Conclusion


The early universe was a dense, hot plasma of photons, electrons and protons.
As time passed it expanded and the temperature fell.
At 400,000 years it was cool enough that the electrons and protons formed neutral gas.
After this the photons streamed freely through the universe - we can observe them at Earth today.

The plasma was not perfectly smooth.
It underwent gravity driven acoustic oscillations.
We see the imprint of these as patterns in the CMB sky.
The power spectrum of the pattern tells us about the nature of the universe in which we find ourselves.