Photon-Baryon Fluid

  Before recombination, Thomson scattering between the photons and electrons and coulomb interactions between the electrons and baryons were sufficiently rapid that the photon-baryon system behaves as a single tightly coupled fluid. Formally, one expands the evolution equations in powers of the Thomson mean free path over the wavelength and horizon scale. Here we briefly review well known results for the scalars (see e.g. [25,26]) to show how vector or vorticity perturbations differ in their behavior (§IVA). In particular, the lack of pressure support for the vorticity changes the relation between the CMB and metric fluctuations. We then study the higher order effects of shear viscosity and polarization generation from scalar, vector and tensor perturbations (§IVB). We identify signatures in the temperature-polarization power spectra that can help separate the types of perturbations. Entropy generation and heat conduction only occur for the scalars (§IVC) and leads to differences in the dissipation rate for fluctuations (§IVD).