Keywords: Cosmology, Early Universe, Large-Scale Structure, Galaxy Formation, Galaxy Clusters, Gravitational Lensing

Nick Kaiser has worked on a wide range of cosmological problems related to the formation of galaxies and large-scale structure in the Universe. These include: anisotropy of the microwave background; "bulk-flows"; analysis of galaxy clustering from redshift and angular surveys; and the evolution and clustering of clusters of galaxies. In recent years Nick's research has been mainly focussed on developing the theory and observational techniques for "Weak Gravitational Lensing" as a probe of the dark matter distribution. Weak lensing exploits the fact that images of distant galaxies are distorted by the deflection of light rays passing by intervening structures, resulting in a statistical anisotropy of the apparent shapes of these galaxies. The anisotropy is generally quite weak (on the order of 10% for lensing by galaxy clusters and around 1% for lensing by superclusters) but can be measured to a high background galaxies; approaching 1 million per square degree at the faintest magnitudes. Recognising the unique potential of this technique to directly probe the dark matter distribution on a wide range scales from galaxy-haloes through galaxy clusters to supercluster scales and beyond, Kaiser and collaborators have pioneered the techniques for measuring the "shear field" and for reconstructing from this the 2-dimensional projected mass distribution.

The main observational results in weak lensing have been the measurement of shear around massive clusters of galaxies. In Canada, Nick Kaiser has collaborated with Gordon Squires, Greg Fahlman and David Woods to use the Canada-France-Hawaii Telescope to do weak lensing analyses of a number of clusters including the EMSS cluster ms1224, and the Abell clusters A2218, A2163 and A2390. The lensing analysis has been complemented by X-ray analysis contributed by Hans Bohringer and Doris Neumann from the MPI in Garching. In Hawaii, Kaiser has joined forces with Ger Luppino to exploit the unprecedented power of the 64 Mpixel UH8K camera to extend these studies to larger scales. Results from this new instrument include shear measurement for the cluster A1413 (with Gillian Wilson), which for the first time yield estimates of the cluster mass profile extending to the infall regime, as well as exciting results on a supercluster at redshift 0.4 (with Lev Kofman) which suggest a "bridge" connecting two of the clusters. Another relatively recent development has been the detection important implications for theories for structure formation, and also give powerful constraints on the redshift of galaxies much too faint to allow direct spectroscopic measurements.

Nick Kaiser has been awarded a number of medals and prizes, including: