The Kerr-de Sitter universe

It is now widely accepted that the universe as we understand it is accelerating in expansion and fits the de Sitter model rather well. As such, a realistic assumption of black holes must place them on a de Sitter background and not Minkowski as is typically done in general relativity. The most astrophysically relevant black hole is the uncharged, rotating Kerr solution, a member of the more general Kerr-Newman metrics. A generalization of the rotating Kerr black hole to a solution of the Einstein's equation with a cosmological constant Lambda was discovered by Carter (1973 Les Astres Occlus ed B DeWitt and C M DeWitt (New York: Gordon and Breach)). It is typically referred to as the Kerrde Sitter spacetime. Here, we discuss the horizon structure of this spacetime and its dependence on Lambda. We recall that in a Lambda > 0 universe, the term 'extremal black hole' refers to a black hole with angular momentum J > M-2. We obtain explicit numerical results for the black hole's maximal spin value and get a distribution of admissible Kerr holes in the (Lambda, spin) parameter space. We look at the conformal structure of the extended spacetime and the embedding of the 3-geometry of the spatial hypersurfaces. In analogy with Reissner-Nordstrom-de Sitter spacetime, in particular by considering the Kerr-de Sitter causal structure as a distortion of the Reissner-Nordstrom-de Sitter one, we show that spatial sections of the extended spacetime are 3-spheres containing two-dimensional topologically spherical sections of the horizons of Kerr holes at the poles. Depending on how a t = constant 3-space is defined, these holes may be seen as black or white holes (four possible combinations).