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Bill S. Offline OP
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Recently it occurred to me that my mental image of the CMB radiation, gleaned from a range of popular science books, was probably completely wrong. I set about some thinking. The following notes outline my thoughts so far, and I would appreciate the comments of others.

Thinking about the CMB

What is the cosmic microwave background? Put simply; it is the remnant radiation from the Big Bang.

The theory is that the Universe, up to almost 400,000 years after the Big Bang, was so hot and dense that it was effectively a plasma within which photons were prevented from travelling any distance, light could not travel through it, so it was opaque.

At a little less than 400,000 years (some references give this as 300,000 years), at what is referred to as the photon decoupling event (also known as the period of last scattering), the Universe had cooled sufficiently for protons and electrons to combine into atoms. This changed the state of the Universe such that photons were able to move freely through the it. It became transparent. These photons have been travelling ever since, but are no longer in the wavelengths of visible light. There energy has decreased, and, correspondingly, their wavelength has increased, so that they now appear as microwaves, with a temperature of about three degrees above absolute zero (almost 3K).

Some questions must arise from this, for instance:
1. Where is the radiation coming from?
2. Where is it going?
3. It is travelling at the speed of light, so why had it not passed us long ago?
4. If its origin is in a small spot that must be central to the Universe, does this give a preferred direction to the Universe?

Attempting to answer these questions challenges the lay person's intuitive image of the Big Bang, the expanding Universe and the CMB radiation.

1. Where is the radiation coming from? This implies another question: Where did the Big Bang happen? The answer to this is that it happened everywhere in the Universe. At the first instant, the Big Bang was the Universe; the Universe was the Big Bang. There is no part of the Universe today, nor will there ever be, however long it continues to expand, in which the Big Bang did not occur. The Big Bang was everywhere; so the radiation must be coming from everywhere.

2. Where is it going? If it originated everywhere in the Universe, it follows that it must be going everywhere in the Universe. It is moving at the speed of light from every point in the Universe to every other point in the Universe, without exception.

3. It is travelling at the speed of light, so why had it not passed us long ago? To some extent, this question has already been answered, but if by "us" we mean the point in the Universe at which the Earth is situated, we must accept that it has been passing us ever since the original radiation was able to move freely through the Universe, and it will continue to pass us, and every other point in the Universe, as long as any energy remains in the waves.

4. If its origin is in a small spot that must be central to the Universe, does this give a preferred direction to the Universe? As mentioned earlier, we have to abandon the image of the Big Bang happening at the centre of the Universe, and the radiation emanating from there and moving outward. Viewed from the Earth, the radiation would be seen to be coming from every direction. This does not mean that the Earth is at the centre of the Universe, because, if that image were right, the radiation would appear to be moving away from Earth, and would, therefore, not be visible. Also, the Earth's position is not special. Whatever viewpoint in the Universe an observation might be made from, the radiation would still be observed to be coming from every direction towards that point. Therefore, no preferred direction can be identified by observation of the CMB radiation.

I said that radiation that is receding would not be visible, yet we are all familiar with doing something like shining a powerful flashlight into the sky and seeing a beam of light that is obviously moving away. This is due to the fact that some of the light is reflected back from water molecules, dust particles etc in the atmosphere. The clearer the air, the less visible would be the beam of light. In space, we would not see it at all unless it were shining straight towards our eyes. For this reason, only the CMB radiation moving directly towards a detector can be detected.

One might also ask if the CMB, because it fills the Universe, can be regarded as a "fixed" reference against which motion in the Universe can be measured. If this were the case, motion would no longer have to be regarded as solely relative. Absolute motion could be demonstrated.

The first thing to recognise is that the CMB is electromagnetic radiation, and as such it is observed as moving at the speed of light, irrespective of the position and (relative) motion of the observer. It seems unlikely, therefore, that the CMB could be regarded as an absolute frame of reference, any more than can any other electromagnetic radiation, including visible light.


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We do use the CMB as a frame of reference, since we are now able to measure the wavelengths quite accurately in every direction:

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

CMBR dipole anisotropy

From the CMB data it is seen that our local group of galaxies (the galactic cluster that includes the Solar System's Milky Way Galaxy) appears to be moving at 627±22 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB) in the direction of galactic longitude l = 276±3°, b = 30±3°. This motion results in an anisotropy of the data (CMB appearing slightly warmer in the direction of movement than in the opposite direction). The standard interpretation of this temperature variation is a simple velocity red shift and blue shift due to motion relative to the CMB, but alternative cosmological models can explain some fraction of the observed dipole temperature distribution in the CMB.


What's being measured is not the speed of light which, as you say, is the same for all observers, but the temperature/wavelength/energy of the light (EMR if you prefer) which is subject to Doppler shift.


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In simpler terms, would that be a blue shift in the direction in which the observer was moving; and a red shift in the other direction?

This would establish motion relative to the CMBR, but is still relative motion rather than absolute motion?


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Yes, that's right, a measured blue shift in the direction of motion relative to the 'rest frame' of the CMBR, and a commensurate red shift in the opposite direction.

Regarding the use of the term 'absolute': having scanned a few discussions on the topic in other forums, I've decided not to dwell on it. It's not worth letting my tea get cold smile


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Quote:
.... 'rest frame' of the CMBR


We can't assign a rest frame to a photon, the CMBR is composed of photons, travelling at c. In what way can the CMBR be said to have a rest frame?


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Originally Posted By: Bill S.
We can't assign a rest frame to a photon, the CMBR is composed of photons, travelling at c. In what way can the CMBR be said to have a rest frame?

The CMBR doesn't have a rest frame. It's called a rest frame because a body is considered at rest relative the CMBR if it is receiving the radiation at the same energy level from all directions.


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Okay there are parts of truths in all the answers so lets do an explaination

Under strict General Relativity there is no zero reference frame and there is no absolute space

>>>>> All observers who are moving by constant velocities relatively to each other may use the same laws of physics (the principle of relativity). <<<<<<

So the CMBR can be viewed and interpretted from our reference frame. If you want to check this the CMBR looks the same for earth based radiotelescopes as well as faster moving satelitte radiotelescopes.

So what is our reference frame well it is 2.7K temperature frame of the universe and it is not absolute or a zero reference frame.

There is another reference frame which we hope we may be able to use later this year which is 1.95K frame from the nuetrino background radiation which will come from the planck satelite.

http://en.wikipedia.org/wiki/Cosmic_neutrino_background
http://en.wikipedia.org/wiki/Planck_(spacecraft)


SO THE IMPORTANT LESSON ALL OBSERVATIONS HAVE A RELATIVE REFERENCE FRAME AND THEY ARE NEVER PRIVILIGED OR THE ZERO FRAME.

Even the CMBR observations or other cosmological observations of the universe are not exempt from this law.

So answering your actual original question the CMBR reference frame is an event horizon 380,000 years after the Big Bang taking whatever shape the universe was then and the radiation is travelling in every direction trying to reach the edge of the universe which it never can because of the particle horizon




Last edited by Orac; 02/12/13 02:39 AM.

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Originally Posted By: Orac
>>>>> All observers who are moving by constant velocities relatively to each other may use the same laws of physics (the principle of relativity). <<<<<<
You are quoting Special Relativity, so it's misleading. You need to update it in view of General Relativity, which informs us that the same laws of physics apply for all observers, everywhere, irrespective of relative velocity factors.


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General realtivity generalises special relativity and it is the relative movements in trying to make a reference frame that is causing Bill S problems.

Never forget that GR embodies SR it is important and just saying GR says so doesn't help anyone you need to move back into Special Relativity to see how there is a cosmic reference frame you can use.


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An "aside" question about the cosmic particle horizon.

I believe that when the expansion of the Universe was thought to be slowing, it was reasoned that the CPH would expand with time. Does this mean that the CPH will shrink with time if the expansion of the Universe is accelerating?


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Correct on both accounts Bill S as shown in the second image above .. it appears you have clearly got your head around the reference frame the CMBR is using now.

Note if we are correct and the universe is basically flat the CPH would only start to shrink when the combined speed of the universe edge movement dropped below the speed of the light.

This may sound weird but again we have to go back to special relativity and its extension into GR to understand it

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

Quote:

While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, it places no theoretical constraint on changes to the scale of space itself. It is thus possible for two objects to be stationary or moving at speeds below that of light, and yet to become separated in space by more than the distance light could have travelled, which can suggest the objects travelled faster than light. For example there are stars which may be expanding away from us (or each other) faster than the speed of light, and this is true for any object that is more than approximately 4.5 gigaparsecs away from us. We can still see such objects because the universe in the past was expanding more slowly than it is today, so the ancient light being received from these objects is still able to reach us, though if the expansion continues unabated there will come a time that we will never see the light from such objects being produced today (on a so-called "space-like slice of spacetime) because space itself is expanding between Earth and the source faster than their light can reach us.


There is a similarity you can easily show which is large scale thermal expansion



You can imagine the longer the tube is the faster the speed and distance of the movement of the two lines the student has marked.

Last edited by Orac; 02/12/13 02:08 PM.

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I suspect that comoving coordinates are going to come in here, so I had better check that thoughts on that subject are on track.

The detectable Universe is expanding. Galaxy groups are in motion relative to one another, such that they are constantly separating. Any two galaxy groups will be further apart at time = t2 than they were at t1.

The rate of separation of the galaxy groups is proportional to the rate of expansion of the Universe. Thus, if the distance between them is considered as a proportion of the size of the Universe, it remains constant.

A corollary of this is that everything in the Universe is comoving unless it is moving in such a way that its motion is independent of the expansion of the Universe.

The relevance of comoving coordinates to the CMBR is that a comoving observer will observe the CMBR as isotropic, whereas a non-comoving observer will see a blueshift in the forward direction and a redshift in the other.


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Quote:
Note if we are correct and the universe is basically flat the CPH would only start to shrink when the combined speed of the universe edge movement dropped below the speed of the light.


Lost!!!

I had the CPH growing if the Universe's movement was less than c.


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Consider three bodies (A, B & C) in expanding space. At t1 A and B are 1ly apart, C is at such a distance, and is moving at such a speed that its light stops reaching A at t1. Presumably the light from C will stop reaching B one year later, at t3.

In this scenario, C will have crossed the cosmic particle horizon long (possibly billions of years) before its light stops reaching A and B, but it is the time and manner of its stopping reaching A and B that is of interest here.

At t2 (half way between t1 and t3) light is still passing B at c, and at a little beyond the mid-point it is still passing at c. What happens to prevent it from reaching A?


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Originally Posted By: Bill S.
Quote:
Note if we are correct and the universe is basically flat the CPH would only start to shrink when the combined speed of the universe edge movement dropped below the speed of the light.


Lost!!!

I had the CPH growing if the Universe's movement was less than c.


Haha that english thing again

My reference point was the universe so the CPH would shrink relative to the universe was how I was looking at it.

However you looked at it from the CPH reference point so you say the CPH is expanding which it is from that point of view.

Same thing different reference .... but slightly confusing smile

Anyhow we both agree the drop below c for the universe expansion would bring about the change.

Last edited by Orac; 02/15/13 04:06 AM.

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I was ferreting around on NASA site this evening for some detail on a cosmic ray paper but ran across this site

http://map.gsfc.nasa.gov/resources/camb_tool/index.html

Might interest you what the CMB would look like if you alter the universe structure.

Quite an interesting applet.

If you want some interesting background on the CMBR power spectrum start here

http://backreaction.blogspot.com.au/2007/12/cmb-power-spectrum.html

Last edited by Orac; 02/15/13 04:25 AM.

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