# Does the CMB allow special rest frames to be defined?

1. Mar 30, 2009

### Spinnor

Does the CMB allow special "almost" rest frames to be defined? In this sense, use a rocket to put yourself in a frame of reference such that in this frame observations of the CMB would average to zero (average over photon momentum) over your celestial sphere.

In such a rest frame is the total momentum of the Universe is about zero?

Such a rest frame could never be found precisely by experiment? How close could we get with present day observations of the microwave background radiation?
Thank you for any thoughts.

2. Mar 31, 2009

### atyy

3. Mar 31, 2009

### clem

The CMB is radiation, not a preferred reference system.

4. Mar 31, 2009

### Dmitry67

CMB alows you to define a special preferred frame in every point, but not globally in the universe. Frames, defined based on CMB will be different in different parts of the Universe

So it does not help to defined an absolute rest frame, but observing CMB we see a spontaneous symmetry breaking on the universe level.

5. Mar 31, 2009

### phyti

Do events move?

6. Apr 2, 2009

7. Apr 2, 2009

### marcus

Certainly. The solar system is moving at 370 km/s relative to CMB rest. The current direction in the sky is marked by the constellation Leo. So if you superimpose a map of CMB temperature over a star map, there is a slight Doppler hotspot around Leo. And a coldspot in the opposite direction.

This is called the CMB dipole. Here's what Ned Wright has to say about it:
http://www.astro.ucla.edu/~wright/CMB-dipole-history.html

One way to understand it is to make a very rough order of magnitude calculation of the Doppler temperature shift. The speed of light is around 300,000 km/s and the solar system's speed is very roughly 300 km/s which is a factor of 1/1000----a tenth of a percent. That means that the CMB temperature should be about a tenth of a percent hotter in the direction of Leo.

That Doppler dipole totally swamps all the smaller little fluctuations in the CMB sky, so they subtract it out. They compensate for the solar system motion in the data, so when you see those beautiful oval blue and red maps of the CMB temperature all you see are the small fluctuations, speckles and blotches, that are really there in the CMB. They are not merely a Doppler result of the earth and solarsystem motion relative to background.

The small fluctuations are more on the order of a hundredth of a percent, compared with a tenth percent.
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Be sure you are not imagining big bang expansion cosmology as an explosion, with stuff flying away from some point in space. This is the most common mistake people have and it cripples understanding. The universe has no collective momentum as far as we know, the CMB has no overall momentum, it isn't going anywhere. It is just a nearly uniform bath of radiation coming equally from all directions. And that is how it was back 13 billion years ago, the same only hotter.

Here's a quote from Scott's FAQ:
"How come we can tell what motion we have with respect to the CMB?

Doesn't this mean there's an absolute frame of reference?
The theory of special relativity is based on the principle that there are no preferred reference frames. In other words, the whole of Einstein's theory rests on the assumption that physics works the same irrespective of what speed and direction you have. So the fact that there is a frame of reference in which there is no motion through the CMB would appear to violate special relativity!

However, the crucial assumption of Einstein's theory is not that there are no special frames, but that there are no special frames where the laws of physics are different. There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe. But for doing any physics experiment, any other frame is as good as this one. So the only difference is that in the CMB rest frame you measure no velocity with respect to the CMB photons, but that does not imply any fundamental difference in the laws of physics."

There is no overall reference frame for the universe. Too big, too curved, too dynamic. But there is a global criterion of being at rest. So you can construct local frames that work in your immediate neighborhood and you can make your local frame be at rest relative CMB, if you want.

Naty cited the Wikipedia article on CMB:
This gives the speed of our local group of galaxies relative to CMB rest.
"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) in the direction of galactic longitude l = 276° ± 3°, b = 30° ± 3°."

Our own galaxy has pretty much the same velocity as the local group---call it 600 km/s. Its velocity vector points towards the southern constellation of Crater (near Corvus, in the general direction of Hydra and Centaurus).
Our solarsystem velocity vector is a combination of the 600 km/s movement of our galaxy, partly canceled by the orbital velocity of the sun as it circles around the center of our galaxy. These cancel to a certain extent, which makes our solarsystem 370 km/s speed come out less than the overall speed of the galaxy.
You can picture it somewhat like when you toss a Frisbee the overall speed of the Frisbee can be greater than the instantaneous speed of a point on part of the edge that happens to be whirling somewhat back towards you as the Frisbee is sailing forwards away from you.

Last edited: Apr 2, 2009
8. Apr 9, 2009

### zonde

Body that is moving relative to CMB will experience radiation pressure from blue shifted CMB and will decelerate towards CMB rest frame.
At least it seems so to me.