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I've read in scientific papers, that at the time of decoupling the baryonic sound speed was 5 km/s. Can anyone tell me how this is deduced or point to textbook/papers that do a derivation? I want to try to understand it from first principles

Qmechanic
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user745730
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The decoupling of the baryonic sound speed in the early universe is a result of the interaction between photons and baryons. This process is well-studied in cosmology literature. One seminal paper you can refer to is "Cosmological Perturbation Theory in the Synchronous and Conformal Newtonian Gauges" by Wayne Hu and Nicholas Sugiyama (1996). It explains the derivation of the sound speed in the context of cosmological perturbation theory. Another useful resource is the textbook "Principles of Physical Cosmology" by P.J.E. Peebles. These sources should provide you with the detailed derivation you're looking for.

Mohamad Syahmi
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