a likelihood function for Gaia

I spent time on the long weekend and today working on a short paper or note on the Gaia likelihood function. My point is trivial: It is just to say that we do inference with Gaia by constructing, from the Catalog, a surrogate likelihood function. This is what everyone does but is rarely made fully explicit. I can't tell whether it is worth publishing. But I find myself compelled to write it nonetheless. I guess it makes the additional point that catalogs from projects like Gaia should be based on likelihood information, not posterior information. Why? Because users need to multiply in new likelihoods, not new posteriors, to update their beliefs!

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