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Keywords:

  • Topological censorship;
  • chronology protection;
  • closed timelike curve;
  • Cauchy horizon;
  • quantum energy inequality.

Abstract

Over the past two decades, substantial efforts have been made to understand the way in which physics enforces the ordinary topology and causal structure that we observe on observed scales – from subnuclear to cosmological. We review the status of topological censorship and the topology of event horizons; chronology protection in classical and semiclassical gravity; and related progress in establishing quantum energy inequalities.