constrained realizations, Wiktionary

I continued to work on my review for Marseille. In working on the end part, where I exhort my colleagues to work on certain crucial problems, I have decided that I am going to say something about constrained realizations. There is nothing—in principle—to stop us from having simulations not just of the cosmological model, but of our actual Universe, constrained by all the multi-wavelength things we know about all lines of sight we have observed (including the actual galaxy properties, their positions, the structures in which they reside, and, in principle, our actual CMB).

There are huge degeneracies, both because the observations are highly incomplete and because the model is extremely uncertain (where it comes to galaxy formation especially). But it is possible—in principle, and soon in practice—to integrate over these degeneracies statistically with Markov chains or Gibbs samplers and the like. The chain of allowed realizations would make predictions (those things that were common to all models in the chain but not yet observed) and direct decisive new observations (those things that varied enormously among models in the chain). And each link in the chain would give specific initial conditions and formation history for every observed object, and all the not-yet-observed ones too! It is hard to imagine this future, but it is neither impossible nor ridiculous. Indeed, once simulations get a bit faster and more user-friendly, I will start to pursue it myself!

I also adjusted Wiktionary entries on physics subjects (I don't think this counts as research, but hey, it's the weekend).