How is speed of Milky Way determined relative to CMB dipole?

Question

I read that our galaxy the milky way is moving at approx 630km/s relative to CMB dipole, I think they applied Doppler shift to find this number but how do they do that? I know when universe is a hot soup of plasma matter and antimatter are spontaneously being created and annihilated then mysteriously a small but significant portion of ordinary matter and vast amount of byproducts, pure energy or what we called CMBR today are the leftovers. This CMB dipole is ancient and how do you use it to measure our galaxy speed?

Answer 1

There is nothing mysterious about the cosmic microwave background radiation (CMBR). It is simply the sea of red-shifted photons that were emitted when the early universe became transparent to electromagnetic radiation (about $379,000$ years after the Big Bang) and have not been absorbed by any matter since then.

The observed CMBR contains a dipole anisotropy (also known as the first spherical harmonic) which is due to the motion of the observer (in this case, the earth or a satellite orbiting the earth) relative to the CMBR. This dipole has three components:

1. A component due to the earth’s orbital motion around the sun. This component can be directly measured by comparing CMBR observations taken at different times of the year.
2. A component due to the sun’s orbital motion within our galaxy. The magnitude and direction of this motion can be accurately estimated from observations of the relative motions of our galactic neighbours.
3. A residual component which is due to the motion of our galaxy relative to the CMBR. This is estimated by subtracting the first two components from the total.

The dipole anisotropy must be subtracted from the CMBR before an analysis of other higher order anisotropies can be made, which gives us information about variations in the density and make-up of the early universe.

For more details see this Wikipedia article.