What is 'in front' of the CMB?

[Gerinski]

The furthest away (back in time) we can see is the CMB. This was the surface of last scatter, the surface of a hot gas bubble which due to redshift appears today as CMB. The actual temperature of that surface was around 3000 K.
Moving from the CMB towards us, the next thing we can observe are early galaxies, I believe that the most distant object detected is a galaxy from around 600 million years after the BB.

In my layman's ignorance, it seems as if we can see a ball at 3000 K, then getting closer towards us suddenly we see nothing at all, just black emptiness, and then suddenly 600 million years later we can see some burning stars (which I presume would have temperatures of several thousand K).

It would seem that the 3000 K bubble would gradually cool down but still radiate for a while until becoming cold enough as to not radiating anything detectable, and then remain 'black' until stellar fusion would ignite and start radiating visible light again. Right?
So it seems to me that in between the CMB and the first visible stars there should still be layered spheres of gradually less intensity, somehow 'blurring' or 'obscuring' the 'sight' of the CMB, as if they were clouds 'just in front of the CMB', until those 'clouds' became thin enough as to become practically undetectable.
What's the mistake? how can we 'see' the CMB? what is 'just a bit closer' than the CMB and why is not it (say the surface of the gas when it was at 1500 K) obstructing the CMB view?

 

[mfb]

In my layman's ignorance, it seems as if we can see a ball at 3000 K, then getting closer towards us suddenly we see nothing at all, just black emptiness, and then suddenly 600 million years later we can see some burning stars (which I presume would have temperatures of several thousand K).

Well, those where the temperatures at the time light was emitted. Due to the expansion of space, this got redshifted significantly, so now it appears as microwave background with a temperature of ~3K and the first stars appear significantly hotter (but they are not bright enough to see them in such a large distance with current telescopes).

It would seem that the 3000 K bubble would gradually cool down but still radiate for a while until becoming cold enough as to not radiating anything detectable, and then remain 'black' until stellar fusion would ignite and start radiating visible light again. Right?

After the temperature dropped below 3000K, the atoms quickly (~30000 years) had no way to interact with the light any more (apart from the transition in its hyperfine structure, but that gives an extremely large wavelength). Atomic hydrogen at 2000K does not glow like a metal would, for example. It is just transparent.

 

[phinds]

Gerinski, mfb has given a good answer to your question but just to be sure you understand one other thing, there was NO "BUBBLE". The expansion did not happen from a point as the term "bubble" would seem to imply.

 

[mfb]

I assumed the "bubble" refers to the origin of the CMB we see today - matter distributed as a sphere ("bubble") around us.

 

[Gerinski]

Yes, I mean 'bubble' as the sphere forming the surface of last scattering.

Thanks for the reply, so if I understand well it just happens that hydrogen at 3000 K (at that time) radiates enough so we can still see the glow (today looking like at 3 K due to redshift) but as soon as it cooled down below the 3000 K, say when it was at 2500 K it did not radiate anymore at any level which can be detectable today. So the CMB was like some 'instant flash of light' sandwiched by 'darkness' (= opacity) before and darkness (no emission of any detectable radiation) immediately after (lasting for millions of years until the first stars started to burn). Right?

 

[phinds]

I assumed the "bubble" refers to the origin of the CMB we see today - matter distributed as a sphere ("bubble") around us.

Yep, reasonable assumption and I see you were right, but you KNOW how many "point origin" posts we get here, so I was making sure.

 

[marcus]

Yes, I mean 'bubble' as the sphere forming the surface of last scattering.

Thanks for the reply, so if I understand well it just happens that hydrogen at 3000 K (at that time) radiates enough so we can still see the glow (today looking like at 3 K due to redshift) but as soon as it cooled down below the 3000 K, say when it was at 2500 K it did not radiate anymore at any level which can be detectable today. So the CMB was like some 'instant flash of light' sandwiched by 'darkness' (= opacity) before and darkness (no emission of any detectable radiation) immediately after (lasting for millions of years until the first stars started to burn). Right?

glowing = opacity

if it's hot enough to glow, it's hot enough to scatter
if it's not hot enough to scatter, it's not hot enough to glow.
absorbing and (re-) radiating are the same process just reverses of each other

so the surface of a star appears like a crisp transition because above it the gas is transparent and you can't see what's below because the surface is glowing, how else could it be?

the surface of a star is like a miniature "surface of last scattering"

I'm oversimplifying but I'm saying that it's not dreadfully surprising that CMB is a one-time flash.It's a tautology--kind of has to be that way.

 

[Gerinski]

Mmmm... the CMB seen as an equivalent to the Sun's surface but applied to the whole universe... not sure I still get it but I'l give it some thought, thanks!

 

[Chalnoth]

So it seems to me that in between the CMB and the first visible stars there should still be layered spheres of gradually less intensity, somehow 'blurring' or 'obscuring' the 'sight' of the CMB, as if they were clouds 'just in front of the CMB', until those 'clouds' became thin enough as to become practically undetectable.
What's the mistake? how can we 'see' the CMB? what is 'just a bit closer' than the CMB and why is not it (say the surface of the gas when it was at 1500 K) obstructing the CMB view?

That's more or less right. The transition of the plasma in the early universe to a gas wasn't instantaneous. This has the effect of blurring our image of the CMB, meaning that shorter wavelengths are suppressed by this effect. This can be seen directly in the CMB power spectrum:
http://lambda.gsfc.nasa.gov/product...s/nineyear/cosmology/images/med/gh9_f01_M.png

The shorter wavelengths are to the right. If the cooling of the CMB was instantaneous, then the even and odd peaks would have different heights, but the third, fifth, seventh, etc. peaks would all be as high as the first, and the fourth, sixth, etc. peaks would all be as high as the second.

Instead, the power tapers off pretty strongly as you get to shorter and shorter wavelengths.

So the transition happened fast enough that it isn't blurred out entirely, but slow enough that we can see the effect very strongly in the data. One thing to bear in mind is that the transition from a plasma to a gas is a phase transition: by the time the gas was at 1500K, it was not a plasma at all, and quite transparent.

 

[D H]

It would seem that the 3000 K bubble would gradually cool down but still radiate for a while until becoming cold enough as to not radiating anything detectable, and then remain 'black' until stellar fusion would ignite and start radiating visible light again. Right?

There's a huge difference between an ideal black body and an ideal transparent body. An ideal black body radiates electromagnetically as a function of its temperature, per Planck's law. An ideal transparent body doesn't radiate according to Planck's law. It don't radiate, period.

While there is no such thing in nature as an ideal black body or an ideal transparent body, the early and current universe are pretty close to those two opposing ideals. The transition from opaque to transparent, while not instantaneous, was quite short in duration.

This gives an answer to the question posed in the title of this thread, "What is 'in front' of the CMB?". By looking "in front" of the CMB we are looking at some point in time after recombination. What we would see is a transparent warm gas if we could see it. We cannot see that transparent warm gas precisely because it's transparent.

 

[Gerinski]

Thanks, it's much clearer now (no pun intended).
So it seems we are quite lucky that the transition is so sudden, otherwise the CMB would have been 'obscured by clouds' (for those of my generation, that's an early Pink Floyd album).

 

[D H]

No luck. It was a phase transition driven by the ongoing expansion of the universe. What's more remarkable is how uniform it is.

 

[mfb]

That's more or less right. The transition of the plasma in the early universe to a gas wasn't instantaneous. This has the effect of blurring our image of the CMB, meaning that shorter wavelengths are suppressed by this effect. This can be seen directly in the CMB power spectrum:
http://lambda.gsfc.nasa.gov/product...s/nineyear/cosmology/images/med/gh9_f01_M.png

The shorter wavelengths are to the right. If the cooling of the CMB was instantaneous, then the even and odd peaks would have different heights, but the third, fifth, seventh, etc. peaks would all be as high as the first, and the fourth, sixth, etc. peaks would all be as high as the second.

Instead, the power tapers off pretty strongly as you get to shorter and shorter wavelengths.

So the transition happened fast enough that it isn't blurred out entirely, but slow enough that we can see the effect very strongly in the data. One thing to bear in mind is that the transition from a plasma to a gas is a phase transition: by the time the gas was at 1500K, it was not a plasma at all, and quite transparent.

Your image shows multipole moments of the temperature fluctuations of the CMB. How are those related to the wavelength of the microwaves?
The length scales of the fluctuations exceed the wavelength of the photons by many, many orders of magnitude.

 

[Naty1]

Interesting discussions...

Chalnoth posted a CMB power spectrum:

http://lambda.gsfc.nasa.gov/product/.../gh9_f01_M.png

How does one interpret that plot relative to the usual 'ideal black body spectrum' of the CMBR??

The usual plot I see is like this: [Intensity versus frequency]

http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation#Features

 

[Chalnoth]

Your image shows multipole moments of the temperature fluctuations of the CMB. How are those related to the wavelength of the microwaves?
The length scales of the fluctuations exceed the wavelength of the photons by many, many orders of magnitude.

This plot is a plot that is purely showing the temperature differences at different places on the sky. It isn't about the wavelengths of the photons, but about the wavelengths of the sound waves in the early universe.

The CMB power spectrum is a visualization of these sound waves (specifically, the average amplitude of sound waves of various wavelengths), and has nothing to do with the black body spectrum.

The way to reconcile the two spectra is that at each point in the sky, the CMB photons at that point have a black body spectrum. At different places in the sky, that black body spectrum is at a slightly higher or slightly lower temperature (the variations are up to around 300 micro-Kelvin, so quite tiny). It is these small differences in temperature that stem from the sound waves in the early universe, and are represented by the CMB power spectrum.

 

[Naty1]

The way to reconcile the two spectra is that at each point in the sky, the CMB photons at that point have a black body spectrum.

Duh, of course...Thanks...My 'senior moments' seem to be lasting longer!

 

[Skolon]

This thread is very appropriate with what I intend to ask (at least for the first two questions), so please allow me to do it even it is an old thread.

From this forum I learned that the CMB radiation we receive today was emitted by primordial plasma when it was at 41 million ly away from our position at that moment. It is this accurate?
If yes, I asked myself what about all radiation emitted by the plasma between this two positions? I suppose all just simple passed us, I mean it hit us in the past and we will never see it again. Right?

The second question is: considering the above answers, how long (from now) we will be able to detect and measure the CMB radiation?

The third question I have about CMB refers to relation between measured temperature of CMB from different location in the sky and the change of our current position relative to the position we had when the measured radiation start to go toward us (let's name this position "Ground zero"). I expected this movement affect the measured values of CMB from different locations on the sky.
I know that the "natural" change of our position was done with too little "speed" compared with expansion "speed", but any motion must been largely affected by the expansion itself because we had 13.7 billion years for that. As we can read here, our motion relative to CMB is 360 +/- 20 kilometers/sec in the direction of constellation Leo. I don't know if we can consider that this was always our motion in respect to CMB, but let suppose it was. So, the Google calculator give for "(360 km / s)*(13.7 billion years) in light years" calculation the 16 451 381.2 light years result. This is almost 16.5 Mly. How much this distance was increased by 13.7 billion years of expansion? I don't know and maybe we will never know for sure. But I think that, consider all this, the distance from our current position and the "Ground Zero" it is large enough to alter the measured values of CMB in such way to see an anisotropy across the sky. And as I know, we actually see that.
So, the third question is: can our "proper" motion (because the complex action of gravity) increased by expansion during a large period be the cause of this anisotropy?

 

[Bandersnatch]

1) yes, it has since passed us
2) forever, but more and more redshifted; just as some of it has already passed us, just as some of it is passing us now, just so more will pass us in the future
3)There are a few issue here, including the meaning of the velocity quoted and the exact meaning of "we" in the early universe, but to the point in question - no. The anisotropy is due to doppler shift, which is dependent on current velocity w/r to the source. The distance traveled doesn't change anything as far as the cmbr anisotropy goes.

 

[Chronos]

The short answer is the observable universe. But, the CMB is no absolute 'horizon'. Gravity waves and neutrinos can penetrate the EM observational barrier.

 

[mfb]

"ground zero" has no special meaning. The 41 million light years correspond to the position "then" of an observer that never moved relative to the CMB and happens to be at our place now.

The speed of our sun relative to the CMB depends on the motion of our galaxy (relative to the CMB) and our motion within the galaxy (~200km/s, significant!) and varied a lot in the past. That's not relevant, however, and more than 5 billion years ago the sun didn't even exist so this position tracking breaks down.
Motion just influences the dipole contribution of the CMB, and this is ignored in all analyses exactly because we cannot distinguish it from motion relative to the CMB. All other contributions are independent of our motion.

 

[nikkkom]

No luck. It was a phase transition driven by the ongoing expansion of the universe. What's more remarkable is how uniform it is.

IIRC the recombination was quite far from instantaneous. The Universe was 380 thousand years old when it started, and it took many thousands of years for fully ionized hydrogen to become fully neutral.

 

[Chronos]

The figure of z=1089 as the surface of last scattering is a little deceptive. It represents about the 50% point of recombination, where about half of the neutral hydrogen gas 'froze' out of the former 'soup' of hot plasma. It appears neutral hydrogen began to freeze out around z = 1587 (based on the Saha equation) and the process was not completed [99%] until about z = 820 (based on the Peebles equation) - re: http://folk.uio.no/hke/AST5220/v10/AST5220_recombination_2010.pdf . This corresponds to a time after the big bang from about 196,000 years to about 600,000 years (re: Jordie's cosmo calculator). So, we could say the era of recombination lasted about 400,000 years. Does that mean the 'last' CMB photon will whiz past us in a couple hundred thousand more years? Of course not, it would be equally naive to suppose they never started reaching Earth until a couple hundred thousand years ago. CMB photons have been arriving here since before the solar system formed and will continue arriving after it is gone. They will merely redshift into obscurity some time in the distant future.

 

[Pedro Paixao]

1) yes, it has since passed us
2) forever, but more and more redshifted; just as some of it has already passed us, just as some of it is passing us now, just so more will pass us in the future

How can we continue to see the CMB is its photons have passed us by? Why do we keep seeing it, if the "flash" that generated it is over?

 

[phinds]

How can we continue to see the CMB is its photons have passed us by? Why do we keep seeing it, if the "flash" that generated it is over?

What "flash"? There was no such thing. We simply see the "surface of last scattering" that existed farther and farther away from us. It happened everywhere in the universe at the same time so first the spot the Earth is now in would have seen the CMB from nearby (and relatively hot) and then over time, from farther and farther away (and cooler). That will never stop, although the redshift will eventually become so severe that the CMB will not be detectable. At some point the size of the detector required would exceed the diameter of the solar system. Not very practical.

 

[russ_watters]

How can we continue to see the CMB is its photons have passed us by? Why do we keep seeing it, if the "flash" that generated it is over?

The surface of last scattering was a specific instant in time, but it wasn't a specific location, it was everywhere. So as time passes, we see light from that "flash" from progressively further distances.

 

[russ_watters]

What "flash"? There was no such thing.

I think insofar as it existed at a single point in time, it can be considered a "flash".

 

[phinds]

I think insofar as it existed at a single point in time, it can be considered a "flash".

My understanding is that it evolved over at least thousands of years. Is that wrong?

 

[Chronos]

Agreed phinds, CMB photons were emitted over many thousands of years according to theory as already cited. It's duration was, however, only a flash relative to the present age of the universe.

 

[phinds]

Agreed phinds, CMB photons were emitted over many thousands of years according to theory as already cited. It's duration was, however, only a flash relative to the present age of the universe.

Well, that's fair enough but I think Pedro was implying a single point in time which is what I was responding to.

 

[Chronos]

A graceful exit is still preferred, even for inflation.

 

[russ_watters]

My understanding is that it evolved over at least thousands of years.

Fair enough: the [checks dictionary] dictionary definition of "flash" just says a very non-scientific "brief". It's just a colloquialism and precisely zero time would be an impossible constraint on basically anything considered a "flash" (lightning, a camera light, bomb explosion) if one wanted to zoom in far enough on the "event" (also scientifically incorrect usage). For exactly the same reason (the same cause) it would be technically incorrect to call the CMB "homogeneous", or even to call the SLC a "surface", but I wouldn't quibble with that either if someone left off a non-scientific qualifier like "almost".

Regardless, none of this has any bearing on the question being asked and if anything applying a duration can lead him on the wrong track.

 

[phinds]

Regardless, none of this has any bearing on the question being asked and if anything applying a duration can lead him on the wrong track.

I agree, which is why I gave the longer answer explaining what is actually going on after pointing out that there was no "flash" (and I agree w/ what has been said about the actual duration not be a flash in the strictest technical meaning of that word but still an extremely brief time cosmological terms).

 

[phinds]

@Pedro Paixao I think my response could have been more courteously worded. Something like "if by 'flash' you mean one point in time, that's not quite right. The surface of last scattering evolved over thousands of years" followed by the rest of what I said.

 

[russ_watters]

Let me try to paint a new picture:

Typically the first time a person hears about the Big Bang it's an explosion, which is where I think people get the idea of a flash that happens/is seen and then is gone. Regardless of the actual duration, it is a short event - a flash. If you are far away from an explosion, that's what you see: the light from the explosion travels outward in a sphere until it reaches you, then it is gone.

For the Big Bang and CMB, imagine you are a long-lived being who was capable of surviving in the very early universe. Early on, the universe would have looked similar to what it is like flying through a cloud in a plane; nothing but bright light in all directions, from close by.

When the universe suddenly* became transparent, it would have been like a suddenly dissipating cloud. But because this cloud is really, really big, you can watch it dissipate over time, as the last vestiges of light from further and further away reach you. A billion years after it dissipated, you see light that had to travel a billion light years to reach you at the moment of dissipation. Ten billion years after it dissipated, you see light that had to travel ten billion light years to get to you. You're seeing the cloud dissipate, from the inside, as an expanding sphere of transparency. That is why you can witness such a brief event for a long time.

*not instantly, but close enough relative to the timescale of the story.

 

[Pedro Paixao]

Got it. Thank you guys!