Kip Thorne gave an excellent public lecture yesterday evening, focussing on impressive new work in numerical relativity (such as black hole collision simulations) and the search for gravitational waves. First up this morning was an informative talk by Schneider on probing cosmology with gravitational lensing techniques, covering mass determinations, direct estimates of the Hubble parameter $H_0$, substructure, the collisionless nature of Dark Matter from Bullet cluster observations and cosmic shear. Over 200 strong lensing multiple-image systems are now known. Fifteen lens systems were used to determine $H_0 = 72$ from the time delay dependent on mass distribution and background geometry, in agreement with the WMAP result and other estimates. It appears that lensing has firmly established the existence of dark halo objects for the Milky Way, expected from the visible undercount of only 20 such objects. Other examples of substructure systems included B1422+231, MG2016+112, for which one can visually see the small perturbing satellite galaxy, and the classic recent result from CL0024+17. Cosmic shear looks at the lensing effect of the 3D distribution of matter on large scales. This was detected by four groups in 2000. With more extensive surveys, it promises to further constrain cosmological parameters in the near future.
Renate Loll gave a clear introduction into basic problems with quantum gravity, and then outlined recent successes of the Causal Dynamical Triangulation approach, in particular the recovery of four dimensions at large scales for the Lorentzian path integral with pathological configurations removed and two dimensionality at small scales. Then Francis Everitt entertained us with his wry sense of humor, as he outlined the success of the Gravity Probe B mission. I caught a better glimpse of the unofficial frame-dragging result, and it looked possibly a little higher than the GR prediction by about 10%, but this will probably disappear in the final error analysis. He didn't really comment on this except to tentatively offer a December deadline for final results.
As expected, the poster session involved a notable lack of interest in Category Theory, but the sandwiches were yummy and the company pleasant.