Hubble Constant

Yesterday (Monday) was the first day of the Oort Workshop in honor of Roger Blandford (Stanford) and about gravitational lensing. Blandford opened the meeting with five questions and has encouraged the group to split into teams to answer them by the end of the week. I joined the Hubble Constant group, where the question is Can we use lensing to determine the Hubble Constant? At first we were all extremely negative, but slowly a consensus emerged, that each lens can, in principle, give you a very strong upper limit on the Hubble Constant. This is because the predicted time delays among images depend on the radial concentration of the mass, and the mass profile is very unlikely to be more concentrated than the stars.

Wayne Hu (Chicago) convinced us that this was pretty important, because the CMB only measures the Hubble Constant if you assume flatness and vanilla cosmological constant. A strict upper limit can rule out strange models (like NYU-born DGP), and a strict lower limit (also possible, we think) can rule out spatial curvature and scalar fields that have different redshift evolution from the cosmological constant.

The upshot is that the Hubble Constant group is working towards writing a paper on the subject, which is ambitious, but not obviously impossible.