Kerr-de Sitter spacetime, Penrose process and the generalized area theorem

TL;DR

This paper analyzes energy extraction via the Penrose process in Kerr-de Sitter spacetime, showing how a positive cosmological constant affects efficiency, horizon areas, and introduces a new mass function to generalize the area theorem.

Contribution

It introduces a new local mass function for Kerr-de Sitter spacetime and proves a generalized area theorem accounting for the cosmological constant and frame dragging effects.

Findings

Increasing $\Lambda$ reduces Penrose process efficiency.

Penrose process cannot occur in the cosmological ergoregion.

Both black hole and cosmological horizon areas increase during the process.

Abstract

We investigate various aspects of energy extraction via the Penrose process in the Kerr-de Sitter spacetime. We show that the increase in the value of a positive cosmological constant, $\Lambda$, always reduces the efficiency of this process. The Kerr-de Sitter spacetime has two ergospheres -- associated with the black hole and the cosmological event horizons. We prove by analysing turning points of the trajectory that the Penrose process in the cosmological ergoregion is never possible. We next show that in this process both black hole and cosmological event horizons' areas increase, the later becomes possible when the particle coming from the black hole ergoregion escapes through the cosmological event horizon. We identify a new, local mass function instead of the mass parameter, to prove this generalized area theorem. This mass function takes care of the local spacetime energy due to the cosmological constant as well, including that arises due to the frame dragging effect due to spacetime rotation. While the current observed value of $\Lambda$ is much tiny, its effect in this process could be considerable in the early universe scenario endowed with a rather high value of it, where the two horizons could have comparable sizes. In particular, the various results we obtain here are also evaluated in a triply degenerate limit of the Kerr-de Sitter spacetime we find, in which radial values of the inner, the black hole and the cosmological event horizons are nearly coincident.