Remarks on matter-gravity entanglement, entropy, information loss and events

I recall my ‘matter-gravity entanglement hypothesis’ and briefly review the evidence for it, based partly on its seeming ability to resolve a number of puzzles related to quantum black holes including the black hole information loss puzzle. I point out that, according to this hypothesis, there is a quantity, i.e. the universe’s ‘matter-gravity entanglement entropy’ – which deserves to be considered the ‘entropy of the universe’ and which, with suitable initial conditions, will plausibly increase monotonically with cosmological time. In the last section, which is more tentative and raises a number of further puzzles and open questions, I discuss the prospects for a notion of ‘events’ which ‘happen’ whose statistical properties are described by this entropy of the universe. It is hoped that such a theory of events may be a step on the way towards explaining how initial quantum fluctuations convert themselves into inhomogeneities in a seemingly classical universe.