I am working on Weinberg, Cosmology book.(adsbygoogle = window.adsbygoogle || []).push({});

I am currently reading Chapter 6.

At the last paragraph of Page 257, Weinberg said these equations take a simple hydrodynamic form for cold dark matter and ........, but for calculations of high accuracy it is necessary to use the Boltzmann equations of kinetic theory .....

In addition, right below sub chapter "Photons" at Page 260, he said the density of free electrons was high enough so that photons could be described hydrodynamically : Thomson scattering gave the photons a total momentum locked to that of the baryonic plasma and a momentum distribution that was isotropic in the co-moving frame.

At this moment, I have some questions.

First one might be silly, but I have no idea about the meaning of hydrodynamical description.

The author said hydrodynamical description gives us good approximation, but I don't know what kind of assumption or situation gives us the right to use hydrodynamical description. Besides, in case of photons, high density of free electrons gives us legitimacy, but why? Could you explain in more detail physically?

Second one is why Thomson scattering gave the photons a total momentum locked to that of the baryonic plasma and a momentum distribution that was isotropic in the co-moving frame.

As far as I know, Thomson scattering is elastic one, so just energy of each particle is conserved. Why photons and baryonic plasma come to have the same total momentum and photon momentum distribution comes to isotropic?

My question becomes so messy, but I really appreciate for your answer

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# Hydrodynamical description of photons

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