Could Dark Mass Be a Possible Explanation for Missing Particles in the Universe?

[inflector]

We know particles have mass. Thusfar we don't know of anything that has mass which is not a particle so we assume the likeliest explanation for apparent missing mass must be missing particles. This makes some good sense. But what if this assumption is wrong?

Has anyone looked into the idea of dark mass instead of dark matter?

Let me illustrate one possible idea that shows the difference:

1) We have some instances where fields themselves can contain energy, like the EM field, if I am not missing something. So a charged particle can transfer its kinetic energy to an EM field thereby slowing down the charged particle and increasing the energy of the field.

2) Could there not be an equivalent ability for a field defining mass (Higgs? or something else) to contain mass itself?

3) We know that as Wheeler said: mass tells spacetime how to curve and spacetime tells mass how to move.

4) What if spacetime was springy and wanted to be straight so that the curvature itself implied some stored energy that shows up in our observations as missing mass at very low accelerations?

That's just one possible idea, but I'm sure there could be many other similar ones I've overlooked. The basic idea is that spacetime itself or a mass field might be able to store/contain mass.

Are there any papers discussing this idea or refuting it? Or are there obvious problems with it that I'm not seeing that make it such a silly idea that no one would have even bothered?

 

[Ich]

What if spacetime was springy and wanted to be straight so that the curvature itself implied some stored energy

Spacetime is springy and wants to be straight, and so curvature itself implies stored energy. That is what makes GR nonlinear. But it does contribute almost nothing at the low densities we're talking about.

The problem with DM is that it's cold. That means that its constituents should be stable, massive particles - unlike the quanta of all the fields we know.
Maybe it is some exotic field, but that's far more speculative than simply assuming there's another stable particle.

 

[Upisoft]

That means that its constituents should be stable, massive particles - unlike the quanta of all the fields we know.

What about Higgs? We don't know if it is stable. As Higgs will be the only scalar boson known (if it exists) it may have some "scalar" conservation factor, making it stable.

 

[inflector]

Spacetime is springy and wants to be straight, and so curvature itself implies stored energy. That is what makes GR nonlinear. But it does contribute almost nothing at the low densities we're talking about.

Thanks Ich, it sounds like I need to keep working my way through Tensor Calculus and Riemann Geometry so I can really understand GR.

 

[Chalnoth]

What about Higgs? We don't know if it is stable. As Higgs will be the only scalar boson known (if it exists) it may have some "scalar" conservation factor, making it stable.

Seems unlikely to me. From what I understand, scalar particles act as a mediator of a fifth force that acts very much like gravity, except without interacting with radiation. I suspect this means that if there was any significant amount of the stuff around, we would have seen its effects by now in gravitational lensing observations. I could be wrong, however.

That said, any specific dark matter hypothesis is unlikely at this point, because we have so many, and none that are obviously more likely than any others.

 

[Phrak]

Spacetime is springy and wants to be straight[...]

get out of town. How do yo figure this? When I think of a very dense chunk of matter, I think if infalling spirals of more matter that will increase the curvature. What did you have in mind that would tend to flatten things out?

 

[Upisoft]

Seems unlikely to me. From what I understand, scalar particles act as a mediator of a fifth force that acts very much like gravity, except without interacting with radiation. I suspect this means that if there was any significant amount of the stuff around, we would have seen its effects by now in gravitational lensing observations. I could be wrong, however.

That said, any specific dark matter hypothesis is unlikely at this point, because we have so many, and none that are obviously more likely than any others.

Since we have not observed any scalar particles, it is safe to say we don't know how they act. If you don't expect an interaction between them and radiation (photons), what lensing are you talking about?

 

[Chalnoth]

Since we have not observed any scalar particles, it is safe to say we don't know how they act.

No, it's not, really. The mathematics of a scalar field determine quite a lot about their behavior.

If you don't expect an interaction between them and radiation (photons), what lensing are you talking about?

Well, as I said, scalar particles act as an intermediary force very much like gravity, except that they couple to the trace of the stress-energy tensor instead. Since the stress-energy tensor for radiation is traceless, this force doesn't interact with radiation, just slower-moving particles.

Though I do have to admit that I'm not entirely clear on whether or not having a gas of scalar particles around would actually affect the strength of the force felt.

 

[inflector]

get out of town. How do yo figure this? When I think of a very dense chunk of matter, I think if infalling spirals of more matter that will increase the curvature. What did you have in mind that would tend to flatten things out?

I'm interested to see Ich's response but in thinking about it, it's obvious that spacetime moves back to straight/flat if you take the matter away, so in that sense, it wants to be straight/flat.

 

[Upisoft]

No, it's not, really. The mathematics of a scalar field determine quite a lot about their behavior.

And what specific scalar field do you talk about? As far as I know we have advanced the science by stretching the known theories and then finding they don't work there. My idea that Higgs can be stable is a shot in the dark, I agree. However shooting it down with a theory that have never been confirmed will not help either.

 

[Ich]

How do yo figure this?

it's obvious that spacetime moves back to straight/flat if you take the matter away

Yes.
I was referring to the common analogy of those "dents in a rubbersheet". You have this flat, undisturbed background, and whenever you go away with your matter and things, it gets back to this state. Like something springy.
And if you bend it quickly, you really have to expend energy, and the deformation you caused will propagate away, like phonons in some springy crystal.
It's an analogy which guides intuition rather than hamper it, IMHO.

That said, I'm aware of what happens in the long term if you let both spacetime and matter interact. The result is quite the opposite of the flatness that I said is spacetime's "natural" state. And indeed, that's where you get entropy from.
So I won't argue if you don't like the analogy.

 

[Chalnoth]

And what specific scalar field do you talk about? As far as I know we have advanced the science by stretching the known theories and then finding they don't work there. My idea that Higgs can be stable is a shot in the dark, I agree. However shooting it down with a theory that have never been confirmed will not help either.

You can't separate a hypothetical explanation from its other implications. However, after a bit more thought, the connection between Higgs = dark matter and a fifth force may be less of a consideration. At any rate, it seems unlikely to me, as any Higgs particle must interact with normal matter in order to give it mass, which would seem to undercut the fact that dark matter is largely non-interacting.

 

[Phrak]

Yes.
I was referring to the common analogy of those "dents in a rubbersheet". You have this flat, undisturbed background, and whenever you go away with your matter and things, it gets back to this state. Like something springy.
And if you bend it quickly, you really have to expend energy, and the deformation you caused will propagate away, like phonons in some springy crystal.
It's an analogy which guides intuition rather than hamper it, IMHO.

That said, I'm aware of what happens in the long term if you let both spacetime and matter interact. The result is quite the opposite of the flatness that I said is spacetime's "natural" state. And indeed, that's where you get entropy from.
So I won't argue if you don't like the analogy.

Thanks. That's what I was interested in hearing. Like inflector, I imagine, I was more interested in the idea behind your statement rather than if it were arguably right or wrong.

 

[zonde]

4) What if spacetime was springy and wanted to be straight so that the curvature itself implied some stored energy that shows up in our observations as missing mass at very low accelerations?

That's just one possible idea, but I'm sure there could be many other similar ones I've overlooked. The basic idea is that spacetime itself or a mass field might be able to store/contain mass.

I believe that idea about spacetime being springy is mainstream viewpoint.

I fancy a bit different idea. What if gravitation of matter is not property of matter but it's state. Meaning that spacetime by itself is completely passive and it requires matter in it's opposite state (antigravitating) to uncurl spacetime.

Concentration of matter in this opposite state at certain places would alter gravity quite radically. I believe that you would require far less "dark matter" if you put it at the opposite side of the "fence" i.e. outside (disk-like) gravitating system of matter in ring-like formation.

 

[Tanelorn]

Isn't simple ionized hydrogen (protons) a candidate for dark matter?

 

[Chalnoth]

Isn't simple ionized hydrogen (protons) a candidate for dark matter?

Nope. First, ionized hydrogen interacts rather strongly with photons, so we can actually see the stuff, as long as we look for it in the right frequency range. But the real clincher is the CMB evidence.

Before the CMB was emitted, the normal matter in our universe was a plasma: everything was ionized. Such a plasma interacts rather strongly with light, so when it fell into a gravitational well, it would bounce back out again. In the CMB, we can see evidence of this bounce, and it turns out that the bounce is much smaller than would be explained by a purely matter-dominated universe: we need roughly five times as much dark matter (which doesn't bounce) as normal matter.

 

[Tanelorn]

Chalnoth, I had not come across the term bounce before, or that dark matter doesn't bounce. Are these effects and properties speculative or strongly certain? Do you have a list of all known properties of dark matter including any speculative properties?

 

[Chalnoth]

Chalnoth, I had not come across the term bounce before, or that dark matter doesn't bounce. Are these effects and properties speculative or strongly certain? Do you have a list of all known properties of dark matter including any speculative properties?

The proper name for the physics at work here is called "Baryon Acoustic Oscillations", and yes, it is highly certain because the only physics we're talking about here is simple electromagnetic interactions. The WMAP data show beyond a reasonable doubt that around 5/6ths of the matter distribution doesn't interact electromagnetically.

 

[Tanelorn]

WMAP data meaning the magnitude of fluctuation of the temperatures in different directions is lower than expected for more ordinary matter?

Since you are familiar with the WMAP, I have a question regarding the first year map of the CMB. I see a single blue patch and three patches of red. What is causing this, are they statistically relevant?

http://en.wikipedia.org/wiki/WMAP

 

[Chalnoth]

WMAP data meaning the magnitude of fluctuation of the temperatures in different directions is lower than expected for more ordinary matter?

It's a bit more complicated than that. Basically, this information can be gleaned from the power spectrum, which is a measure of the typical amplitude of fluctuations of a certain size on the sky. Here is a plot of this power spectrum from the seven-year WMAP data:
http://lambda.gsfc.nasa.gov/product...nyear/powspectra/images/med/dl7_f01_PPT_M.png

The plot goes from large-angle fluctuations on the left, to small-angle fluctuations on the right. The first peak is at about one degree on the sky, the second peak at about half a degree, and so on. This first peak represents potential wells that were large enough that matter had just enough time to fall into them. The second peak represents potential wells that matter had enough time to fall into, then bounce back out of. Because the dark matter doesn't bounce, this second peak is suppressed by the existence of dark matter. In fact, every even-numbered peak is reduced by the existence of dark matter.

Since you are familiar with the WMAP, I have a question regarding the first year map of the CMB. I see a single blue patch and three patches of red. What is causing this, are they statistically relevant?

http://en.wikipedia.org/wiki/WMAP

Well, there has been some discussion about the large angular scale correlations in the WMAP data, but so far none of these have been demonstrated to be anything significant. So basically, WMAP is, so far, fully-consistent with standard models of cosmology. This is the paper where the WMAP team went through these issues for the 7-year data release:
http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/anomalies/wmap_7yr_anomalies.pdf

 

[zonde]

The WMAP data show beyond a reasonable doubt that around 5/6ths of the matter distribution doesn't interact electromagnetically.

You can't be serious. How long is chain of untested or poorly tested assumptions behind this interpretation of observations?

I believe that search for dark matter relies on observations with much more direct interpretations than this. Like galaxy rotation curves and anomalies in gravitation lensing around colliding galaxy clusters.

 

[Tanelorn]

Just wrote a really long reply and it got lost..

 

[Chalnoth]

You can't be serious. How long is chain of untested or poorly tested assumptions behind this interpretation of observations?

Um, the basic assumptions are:
1. General Relativity is valid.
2. There is some amount of normal matter.
3. There is some matter that doesn't interact electromagnetically.

That's about it. While there are other parameters that effect our interpretation of the CMB, they don't have much of any impact upon the measured ratio of normal matter to dark matter, because they have extremely different signatures in the CMB power spectrum.

I believe that search for dark matter relies on observations with much more direct interpretations than this. Like galaxy rotation curves and anomalies in gravitation lensing around colliding galaxy clusters.

The CMB evidence is actually some of the strongest evidence there is for the existence of dark matter. Galaxy rotation curves, for instance, can somewhat easily be explained by modified gravity. But it is extremely difficult for modified gravity to explain the CMB results without dark matter.

 

[Tanelorn]

Chalnoth, thanks for taking the time to discuss this, I really appreciate it.

So this plot is showing power/temperature variation with angle which is not random, but which is a repeatable pattern every degree of so. This repeatable pattern shows the effect that dark matter is having on suppressing "bounce" of the ordinary matter in the plasma.

Wouldn't the plasma dark matter mixture be so finely and randomly mixed as to prevent any such patterns? Also wouldn't noise, galactic lensing, variations in space time expansion, or other radio sources cause any such pattern to be lost in noise?


The WMAP data is obviously a major cornerstone of modern cosmology and I would to be certain of the measurements and assumptions.


As an Radio engineer myself I would be concerned about the instrumentation. If the sky was a couple of orders of magnitude smoother than measured would the instrumentation still see this? Also if the angular variation in the spectrum were a couple of orders of magnitude smaller than measured would the instrumentation also see this?

Regarding other interferer RF sources I see that the galactic plane is very strong in some of the plots. Is this the effects of the galactic plane removed from other plots? However, it shows how much radio noise a single galaxy contributes to the picture. Now if you add in there a quasi infinite number of galaxy sources from every area of the sky with lensing and dispersion from dust does this not also add a measure of uncertainty to the WMAP data?

Thanks again!

 

[Chalnoth]

So this plot is showing power/temperature variation with angle which is not random, but which is a repeatable pattern every degree of so. This repeatable pattern shows the effect that dark matter is having on suppressing "bounce" of the ordinary matter in the plasma.

Actually, it is very random. It's just that the plot I posted above, the power spectrum, is the correlation between pixels separated by some angle on the sky. I don't think it would be useful to go into the mathematical details here (they're pretty simple to write out, but can be difficult to understand if you're not familiar with spherical harmonics).

But the basic idea is that each $C_\ell$ is the variance of the amplitude of a wave on the sky of a particular wavelength. The larger the variance, the greater the typical amplitude of the waves. To our knowledge, the CMB is comprised of such waves drawn with a completely random distribution, but with different mean amplitudes at different length scales.

Wouldn't the plasma dark matter mixture be so finely and randomly mixed as to prevent any such patterns? Also wouldn't noise, galactic lensing, variations in space time expansion, or other radio sources cause any such pattern to be lost in noise?

Well, the plasma itself tends to suppress the small angular scale fluctuations, because the phase transition from an opaque plasma to the transparent CMB took some finite amount of time, so when we look at the CMB we're actually looking through some finite distance of semi-transparent material. This causes the power spectrum to rather rapidly die away at $\ell$ values above a thousand or so (visible with many ground and balloon-based experiments, as well as Planck).

Instrument noise itself tends to increase the error at the higher-frequency end as well. In the WMAP data, you can see this effect blowing up the noise around $\ell=1000$ or so. Planck is expected to be able to measure the power spectrum at least out to $\ell=2000$, and many ground-based and balloon-borne experiments go to even higher resolutions, but the ground-based and balloon-borne experiments only see a fraction of the sky, and as a result aren't able to as effectively measure the variance of the fluctuations, because they aren't looking at as many fluctuations as WMAP or Planck.

Galactic lensing and radio sources also provide significant effects, but again mostly at small angular scales. These effects need to be corrected for to get accurate estimates of the CMB at small angular scales.

Variations in space-time expansion basically have zero effect on the CMB signal, as for the most part we only see the integrated amount of expansion, not how much it has varied over time. Still, there is a weak signal at the large angular scale range due to the space-time expansion effecting the growth of structure. This is known as the Integrated Sachs-Wolfe Effect.

As an Radio engineer myself I would be concerned about the instrumentation. If the sky was a couple of orders of magnitude smoother than measured would the instrumentation still see this? Also if the angular variation in the spectrum were a couple of orders of magnitude smaller than measured would the instrumentation also see this?

Well, the WMAP instrument is a differencing instrument. It is actually two telescopes sitting back-to-back looking at two different directions of the sky at any given time. It then takes the difference between the signal from corresponding radiometers in each telescope. The differencing allows WMAP to very accurately measure small differences in temperature on the sky without any active cooling, though it is completely insensitive to the absolute temperature. Obviously if the level of variations was substantially lower, WMAP wouldn't have been able to measure them nearly as well. But we knew by the time WMAP launched what the level of variations was, because they were measured by COBE.

Planck, by the way, uses active cooling. This allows it to get much better signal-to-noise for its radiometers, and also allows it to have bolometers to capture higher-frequency signals.

Regarding other interferer RF sources I see that the galactic plane is very strong in some of the plots. Is this the effects of the galactic plane removed from other plots? However, it shows how much radio noise a single galaxy contributes to the picture. Now if you add in there a quasi infinite number of galaxy sources from every area of the sky with lensing and dispersion from dust does this not also add a measure of uncertainty to the WMAP data?

Thanks again!

I believe the majority of the signal along the galactic plane on the WMAP data is from synchrotron emission from our galaxy. This stems from electrons emitting radiation as they are accelerated by the magnetic fields around the galaxy. I don't think the stars themselves emit very much radiation at these wavelengths, so they aren't much of a concern. The cold dust in the galaxy also emits radiation, but most of that is at slightly higher frequencies than WMAP is sensitive to.

However, Planck is sensitive to these frequencies, and gets a very nice picture of the dust, as you can see in this image. This image was composed from multiple Planck channels, with red coming from the lower-frequency channels, and blue coming from the higher-frequency channels. So the dust you can see as blue emission, and the synchrotron looks more red (note that due to the particular frequency scaling chosen, the CMB itself looks very red in this image). There are a number of point sources visible as well, but not as many as you might think.

 

[Tanelorn]

Chalnoth, thanks again for your very helpful reply. The number of questions I have are growing exponentially with each answer!

However, there is one question I want to be clear about before the others and that is, why do some pictures show the Galactic plane and some do not? At the moment I am assuming that the galactic plane is removed later mathematically from some of the plots? This is an example here on page 32, one with the plane and one without:

http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/anomalies/wmap_7yr_anomalies.pdf


PS. I really liked the S.H. initials. So unlikely and yet shows that unlikely things can do happen, but hopefully do not get built into theories! I also saw that the quadrupole effect looks like it could pose a problem of sorts.

 

[Chalnoth]

However, there is one question I want to be clear about before the others and that is, why do some pictures show the Galactic plane and some do not? At the moment I am assuming that the galactic plane is removed later mathematically from some of the plots?

Right, so the image that doesn't show the galactic plane used a component separation technique known as Internal Linear Combination (ILC). ILC makes use of the fact that we know how the CMB scales with frequency. The WMAP maps were calibrated off of the CMB dipole due to our own motion, which is an order of magnitude larger than the other anisotropies in the CMB, but has the same dependence on frequency (this dipole is removed from all maps you see, by the way). With this assumption, the ILC technique can be described as follows:

1. Let's assume that the signal for each map at every pixel can be represented as:

$$d_i = a_i s + n_i$$

Here $$i$$ indexes the frequency band, $$a_i$$ is the frequency scaling of the particular signal, $$s$$ is the signal you're interested in (in this case CMB), $$n_i$$ is everything else, and $$d_i$$ are the individual frequency maps.

The goal is to find a linear combination of the maps $$d_i$$ that retains the signal you want while minimizing everything else:

$$\tilde{s} = \sum_i w_i d_i$$

If we exploit the fact that $a_i = 1$ for the CMB, then we know that if $\sum_i w_i = 1$, $\tilde{s} = s + \sum_i w_i n_i$. So if we use this constraint that the sum of the weights of the channels must equal to one, then we just minimize the variance of the output and we are left with a signal that is mostly CMB. This can be done analytically with just a tiny bit of linear algebra.

There are many other techniques for distinguishing between the CMB and the foregrounds in multi-channel maps, but this is the simplest and fastest.

PS. I really liked the S.H. initials. So unlikely and yet shows that unlikely things can do happen, but hopefully do not get built into theories! I also saw that the quadrupole effect looks like it could pose a problem of sorts.

Yeah, the SH initials were cute. But the quadrupole really isn't a problem, because statistically speaking, it's not a terribly unlikely value in the standard cosmology. There has been some work that may indicate it's just the result of correlated noise in the WMAP instrument, though, so with Planck we should be able to confirm or discount whether or not this quadrupole is real.

 

[Tanelorn]

Chalnoth, thanks again for your reply.

Regrettably my maths isn't what it needs to be. e.g. di = ais + ni
Often I am not sure if I am looking at a simple algebraic equation where I can substitute di=1, ai=2 etc.

However, I am a pretty good RF engineer including radar receiver design so I can speak to the practical limitations of trying to extract a wanted signal from an interferer. Firstly there are frequency selectivity techniques using narrow band filters which can attenuate sources at other frequencies. Then there are FM techniques and FM chirp techniques which can extract a correlated signal right out of the noise. Similarly there are also digital spread spectrum techniques which can extract energy out of the noise by correlation, not possible here. I can't think of any other way of removing unwanted energy. Perhaps the mathematics you mention describes one of these methods?

 

[Chalnoth]

Well, perhaps I can explain it a bit.

The equation I wrote before is:

$$d_i(p) = a_i s(p) + n_i(p)$$

I've added in the fact that the data, CMB signal, and noise all vary from pixel to pixel, so they are represented here as functions of a pixel. Yes, you can understand these as being simple numbers. $d(p)_i$ are the actual sky maps that WMAP observes at each frequency (WMAP observes the sky at 5 frequencies), and we are trying to extract a single map of the CMB, $s(p)$, out of these five maps.

Now, as I explained, we calibrate the instrument off of the CMB itself, so that the CMB signal is the same in all channels, so we can simplify it as:

$$d_i(p) = s(p) + n_i(p)$$

This is just a statement that in each data channel, the data in each pixel is a combination of CMB and other crap. The other crap will vary from channel to channel, but the CMB contribution is the same.

The idea with ILC is that we can extract the CMB by considering a linear combination of the channels:

$$\tilde{s}(p) = \sum_i w_i d_i(p)$$

Here $\tilde{s}(p)$ is our estimate of the CMB, the result of the ILC. And we select the weights $w_i$ such that the variance of the other crap that isn't CMB is minimized. In other words, each $w_i$ is just a number we multiply each map that WMAP observes. We then add these weighted maps together to get our estimate of the CMB.

Does that help?

 

[Tanelorn]

Thanks Chalnoth, Is this perhaps the equivalent of automatic gain control, so that the average signal level in the galactic plane is the same as the average signal elsewhere in the sky, or perhaps you subtract a value to bring the average value in the galactic plane down to the same as the average value in the rest of the sky. The aim being to focus on the detail or difference between pixels which should then be the same background radiation as elsewhere?

You mentioned earlier about the background radiation is being measured at several frequencies. However doesn't this then equate to a range of background temperatures? I had understood that CMB temperature needed to be an exact figure for the standard model?

I just watched a BBC Horizon documentary "Before the big bang". Fascinating stuff, it seems that the standard model is not acceptable as a complete explanation to many of the original believers. "Plenty of effect and not enough cause". Well this is my outrageous contribution to such a speculative program :) https://www.physicsforums.com/showthread.php?t=440065

 

[Chalnoth]

Thanks Chalnoth, Is this perhaps the equivalent of automatic gain control, so that the average signal level in the galactic plane is the same as the average signal elsewhere in the sky, or perhaps you subtract a value to bring the average value in the galactic plane down to the same as the average value in the rest of the sky. The aim being to focus on the detail or difference between pixels which should then be the same background radiation as elsewhere?

I don't exactly know what automatic gain control is. But the idea here might be described as thus:

Imagine you have a bunch of microphones in a crowded room. There are lots of people talking, but one thing you know is that they are all the same distance from George, so there's noise all over the place, and the microphones all measure different signals, but they each carry the exact same contribution from George.

The idea here would be that you can obtain the sound from George, to some degree of accuracy, by taking a weighted sum of the signals from each microphone. If you make sure that the weights sum to one, then you ensure that George's contribution to the noise is retained. Then you just need to minimize the amount of sound in the output. Since George's contribution is enforced by keeping the weights summed to one, minimizing the output minimizes everything but George's contribution, so that he comes in as clearly as possible.

You mentioned earlier about the background radiation is being measured at several frequencies. However doesn't this then equate to a range of background temperatures? I had understood that CMB temperature needed to be an exact figure for the standard model?

The CMB has a thermal black body spectrum, the most precise black body spectrum known to man. This spectrum corresponds to a temperature of 2.725K, with a peak at 160GHz, though it emits significant amounts of radiation at both lower and higher frequencies. Broadly, the CMB is most visible between about ~15GHz and ~500GHz. WMAP measures the sky between 23GHz and 94GHz (it stays in the lower frequency range because radiometers have difficulty measuring higher-frequency signals).

 

[Tanelorn]

Thanks for your reply Chalnoth.

I just found this WMAP system block diagram, which I hope may also help me understand how the contribution for the galactic plane can be neglected.

http://wmap.gsfc.nasa.gov/mission/observatory_rec.html

In your description, why would the microphones all measure different signals?
Any antenna looking in the direction of the galactic plane is still going to see the noise from the galactic plane. Perhaps polarization comes into this?
Anyway, I think I will have to move on from this question until I can get a more detailed picture of the actual RF signals entering each of the difference receivers. There is plenty of other sky to consider.It appears that with the center of the frequency peak at 160GHz so we would prefer to measure the signal there, but this is not easy or as accurate with today's technology. eg. The highest frequency I have worked on is 18GHz. What method was used to determine this frequency peak?

The WMAP is looking for tiny differences in RF amplitude, but shouldn't we be measuring the frequency of the frequency peak to determine the actual CBR temperature?

Has there been any resolution of the following: Since CBR is received from the edge of the observable universe, and we are at the center of our observable universe, why is there a large red shift on the CBR? How does the red shift affect the WMAP measurements?

Also what would be the average attenuation of the CMB after traveling 13.8 Billion light years? And what is the spread on this attenuation and how might this affect the WMAP measurements?

 

[Chalnoth]

In your description, why would the microphones all measure different signals?

In WMAP, the reason is that they get different amounts of the signal from the galactic plane. The reason they get different amounts of the galactic signal is that the galactic signal is not a black-body spectrum at a temperature of 2.725K.

In the microphone analogy, this is similar to there being another person in the room, Fred, who is also talking, sometimes much louder than George, but Fred is not situated at the same location, so some microphones pick up Fred's voice louder than other microphones. Thus we can use a linear combination of the microphones to cancel out Fred's voice while keeping George's. As long as Fred isn't moving around, this is a relatively simple operation.

Anyway, I think I will have to move on from this question until I can get a more detailed picture of the actual RF signals entering each of the difference receivers. There is plenty of other sky to consider.

Well, understanding the RF signals themselves isn't all that likely to give you an understanding of how the WMAP team went back and subtracted the galaxy from the maps to get the CMB.

But the basic idea is that each detector is a radiometer which detects the amount of radiation impacting the telescope from a particular direction within a small range of frequencies. The central frequencies of the detectors are 23GHz, 33GHz, 41GHz, 61GHz, and 94GHz. The instrument subtracts the signal from a detector looking at one part of the sky from the value of a corresponding detector (which measures the sky at the same frequency) looking at another part of the sky. As WMAP orbits the Sun, WMAP spins around, scanning the sky so that it is always pointed away from the Sun. Every six months, WMAP scans the whole sky once (well, almost...some holes are left, but these are covered in the next six months).

From the time-ordered data, the WMAP team produces maps of the whole sky at each frequency. These are not maps of absolute amount of radiation hitting the telescope at each frequency, but of the deviation from the average amount of radiation hitting the telescope.

It appears that with the center of the frequency peak at 160GHz so we would prefer to measure the signal there, but this is not easy or as accurate with today's technology. eg. The highest frequency I have worked on is 18GHz. What method was used to determine this frequency peak?

This was done earlier with the COBE satellite's FIRAS instrument, which measured the spectrum of the CMB to tremendous accuracy:
http://lambda.gsfc.nasa.gov/product/cobe/cobe_images/firas_spectrum.jpg

On this plot, the error bars are smaller than the trend line at every point. Not just slightly smaller, but mind-bogglingly, absurdly smaller. The measurement error at the peak is around 0.0035%. All but the last three data points have an error less than 1%.

The WMAP is looking for tiny differences in RF amplitude, but shouldn't we be measuring the frequency of the frequency peak to determine the actual CBR temperature?

Well, this was done by FIRAS to tremendous accuracy, and so isn't really that interesting to scientists any longer. We want to know the small deviations in temperature across the sky, and in order to distinguish between the CMB and other sources, we need to look at the sky at multiple different frequencies.

Planck, by the way, should measure the CMB near the peak. It has detectors that look at the sky in nine different frequency bands from 30GHz to 857GHz. The 30GHz-70GHz detectors are radiometers, like WMAP (but actively cooled instead of using the differencing strategy). The 100GHz-857GHz detectors are bolometers, which instead of being antennas are designed so that energy deposited in the bolometer increases the temperature of the bolometer slightly, leading to a change in electrical resistance that can be measured. I believe the frequency of the radiation that strikes each detector is singled out by constructing a horn which only permits the passage of radiation within a specific wave band.

Has there been any resolution of the following: Since CBR is received from the edge of the observable universe, and we are at the center of our observable universe, why is there a large red shift on the CBR? How does the red shift affect the WMAP measurements?

The redshift comes from the expansion of the universe. The universe has expanded by a factor of about 1090 since the emission of the CMB, which has multiplied the wavelength of each photon emitted at that time by a factor of 1090.

Also what is the average attenuation of the CMB from after traveling 13.8 Billion light years? What is the spread on this attenuation and how does this affect the WMAP measurements?

About 8% of the radiation is lost in transit. This can be measured due to the correlation between the polarized signal and the unpolarized signal, as the ionized gas in between us and the CMB is sensitive to the polarization of the light.

 

[Tanelorn]

Thanks Chalnoth. I still have one question regarding "We want to know the small deviations in temperature across the sky". By this you mean variation in the center frequency?

The WMB is measuring intensity how does this translate to frequency variation?

I am astonished that only 8% of the energy is lost in transit. Even a 10ft coax would lose more than this!

On WMAP what is the relationship in angle or direction for the two inputs used to generate the relative or difference temperature?Regarding redshift I understood that all directions are moving away from us equally fast. Yet measurements show our galaxy is moving at a considerable speed causing a CBR red shift in one direction and a CBR blue shift in the other. I read that something about this could not be explained since we are at the center of our observable universe.

http://www.astro.ucla.edu/~wright/CMB-DT.html

 

[Chalnoth]

Thanks Chalnoth. I still have one question regarding "We want to know the small deviations in temperature across the sky". By this you mean variation in the center frequency?

Well, that's not so easy to measure. But small differences in temperature do lead to different amounts of radiation at every frequency, and the amount of radiation of a particular frequency striking the telescope we can measure. The way the maps are calibrated is that if the CMB deviates from its central temperature of 2.725K by 100uK in one direction, then the corresponding pixel takes on the value of 100uK at every frequency, up to instrument noise and contamination from other sources.

I am astonished that only 8% of the energy is lost in transit. Even a 10ft coax would lose more than this!

Yes, the universe has been extremely transparent since the CMB was emitted!

On WMAP what is the relationship in angle or direction for the two inputs used to generate the relative or difference temperature?

They observe the sky at an angle of 141 degrees from one another. The satellite rotates and precesses so that each detector points at each direction in the sky over the course of a little over six months.

Regarding redshift I understood that all directions are moving away from us equally fast. Yet measurements show our galaxy is moving at a considerable speed causing a CBR red shift in one direction and a CBR blue shift in the other

Well, things are, on average, moving away from us at a rate proportional to distance that is the same, again on average, in all directions, once we have subtracted our own motion.