The prize is sponsored by the Gruber Foundation and the International Astronomical Union (not always the favorite scientific organization in my household owing to the whole Pluto thing). Rob was a part of the Supernova Cosmology Project and a co-author of a 1999 paper (whose first author is Saul Perlmutter) that helped establish that the expansion of the Universe is accelerating.
There's a bit of irony in the timing here, since Rob found out he's sharing the in award just after he decided to leave academia because his dean made it clear he wasn't going to get tenure unless he brought in buckets of grant money ASAP. It really does seem sometimes like the people at the top of the hierarchy in universities see the whole endeavor in terms of making money rather than making knowledge.
Anyway, go give Rob an "atta boy!"