The behavior of Dark Matter is well-described by that of non-interacting massive particles, and can be "seen" with gravitational lensing effects when in high concentrations, such as in galaxies or galaxy clusters. Nearly nothing is known about Dark Energy, however, and observing the ubiquitous WL effects due to the uneven mass distribution across the universe is expected to provide strong evidence of the behavior of Dark Energy across the history of the universe.

I am currently working on three specific projects:

1. Using WL to constrain the galaxy cluster X-ray temperature (Tx) and cluster mass (M) relation.

Counting the number density of galaxy clusters, the largest structures due to gravitational collapse, helps constrain the expansion history of the universe, and hence the behavior of Dark Energy. The largest systematic error in cluster counting methods is in knowing the exact mass of a given cluster. Cluster X-ray temperature can be used as a mass indicator, but its signal is subject to baryonic physics, such as shock heating from mergers. WL signal is proportional to the gravitational effects only, but is affected by the mass projection along the line-of-sight. Our project is to quantify the exact relation between the X-ray temperature and mass, including the intrinsic scatter between the two quantities, from the combination of high-accuracy Tx and WL mass measurement. We have 20 target clusters we would like to observe in WL, of which X-ray temperature measurements exists for all. The WL imaging is currently ongoing.

2. Lensing and photometric redshifts.

The signal for gravitational lensing depends not only on the lens mass, but also on the geometry of the system. Hence, it is important to know the distance/redshift not only to the "lens," but also to the galaxy whose image is being lensed (the "source"). In WL, the more source images one has, the lower the statistical errors. Photometric redshift method ("photo-z") allows redshift information to be obtained for a large number of galaxies at once, although its accuracy is not as high as that obtained from spectroscopy. This project quantifies the photo-z systematic error that is present in galaxy-galaxy lensing using the SDSS data set.

3. Getting the WL technique right.

As stated in the Dark Energy Task Force report (Albrecht et al 2006), weak lensing is possibly the most promising technique currently available in terms of constraining the behavior of Dark Energy. However, it is still not "mature," and there are many technical issues left in terms of removing the sources of systematic errors. For example, the demands for accurately stacking the individual exposure images for WL are quite stringent, since any misalignment appears as a shear in the combined image. Specific WL techniques are still being developed/improved upon so that more (i.e., fainter) galaxy images can be used for estimating the gravitational shear. These developments will be tested to obtain a highly accurate shear estimate for use in WL cosmology.