Temperature fluctuations in an inhomogeneous diffusive fluid
  We discuss metric perturbations of the relativistic diffusion equation around
the homogeneous Juttner equilibrium of massless particles in a homogeneous
expanding universe. The metric perturbation describes matter distribution and
the gravitational wave background in an inhomogeneous universe. We show that
the lowest order perturbation can be treated as a variation of temperature. We
derive a formula expressing temperature fluctuations in terms of the diffusion
and tensor power spectrum. We discuss the multipole expansion of the
fluctuations in the presence of diffusion.
