a reflecting box, and slowly expand the box just as the universe expands. The behavior of that light is identical to what happens to the CMB in an expanding universe. Are there really walls like that? No, it's another effective treatment, like treating the vacuum as a blackbody with a temperature that cools in the same way. It is not literally a blackbody, and there are no literal walls, more like "literary" walls. But yes, that's not the same as being an actual blackbody, so it's fine to clarify to what extent the picture works, and to what extent it can be overextended. Forums are great for hashing out that kind of thing, as long as no one gets too defensive about the process. (Do we get defensive in practice? Oh yes, all the time!)

Andrew D
2010-Jul-15, 12:41 AM
Neither.

An accelerating observer would not record a uniform, all-sky (isotropic) emission.

Suppose the sky were covered with point sources, all of which emit a blackbody SED, at a rest-frame temperature of T, all of which are at the same, very large distance, from our observer.

An observer would see, in an isotropic, homogeneous universe, the point sources all having the same blackbody temperature ... provided the observer were at rest wrt the sources.

If the observer were moving - including being accelerated - the point sources would be seen to have different temperatures, with the pattern of temperatures dependent on the direction of motion, etc.

Note: this is simplified; there are caveats to add, and details to explain ...

Thank you. I would rather receive the details and ask questions if I do not understand them or what I can read about them. I start the AST series at my university in the fall, so hopefully my misunderstandings will be short lived.

Nereid
2010-Jul-15, 12:34 PM
Thank you. I would rather receive the details and ask questions if I do not understand them or what I can read about them. I start the AST series at my university in the fall, so hopefully my misunderstandings will be short lived.
I don't know where you are in your studies, but reading up on the Sachs-Wolfe effect might help you understand how acceleration may affect the perception of the CMB.

Also, have your heard of CMBFAST (http://lambda.gsfc.nasa.gov/toolbox/tb_cmbfast_ov.cfm)? That may be at least indirectly relevant to your questions.