# Are we comoving observers of space expansion?

In cosmology:

A comoving observer is the only observer that will perceive the universe, including the cosmic microwave background radiation, to be isotropic. (Wikipedia)

According to this definition, is Earth considered as a comoving reference frame, or are we supposed to have a "peculiar velocity"?

What is the current precision for measuring if a frame is comoving or not, and for measuring its peculiar velocity? Or: From which speed (with respect to Earth) a frame would be considered as peculiar?

We have a small peculiar velocity with respect to the comoving frame, this can be seen as a dipole in the CMB data. (CMB gets doppler shifted)

This dipole (and the monopole) is usually subtracted before doing further analysis of the CMB. I think (but I am not sure about this) that measuring the CMB-dipole is the best and easiest way to find earths peculiar velocity with respect to the comoving frame.

There is no sharp division between an object with a peculiar velocity and one without, the question is, how large is the peculia velocity compared to what scales we are talking about.

The numerical value for the peculiar velocity is:

(369 $\pm$ 0,9) km/s.

You can find it here: http://arxiv.org/pdf/1303.5087.pdf

• I like it, +1. Adding the numerical value for Earth's mean peculiar velocity and the approximate precision of that number would make this answer perfect. – Jim Jan 20 '15 at 15:22
• Added the actual numerical value, per your request:) – Ihle Jan 20 '15 at 16:15
• That's a phantastic precision! – Moonraker Jan 20 '15 at 16:26
• Yes, the CMB has made cosmology into a precision science! Especially after WMAP and PLANCK. – Ihle Jan 20 '15 at 16:28