Observational Cosmology by Observing Gravitational Lensing

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AST 9624592 Abstract - Bernstein The dark matter which dominates the Universe does not emit detectable radiation, but does distort images of background objects by the gravitational deflection of the passing light rays. The research plan is to measure these "weak gravitational lensing" effects to determine the true shapes of galaxy clusters and the spectrum of the mass fluctuations in the present-day Universe. These weak lensing distortions are now being used to measure the mass distributions of the inner parts of a few rich clusters of galaxies. The mass profiles of 10 clusters of galaxies will be measured to a larger radius. This is of great interest since theoretical N-body simulations show that cluster shapes and profiles are sensitive to the value of the cosmological density parameter Omega, and to the nature of the dark matter particles. A particularly interesting case is the weak cluster 0957+561, which contains a multiply-imaged quasar with known time delay. By mapping the cluster mass, the modeling of this lens will be improved and an improved value for the Hubble constant will be obtained. The timely completion of these observations requires the use of a Charged Coupled Device (CCD) mosaic camera on a telescope with good seeing. A CCD camera for the Michigan-Dartmouth-MIT (MDM) 2.4m telescope will be constructed. Weak gravitations lensing in random parts of the sky has now been tentatively detected. The weak gravitational distortions of several million field galaxies will be measured, the results from these measurements will provide unambiguous quantitative measurements of: the strength of mass fluctuations on 8 Mpc scales, where linear perturbation theory holds and the fluctuations are directly related to quantum processes in the early Universe; the slope of the power spectrum in the in smaller regions, which distinguishes between present rival models of the structure formation; the power spectrum on 100 kpc scales, where clues to the nature of galaxy formation and dark matter halos will be found; and the growth rate of these structures over the last third of the history of the Universe. These measurements are only feasible if imaging devices with extremely large throughput are used. The Large Area CCD (LACCD) mosaic camera will be completed and mounted on the Cerro Tololo Interamerican Observatory (CTIO) 4m telescope in Chile, giving larger throughput than any other instrument there. Both cameras (at MDM and CTIO) to be completed under this grant will become facility instruments and made available to other astronomers. The completed instruments will also be used to enhance the education of students at the University of Michigan. Ph. D. students will obtain previously unavailable instrumentation experience, and selected undergraduate astronomy majors will work with instruments in Michigan and at observatory sites. The introductory courses for non-science major undergraduates will be revamped, with an emphasis on improving the students' visualization and problem-solving abilities by introducing more modern laboratory exercises and improved classroom demonstrations. The results of this program, and current results by other Astronomy Department members, will be communicated to high school science teachers and students throughout the State of Michigan via a monthly full-color "Image-of-the-Month" club mailing.