### [astro-ph/0601559] Aberration of the Cosmic Microwave Backgr

Posted:

**January 26 2006**This paper discusses angular aberration of the CMB (well known; see also astro-ph/0112457, a reference the authors seem to have missed). If an observer moves with respect to the CMB they see a dipole-like rotation of angles on the sky, leading to a magnification and demagnification in opposite hemispheres.

As it happens I've also been thinking about this the last week. Two points:

* Gravitational lensing of the CMB in general has a dipole convergence which is exactly the same as lowest order abberation; if you want to measure the primordial dipole (in principle) you have to get round this.

* I believe you can show that the frame in which the kinetic aberration vanishes is the same frame as the one in which the kinetic contribution to the dipole CMB vanishes, and coincides with the Newtonian Gauge (in linear theory). This seems to be an interesting physical identification of the CMB frame with the Newtonian gauge.

As it happens I've also been thinking about this the last week. Two points:

* Gravitational lensing of the CMB in general has a dipole convergence which is exactly the same as lowest order abberation; if you want to measure the primordial dipole (in principle) you have to get round this.

* I believe you can show that the frame in which the kinetic aberration vanishes is the same frame as the one in which the kinetic contribution to the dipole CMB vanishes, and coincides with the Newtonian Gauge (in linear theory). This seems to be an interesting physical identification of the CMB frame with the Newtonian gauge.