bias is not local!

I spent most of the day reading and commenting on student work, plus a bit of work with Lang and Price-Whelan on detecting faint sources in multi-epoch imaging (my recent distraction). This was interrupted by a very nice blackboard talk by Roman Scoccimarro (NYU) about galaxy bias—the relationship between the galaxy density field and the mass density field. He showed that even if bias starts out local (that is, if galaxy density depends only on the local value of the density), it evolves with time into a configuration that is non-local. The effect is unavoidable given gravity. This point is obvious, but I had never noticed it before. He showed that it has a big effect on cosmological measurements, especially the three-point function, where the non-locality term enters at leading order.