It was my great pleasure to sit on the PhD defense committee for Adrian Price-Whelan (Columbia) today. He spoke about using phase-space structures, especially streams of stars, to constrain the gravitational field in the Milky Way. He pointed out that we should call it the “gravitational field” not the “gravitational potential” because the latter is only known up to a constant. (I think the argument is deeper: The potential exists only to be differentiated, never observed!)
Two important ideas that were under heavy discussion both in Price-Whelan's talk and in the discussion afterwards were the following: Price-Whelan may have the first ever empirical evidence for dynamical chaos outside of planetary systems. His evidence is in the form of morphological predictions for stellar streams. This argument is weak at present, but if the evidence grows stronger, he will be responsible for a new kind of fundamental measurement.
The other idea is that we might be making big mistakes interpreting the Milky Way entirely in terms of simple, integrable models. As my loyal reader knows, I have ranted about this for years, but my rants have been vapor-ware. Price-Whelan (along with Pearson at Columbia and Bonaca at Yale, I should add) is close to being able to address this point directly and quantitatively.
In addition to all of these things, Price-Whelan has delivered some great probabilistic methods and code, contributed heavily to important open-source astronomy projects, and led our community in education regarding code, data analysis, and research practice. It has been a great pleasure and opportunity for me to be involved in his work for (what his thesis points out is) the last decade!