The Giant Measurement at the End of Time

2.5.6.0.1 Kent and Bousso&Susskind have proposed approaches to defining quantum wavefunction branches based on late-time information. The basic idea is to exploit the way that quantum information propagates in our universe in particular: (a) photons(and gravitons) carry away information at the speed of light, (b) the universe is expanding and cooling, meaning that these photons(in some cosmological environments) may propagate uninterruptedly forever. 2.5.6.0.2 What this means is that photons very late in time are a privileged set of degrees of freedom: they plausibly carry information about everything that has classically "happened" previously. This suggests a way of defining the set of branches: just "measure"(apply a fictitious measurement operator, that is) the photon degrees of freedom very late in time! 2.5.6.0.3 That's the basic idea. There are many subtleties of course, and Kent and Bousso/Susskind have different proposals. I'll review each in turn and then talk about the philosophical merit of these approaches in general. • 2.5.6.0.3.1 The following discussion uses some elementary definitions of objects from general relativity. If you know what a spacelike surface and lightcone are, that should be enough.• 2.5.6.0.3.2* Note that as of the time of writing, QFTs in arbitrary space-times are not totally mathematically well-defined objects -- and what's more, the fully correct theory of the objects we are discussing should be quantum gravitational, where we don't even know what the exact form of the theory should look like. Thus these constructions are a bit sketchily defined, even compared to QFTs which are already mathematically sketchy.2.5.6.1 Quantum Reality via Late-Time Photodetection2.5.6.1.1 Kent has proposed a few variants of this idea. I'll review the version in his paper Quantum Reality via Late-Time Photodetection. 2.5.6.1.2 Kent is interested in defining a set of possible histories of what he calls "beables", which are essentially the classical-like