[Bill S (OP)]

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 more knowledgeable than I.

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.




     

[lostinspace]

The way I try to imagine it is that what we see now is light emmited 13.5byr ago from a sphere about 5b lightyears away from us. As the universe has expanded the remaining distance to travel has continually increased so that it has taken 13.5byr to get here. Meanwhile the source has expanded out to a sphere which is now 45b lightyears radius so the visible universe is  about 90b lightyears across.
This assumes the photon we see was heading our way in the first place and just as many will be heading the other way. Also anything emmited from within this 5b lightyear sphere  and heading our way will have passed us already.
The quoted 5b lightyears is very dependant on the hubble parameter which seems to have given a variable expansion rate through time.

[Bill S (OP)]

I think I shall have to read that a few times to get my head round it.

anything emmited from within this 5b lightyear sphere  and heading our way will have passed us already.

Does this take into account the fact that Earth's location would also have been inside the same sphere at that time?

[JP]

Very astute observations, Bill!

I'll just throw this out there and resist trying to explain it in too much detail, since it's way beyond my areas of expertise, but you can indeed pick a reference frame which makes the CMBR look the same in all directions (isotropic), which has to do with loosely, I believe, flowing along with the "expansion" of the universe rather than choosing a frame that does not flow along with the expansion.  (For example, sitting still and letting the universe carry you along as it expands vs. getting in a rocket and heading off at near-light speed in some direction.) 
http://en.wikipedia.org/wiki/Comoving_distance#Comoving_coordinates

Maybe a cosmologist can pop in and enlighten us both!

[Bill S (OP)]

Thanks JP.

I'm glad you didn't go into "too much detail"; the simple explanation is giving me enough trouble!

Now I'm going to have to come to terms with comoving coordinates.  I guess that means taking the dog for a walk.

[Bill S (OP)]

OK, dog walked, various domestic exigencies dealt with; time to expose my thoughts about comoving coordinates to criticism.

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.

[JP]

Yup, that's basically how I understand it, Bill.  It seems odd that the universe has a preferred set of coordinates in that sense, but I guess it's not too odd.  After all, every day we use our reference frame on the earth to define a preferred set of coordinates with respect to the earth.  It's just that in this case, we're defining it with respect to the universe's expansion.

[imatfaal]

JP can correct me (or is soulsurfer still around haven't seen for a while) - it is important not to think of the cmbr an absolute set of coordinates that go back to very old ideas of absolute time and absolute space; the cmbr is just a useful background and added part of toolkit.  ie both the OS grid references and latitude/longitude are very useful to find way around UK but neither the more local OS nor the global lat/long are anything but arbitrary human tools for measurement

[JP]

Yeah, as I understand it, although we can't claim to have an absolute reference frame, we can always look around us and measure our reference frame with respect to something else.  In this case that something else is the expansion of the universe.

[yor_on]

So space grows bigger, and light gets redshifted losing energy, and that must be the same from any frame you choose to measure it from. Is there a relevance to those two being together, can one assume that it represents the energy needed for a expansion? And it gets even weirder if you imagine yourself co moving with a light quanta, as the definition then becomes one of no change at all to it.