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As sources say, the Cosmic Microwave Background originated as the first electrons and protons recombined in the early Universe to hydrogen atoms. The resulting $\approx$ 13.6eV photons enlarged hugely due to the expansion of the Universe are now the CMB.

However, that was not the only recombination process. For example, about 25% of the young Universe was helium, thus there should have been two additional reactions:

$$\alpha + e⁻ \rightarrow \mathrm{He}^+$$

$$\mathrm{He}^+ + e⁻ \rightarrow \mathrm{He}$$

Beside that, some deuterium, tritium and $\mathrm{He}^3$ also was created, with a slightly different recombination spectra.

Beside that, also higher order reactions should have happened, like the creation of $\mathrm{HHe}^+$ ions, $\mathrm{H_2}$ molecules, and so on.

These reactions happened with a different reaction rate as the Universe cooled.

I think, the result of all these should have been a complex spectrum.

However, the CMB has a nearly perfect black body spectrum.

Why?

Where are, for example, the radio photons coming from the initial helium recombination?

Yannick B
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peterh
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1 Answers1

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The spectrum is almost a perfect blackbody because the path length of photons prior to recombination is much less than the "size" of the universe (or $c$ multiplied by the age). The photons present in the universe at that time are those last emitted by a plasma almost in perfect thermodynamic equilibrium and have a blackbody spectrum. Note that these photons were mostly not produced by recombination.

Modifying that spectrum due to processes occurring close to the main recombination event is very difficult because the photons outnumber the baryons and electrons by more than a billion times.

In principle there may be some very weak recombination emission lines corresponding to the redshifted Lyman and Balmer series and there are plans to search for those. But as I said, the preexisting photons are billions of times more numerous than photons directly from recombination.

Helium recombination lines would not be seen because the universe was opaque to those photons when they were emitted.

ProfRob
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