ON THE THERMODYNAMICS OF TILTED AND COLLISIONLESS GASES IN FRIEDMANN-ROBERTSON-WALKER SPACETIMES

Some features of the thermodynamics of tilted fluids, as witnessed by observers comoving with the fluid, are analysed. The Gibbs equation, e xpressed in terms of quantities measured by preferred observers comovi ng with the metric, is obtained and the role played by the entropy pro duction discussed. The results of Bedran and Calvao are analysed from the the point of view of kinetic theory. We focus our attention on a c ollisionless gases model proposed by Ellis et al and show that for the latter u(i)/T is a conformal Killing vector in accordance with Bedran and Calvao.