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TheNick
2008-Jan-21, 07:36 PM
The millikelvin dipole in the cosmic microwave background. I just want to know where it comes from.

Cougar
2008-Jan-21, 08:11 PM
The millikelvin dipole in the cosmic microwave background. I just want to know where it comes from.
It's just the result of our solar system's motion with respect to the CMB (or the Hubble flow). Here (http://www.physicsforums.com/showpost.php?p=29516&postcount=18) is an informative post on this subject on the Physics Forums.

TheNick
2008-Jan-21, 09:25 PM
So would I be correct to assume the dipole is constantly shifting, as the Earth's direction of motion is constantly changing (as it orbits the Sun, as the Sun orbits the galaxy, etc.)?

Hornblower
2008-Jan-22, 01:56 AM
So would I be correct to assume the dipole is constantly shifting, as the Earth's direction of motion is constantly changing (as it orbits the Sun, as the Sun orbits the galaxy, etc.)?
The annual variation should be observable with a suitable radio instrument. Given enough time, meaning hundreds of millions of years, the galactic orbital cycle should show up even stronger.

Nereid
2008-Jan-22, 02:53 AM
So would I be correct to assume the dipole is constantly shifting, as the Earth's direction of motion is constantly changing (as it orbits the Sun, [...] ?
It is, and it was ... IIRC, some of the COBE (the first space mission to observe the CMB) papers go into this in considerable detail, along with how they removed the Zodiacal foregrounds, and much more ...

Nereid
2008-Jan-22, 01:14 PM
Update:

The CMB dipole was detected before COBE, though I haven't yet found a paper which describes daily and/or annual signals in it.

This 1990 COBE team paper (COBE Differential Microwave Radiometers - Instrument design and implementation (http://adsabs.harvard.edu/abs/1990ApJ...360..685S)) describes the design and operation of the radiometers, and includes a brief introduction to the dipole.

This 1991 COBE team paper (Preliminary results from the COBE differential microwave radiometers - Large angular scale isotropy of the cosmic microwave background (http://adsabs.harvard.edu/abs/1991ApJ...371L...1S)) includes the following:
We correct the calibrated data to solar system barycenter to remove the effects of satellite motion (7.4 km s-1) around Earth and Earth's motion (30 km s-1) around the solar system barycenter.

This 1993 COBE paper (Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps (http://adsabs.harvard.edu/abs/1993ApJ...419....1K)) shows, in Figure 4, a nice application of the Earth's orbital doppler shift signature to independently estimate the temperature of the CMB, using the differential microwave radiometers ("The data follows the expected dipole pattern.")