Discussion on the Hubble constant.

See http://howdy.physics.nyu.edu/index.php/Hubble_Constant
Wikipage from the Leiden meeting.

Key project: Probability distribution made up of around 5 probability distributions, therefore somewhat skewed (higher values come from the fundamental plane). They add them (don't multiply them) - as worried about systematics. Plot is shown on the above mentioned page.

Key project based on 'exquisite' (empirical) knowledge about the cepheids.

Also from Freedman et al paper: shows the gaussians for the distance modulus distribution to the Magellanic clouds. Problem: Distance to the LMC, which is important for the key project, not that well known. Cepheids in the LMC might be different from those in other galaxies, as LMC different.

WMAP: Result assumes universe is flat. Otherwise can vary from 30-100 (assuming wmap data alone). Peak would be at around 35, then comes down quickly. If you can show that H is less than 70, then from wmap this shows that the universe is closed.

Strong lensing: The important thing is the average surface mass density in the annulus between the images used to determine the time delay. The steeper the potential, the larger the Hubble constant (for a given time delay). Can find an upper limit, by assuming the steepest possible mass distribution (which is the constant M/L ratio profile).

Can be many profiles which fit the few constraints - but need input on what is physical. However, do we know what is physical for the CDM?

Will small clumps affect the time delay?

Picked 9 lenses, which are not in clusters and have well determined time delays. Got upper limits which are somewhat higher than the 72 value. Problem: fitting a de Vaucouleurs can give very poorly constraint effective radius. Most extreme would be to say it's all a point mass - but most extreme 'realistic' case is saying all the mass is in the stellar component, i.e. hence the constant M/L ratio. Get upper limit of around 78.

Can we do better? Need measurement with around 10% uncertainty to be of interest to the community? Hard to estimate the convergence for lenses.

Question for cosmography group: Can we improve H estimates?