The CMB refence frame: A moving frame ?

[Jacques]

In an other thread Martin told

the world is full of preferred frames. the expansion of the universe, and the CMB defines one----the FRW metric that all cosmologists use has a built in preferred frame.

 

I have difficulty to imagine the microwave background like a reference system. Microwave are radiations and radiations are not fixed in space...

Can someone help me understand that one ?

Thanks

[YT2095]

I think it refers to the Frequency rather than the Mode.

 

ie/ the 21cm Hydrogen band is used often in radio astromomy as a reference point

[Martin]

In an other thread Martin told

 

 

I have difficulty to imagine the microwave background like a reference system. Microwave are radiations and radiations are not fixed in space...

Can someone help me understand that one ?

Thanks

 

hi Jacques, i will have to get you some links as refs to what i say but the essential is

 

that when the temperature of the CMB was mapped it was found that the overall whole-sky average was some 2.7 kelvin

but there was a large hotspot in the direction of Leo

(and a corresponding coldspot 180 degrees from Leo)

 

as if the earth and sun were moving in the direction of Leo at a speed of

350 km/second

and the doppler effect was heating the CMB microwaves in the Leo direction

and cooling the microwaves in the opposite direction

 

this has been discussed by the directors of the "COBE" program, satellite cosmic background explorer, around 1995

 

the motion is believed to be a combination of the motion of the sun WITHIN our milkyway galaxy and the motion of the galaxy itself, in the background of the CMB.

 

 

so the astronomers have constructed a reference frame of an observer who is compensating by going away from Leo, in such a way that he sees no hotspot and no coldspot.

 

he is at rest with respect to CMB

 

there is more to say but I must go out for a little while, be back later

[Jacques]

OK The CMB is not a reference system but is used to create a reference system.

OK

Thanks

[Martin]

sure no problem,

interestingly enough even before the CMB was mapped enough so one could say "at rest with respect to the CMB"

 

there was an even earlier idea of being at rest

which came from Hubble, he mapped the expansion of space

and from the standpoint of the sun and earth it is lopside

 

If you look in the direction of Leo the distant galaxies are not receding away from us quite as fast as they should be

(there is randomness but I am talking about averages)

 

and if you look in the opposite direction the galaxies at some given distance tend to be receding faster than average, faster than they should be.

 

so the earth and sun are not "at rest with respect to the Hubble Flow" as it is called. there is this statistical lopsidedness.

 

the expansion of space is not symmetric around us, from a sun and earth perspective.

 

so one must correct by the same velocity vector as with the CMB

and then one gets a symmetric expansion

 

============

Indeed General Relativity normally produces solutions for spacetime for which there is (up to rotations) a preferred frame, in other words an idea of rest. The idea that there should be no preferred frame is a notion of the earlier theory the 1905 Special.

 

Classical General Relativity (1915) gives cosmologists the socalled Friedmann Robertson Walker metric which is what they all use. this metric, the FRW metric, is a solution to the main equation of GR and it is what people use when they talk about Inflation, Big Bang, Dark Energy, accelerated expansion, surface of last CMB scattering, etc etc etc.

 

naturally this FRW metric does not have lorentz symmetry of Special Rel (IT IS NOT FLAT after all!) so one cannot say that all observers are equal.

there is in the FRW metric the idea of an observer who is at rest with respect to the metric, and this defines a preferred frame up to rotation at least, and as one expects it corresponds to being at rest also with respect to the CMB and the recession of distant galaxies.

 

the FRW metric dates back to about 1922, Alex Friedmann who lived in what is now St. Petersburg, Russia. Very interesting guy, worth looking up.

Mainstream cosmology has relied on the FRW metric for the better part of a century now. It is what predicted that there would be a CMB etc. Very entrenched. Keeps on fitting the data (esp since 1998 with a positive Lambda term, cosm. const, inserted).

 

the 1905 Special picture dominates the mass media science popularization culture and is applicable as an excellent local approximation in situations where space is locally nearly flat and almost not expanding (i.e. curvature, expansion, geometric effects of gravity negligible). So it is perfect for particle physics, for instance. But one should not take it too seriously or apply it in the large, where it does not fit.

 

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this is just my personal views, dont want to sound like a pedant.

 

i will try to find you a link to something that shows the CMB hotspot in the sky

 

YESSSS!

http://aether.lbl.gov/www/projects/u2/

 

the first discovery of it was actually with a U2 plane, it did not even take the COBE satellite, which was later circa 1990