Theoretical Astrophysics and Cosmology

Bhuvnesh Jain, Ravi K. Sheth, Mark Trodden, Justin Khoury

Dark Matter Halos and Weak Lensing

One of the best ways of studying dark matter is through weak gravitational lensing, which is the shearing and magnification of light we receive from distant galaxies. The effect is very subtle, typically less than a 1% stretch of the image, so very large numbers of g alaxies must be analyzed to detect the effect. We have measured the shapes of 2 million galaxies in 75 square degrees of CCD 4-meter telescope images, detecting the effect and measuring the fluctuations in dark matter to ~10% accuracy. With improved algorithms we will examine these data again, and turn to deeper data being gathered by the Deep Lens Survey ( Our research has focused on understanding how small fluctuations in the early Universe grew to form the large-scale structure observed today. We use theoretical modeling of weak lensing by large-scale structure for different cosmological models and are interested in the measurement of galaxy clustering parameters and their relation to the clustering of dark matter measured by lensing effects.

Dark Energy

The group has a strong interest in dark energy. So far we have developed methods to measure the time evolution of dark energy and unveil its nature by using observations from the cosmic microwave background, the differential ages of galaxies and weak lensing. The Penn group is involved in The SuperNova/Acceleration probe (SNAP,, a proposed 2-meter space telescope designed to measure supernova to redshift 1.7 with sufficient accuracy to measure the evolution of the mysterious dark energy that dominates he present Universe. With its billion-pixel visible/near-infrared imager, SNAP is also the ideal tool for measuring the mass distribution of the Universe with weak gravitational lensing. We are currently working with the SNAP collabo ration to optimize the SNAP design andmission by conducting extensive calculations of its capabilities for supernova and weak-lensing measurements.

Theoretical and string cosmology

The astro group at Penn is unique in that it has very strong ties with the string cosmologists. We have a very dynamic collaboration to address fundamental questions in cosmology: how was the universe created? what is dark energy? some of the leading ideas in theoretical cosmology (e.g. the ekpirotic universe) were thought at Penn.

Galaxy Formation and Evolution

Much work has been done at Penn to understand the evolution and formation of galaxies by analyzing the stellar populations in distant galaxies. We have developed stellar population models and unveil the star formation history of galaxies from the SDSS. We have also studied in detail the evolution of elliptical galaxies over the Hubble time from SDSS and other surveys. We continue our research on stellar populations of galaxies from SDSS, high-redshift surveys and ACT optical follow-up

Large scale structure

The astro group at Penn is involved in studies of large scale structure (2dF and SDSS) to determine the bias other cosmological parameters of the universe. The group is also leading the use of the halo model in studies of large scale structure and analytical mass functions.

Numerical simulations of star and planet formation

Numerical simulations of stars (and recently planets) are performed at Penn in collaboration with the group in Copenhagen (Aake Nordlund) and the group in San Diego (Paolo Padoan & Mike Norman).