fast sampling and the end of time

At group meeting, Hou described our very fast MCMC algorithm; in the tests we have done so far on exoplanets (radial-velocity fitting) it beats Metropolis-Hastings MCMC by a factor of about 100 in speed. He is using affine-invariant sampling that uses an ensemble of parallel chains to build the proposal distribution. It is slick, and not complicated.

In the afternoon, Ben Freivogel (Berkeley) gave an extremely amusing talk about eternal inflation, string theory, and calculation of probabilities in the multiverse. He concludes that the only consistent way to make predictions in the theory is to put a limit on the time, and—assigning reality to his physical model—therefore finds that time has high probability of coming to an end in the next Hubble time. The argument is way out in left field, relying heavily on arguments of realism, which I reject. But I appreciate the candor: He takes the position that if you need to put a limit on time in order to consistently calculate, then time is predicted by the theory to end. I don't necessarily disagree with the latter part, but there are other reactions one can have to the former—the problem of calculating. Like: maybe eternal inflation just doesn't make predictions at all.

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