Cosmological models with fluid matter undergoing velocity diffusion (Q451037)

The author presents a model for diffusion in general relativity and discusses how the inclusion of diffusion effects the global dynamics of simple cosmological spacetimes. The paper is a shorter version of a previous publication by the author [``A kinetic theory of diffusion in general relativity with cosmological scalar field'', J. Cosm. Astrop. Phys. 11, 016 (2011)].NEWLINENEWLINEThe galaxies are represented as fluid particles that undergo velocity diffusion in a background medium provided by a cosmological scalar field. The diffusion process is modeled by an energy-momentum tensor whose divergence is proportional to the fluid flow (number density times the four-velocity). The proportionality constant is called the diffusion constant. In previous work, the author derives this rather ad-hoc looking term from relativistic kinetic theory.NEWLINENEWLINEA cosmological scalar field is added to ensure consistency with the Einstein equations that satisfy the Bianchi identity. However, the canonical energy-momentum tensor of this scalar field is not added as a source to the Einstein equations.NEWLINENEWLINEThe author studies the consequences of this diffusion model for spatially homogeneous and isotropic solutions (Friedman-Lemaître-Robertson-Walker metrics) with vanishing spatial curvature. Depending on the choice of initial data parameters, the global dynamics of the cosmological spacetime can be very different when diffusion is included.NEWLINENEWLINEIt would be interesting to apply the knowledge gained from this analysis of general relativistic diffusion to physical cosmology.