Bulk-boundary decompositions and exact perfect fluid stellar distributions

Abstract

The decomposition of the Ricci scalar in terms of bulk and boundary terms gives rise to an interesting gravitational field theory as proposed by Böhmer and Jensko (BJ). BJ went on to suggest functional forms of the bulk contributions in the Lagrangian. For the purposes of astrophysical modelling the boundary terms are non-dynamical and the entire theory is built from the bulk contribution only. The field equations for a stellar perfect fluid distribution are obtained against the background of a conformostatic metric. Since the linear form of the function of the bulk term is equivalent to Einstein we probe the next level of complexity namely the pure quadratic case. The field equations are substantially more complicated than Einstein's equations. The equation of pressure isotropy proves intractable even with simple choices of one of the geometric potential functions. Accordingly we analyse the case of the potentials varying inversely with the radius. Viable models do not arise in this case and so we postulate a proportional relationship between the potentials. It turns out that a physically reasonable shell of isotropic fluid may be constructed that satisfies elementary physical requirements. Finally we investigate the isothermal fluid where both density and pressure go as the inverse square law of the radius. This results in fluid behaviour that is comparable to that in the corresponding Einstein model.

20 pages, 3 figures

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