Two Boosted Black Holes in Asymptotically de Sitter Space-Time -   Relation between Mass and Apparent Horizon Formation -
  We study the apparent horizon for two boosted black holes in the asymptotically de Sitter space-time by solving the initial data on a space with punctures. We show that the apparent horizon enclosing both black holes is not formed if the conserved mass of the system (Abbott-Deser mass) is larger than a critical mass. The black hole with too large AD mass therefore cannot be formed in the asymptotically de Sitter space-time even though each black hole has any inward momentum. We also discuss the dynamical meaning of AD mass by examining the electric part of the Weyl tensor (the tidal force) for various initial data. 
Takeshi Chiba, Kei-ichi Maeda, Ken-ichi Nakao, Taro Tsukamoto