Looking back


As we look out into space we look back into time...

Carl Sagan  

The CMB photons which we are receiving now set off in straight lines 14 billion years ago when the plasma combined to form neutral atoms.

So they started from a spherical "shell" all around where the Earth would later form
the so called "Surface of Last Scattering".
Variations in their brightness tell us about variations in the density of the plasma at that time...