September 8, 2004 | 3:30 PM | RI 480 | Wednesday colloquium Gravitational lensing Martin White, UC, Berkeley
The study of modern cosmology has been tremendously advanced by probes for which detailed comparison between theory and observation is possible. However, our supply of clean cosmological probes is limited. Weak gravitational lensing is one such probe, combining theoretical control and experimental tractability with sensitivity to interesting cosmological parameters. In this talk I will review the current state of the art in weak lensing theory and experiment, and discuss several areas of current research interest.
September 3, 2004 | 12:00 PM | LASR Conference Room | Friday noon seminar When Was the Universe Reionized? Avi Loeb, Harvard University
The re-ionization history of cosmic hydrogen, left over from the big bang, provides crucial fossil evidence for when the first stars and black holes formed in the infant universe. Current observations provide a mixed message. The large-scale polarization anisotropies of the cosmic microwave background measured by WMAP imply a reionization redshift of 10-20. However, the extent of the ionized regions around the highest redshift quasars indicate a significantly neutral universe at a redshift of 6.4. I will summarize the status of current observational and theoretical studies, and address the possibility that the time evolution of the mean ionization fraction might have been non-monotonic. The truth will likely be revealed over the next decade through observations of the Lyman-alpha spectra of galaxies, quasars and gamma-ray bursts, as well as the detection of intergalactic 21 cm emission from redshifts above 6.
September 2, 2004 | 12:00 PM | LASR Conference Room | Special seminar Galaxy-Galaxy, Galaxy-Matter, and Matter-Matter Power Spectra Mark Neyrinck, University of Colorado, Boulder
The halo model of large-scale structure has proven to be quite useful in understanding the power spectra of dark matter and galaxies, and their cross-correlation. In the standard halo model, a halo is defined as the contents of an overdensity contour at the overdensity of virialization, and may be occupied by many galaxies. Our alternative model, in which all virialized objects (including subhaloes) qualify as haloes, allows simple estimates for various useful quantities, in particular the galaxy-matter cross-correlation coefficient r. This might allow a reconstruction of the dark matter power spectrum from the galaxy-galaxy and galaxy-mass power spectra, with little information assumed about the population of galaxies in the sample. I will discuss these issues, and, if time permits, I will also discuss VOBOZ, our recent N-body simulation halo-finding algorithm.