Wendy Freedman
Observational cosmology, galactic evolution, and the evolution of stellar populations are Observatories’ director Wendy Freedman's principal interests. For over a decade she led an effort at determining the rate at which the universe is expanding as one of three principal investigators involved in the Hubble Space Telescope (HST) Key Project, which was designed to refine the accuracy of the current expansion rate, known as the Hubble constant. In 2001 the group published their results in The Astrophysical Journal, concluding that the universe is expanding at a rate of 72 kilometers/second/Megaparsec, with a total (statistical plus systematic) uncertainty of 10% [Freedman et al. (2001), Astrophysical Journal, 553, 47.] These results agreed excellently with the results of NASA's Wilkinson Microwave Anisotropy Probe, WMAP. The most recent result of that group is a Hubble Constant value of 71 kilometers/second/Megaparsec. Both results are consistent with an age of the universe of about 13.7 billion years, assuming that the universe is accelerating.
Most recently, Freedman, in collaboration with Carnegie staff members Mark Phillips, Eric Persson, Barry Madore, and Carnegie postdoc Chris Burns, has been studying the acceleration of the universe as part of the 5-year, NSF and DOE-funded Carnegie Supernova Project (CSP). Observations of type Ia supernovae by two independent groups have provided evidence that the universe is accelerating, which is consistent with the notion of repulsive pressure or “dark energy.” A physical understanding of dark energy is currently lacking, but the implications of these results for fundamental physics are enormous. As such, accurately characterizing the nature of this dark energy is critical.
The CSP group is observing type Ia supernovae using the 40-inch, 100-inch, and 6.5-meter Magellan telescopes at Las Campanas. The aim of this program is to provide a comparison of near and distant type Ia supernovae for their use as standard references for determining distance and the rate of expansion. Observations are also being made in the infrared, which is much less sensitive to systematic effects such as interstellar dust and chemical composition differences. The team is using PANIC, the infrared camera built by Eric Persson, at high redshift. By comparing nearby and distant supernovae, the researchers are mapping the history of the expansion rate of the universe—an essential component toward understanding the nature of dark energy.