As I previously understood it, before recombination, high energy photons were bound up in interactions with excited electrons, meaning electrons couldn't settle into orbit with protons and other nuclei.

However, there were lots of free protons about, whose electric charge would have made them interact electromagnetically. Were photons as much bound up with protons as electrons during this phase? If so, did those interactions produce electron / positron pairs?

  • $\begingroup$ ( bound photons or bouncing photons ? bound means only that there are excited particles/atoms. ) Atomization is a very particular configuration which minimizes the interactions between near nucleus components $\endgroup$
    – user46925
    Commented Jan 23, 2016 at 15:40

1 Answer 1


From the theory of Thompson scattering (see http://farside.ph.utexas.edu/teaching/em/lectures/node96.html ) we know that a charged particle of mass m interacting with a plane wave electromagnetic field of Strength $E_0$ and frequency $\omega$ has an effective dipole moment of magnitude $$d=\frac{e^2E_0}{m\omega^2}$$

Note that the dipole moment scales inversely with the mass of the charged particle. It is this dipole moment (actually $d^2$) that determines the amount of energy that the charged particle scatters (radiates) from the electromagnetic field. Hence electrons with a mass some 2000 times less than that of the proton are much more efficient at interacting with (scattering) a high energy photon. Technically the protons contribute as well but the overwhelming majority of energy scattering (and hence decoupling) is between electrons and photons.

  • $\begingroup$ typo : 2000 times less ... $\endgroup$
    – user46925
    Commented Jan 23, 2016 at 22:08
  • $\begingroup$ @I Gael Thanks. The mind is quicker than the fingers. $\endgroup$ Commented Jan 23, 2016 at 23:03
  • $\begingroup$ $RobJeffries Thanks. I'll edit to make this explicit. $\endgroup$ Commented Jan 23, 2016 at 23:07

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