Professor, Department of Astronomy and Astrophysics
University of Chicago

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Electron Glue

Key Concepts

In the plasma before recombination, free electrons act to glue the CMB photons to the baryons.

How this works is that the CMB photons can't travel very far before they Thomson scatter off an electron.  The two are therefore tightly-coupled together and behave like a nearly perfect fluid.  Likewise the electrons can't travel very far from a baryon before electromagnetic or Coulomb interactions force the charged baryons to follow:

[Note: This picture isn't quite fair since there are actually substantially more CMB photons than baryons or electrons in the universe.  Despite the inertial mass of the baryons, photons actually dominate the dynamics of the plasma.  The fluid is a relativistic fluid.]