The Recombination Era
Age of Universe: about 300,000 years
The light comes to us from the time when
hydrogen atoms were able to recombine and stay that way. The
interaction of photons with matter was then restricted to the hydrogen spectral
lines, leaving
almost all photons free to fly unimpeded.
We see such
photons redshifted
by z=1000 (meaning the Universe was 1000 times smaller at that time).