Hi Cadie,

     I would recommend that you aim high and invite a nobel laureate.  I know that we invited Leon Lederman to come speak at our meeting and he agreed.  I guess what I'm saying is that you should pick someone high profile and invite them.  The worst they can do is say no.

Dave

Try not to become a man of success, but rather try to become a man of value -- Albert Einstein

I'm not sure where you are, but I keep hearing Dr. William Phillips (spelling might be wrong) is great, and I personally would suggest Dr. Andy Cohen (BU), Dr. Phillip Mannheim (UConn), Dr. Plesser (Duke), or Dr. Alan Guth (MIT).  These last 4 are people I've heard speak or talked to personally, and I've really enjoyed (from an undergraduate perspective).  I'd guess you'd get a very passionate debate if you put them in the same room, but they all can speak on the subject(s) you mention. =)  Good luck!

At the risk of self promoting, you should invite my chapter's advisor, Prof. Ian Shipsey (Purdue). He's a particle physicist but is also involved in LSST and is honestly one of the most well spoken physicists I've ever met (our chapter has lunch every week with the department's weekly colloquium speaker). He's also very approachable and, in my opinion, could probably spend half an hour discussing the minute details of running nails on a chalkboard and have you rivited and laughing the whole time.