Up: Photon-Baryon Fluid
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Differences in the bulk velocities of the photons and baryons
also represent imperfections in the
fluid that lead to entropy generation and heat conduction.
The baryon Euler equations (67) and (71)
which may be iterated to the desired order in .For scalar fluctuations, this slippage
leads to the generation of non-adiabatic
pressure or entropy fluctuations
as the local number density of baryons to photons changes.
Equivalently, this can be viewed as heat conduction in the fluid.
For vorticity fluctuations, these processes do not occur since there
are no density, pressure, or temperature differentials in the fluid.