- Fellow, American Physical Society (2015-)
- Scientific Editor, Galaxies and Cosmology, The Astrophysical Journal (2017-)
- Co-Director, Center for Particle-Cosmology, U Penn (2009-)
- Co-coordinator, DES Weak Lensing Working Group (2007-)
- Co-chair, LSST Weak Lensing Science Collaboration (2006-2013)
- Advisory Board and Management Committee of the DES project (current) and LSST project (2006-2010)
- Edmund J. and Louise W. Kahn Term Chair in the Natural Sciences (2011-2015)
- Spokesperson, LSST Dark Energy Science Collaboration (2012-2015)
Ph.D., MIT (1994)
A.B., Physics (High Honors), Princeton University (1989)
My research area is cosmology and gravitational lensing. The questions I study are: How did small fluctuations in the early universe grow to form the large-scale structure observed today? What is the nature of dark matter and dark energy? And how did these mysterious components of our universe shape the formation of galaxies and clusters?
Gravitational lensing is the shearing and magnification of light we receive from distant galaxies. I use ‘weak’ lensing, the small distortions in the shapes of millions of galaxies, to map the large-scale distribution of dark matter and address cosmological questions. My other interests span a variety of topics in theory and data analysis, largely enabled by massive cosmological surveys. A brief description follows; for details see my publications listed below.
I am actively involved in the Dark Energy Survey (DES) and in the planning of the Large Synoptic Survey Telescope (LSST) and the space missions Euclid and WFIRST. My research group members have carried out lensing measurements and cosmological analyses with DES. Cross-correlations of galaxies and stars with the cosmic microwave background (CMB) is a promising new field of cosmology. We have worked on cosmology and galaxy clusters with DES and the CMB surveys with ACT and SPT. The planned Simons Observatory is an ambitious CMB survey that will enable new advances.
Cosmologists have been intrigued recently by puzzles related to the current expansion rate of the universe (called H0) and the amplitude of mass fluctuations: direct measurements appear to disagree with the predictions of the standard model of cosmology anchored by the CMB. My group’s work with weak lensing is aimed at sharpening the measurement of mass fluctuations. I have also worked on both the theory and data sides of the H0 puzzle.
Galaxy Clusters and Small Scales:
Galaxy clusters are the largest relaxed structures in the universe. A basic question about clusters has vexed cosmologists for decades: what is the boundary of their dark matter ‘halo’? We have analyzed galaxy and CMB survey data to identify a sharp feature called splashback via both the galaxy light and lensing. Splashback marks the cluster halo boundary and provides a promising new probe of both galaxy evolution and fundamental physics, and my group is pursuing these topics.
On smaller scales, I have explored how outer parts of planetary systems can be detected and studied statistically with large surveys. We have constrained debris disks and Oort clouds around nearby stars via their thermal emission using CMB survey data and explored the detection of Planet 9 with forthcoming surveys.
Astrophysical Tests of Gravity:
Theories of modified gravity aim to explain cosmic acceleration without invoking dark energy. In addition to their cosmological effects, these theories typically predict new physical effects on the smaller scales of individual galaxies and clusters. Possible dark matter interactions also lead to signatures on small scales. My group has developed astrophysical tests of gravity and dark sector interactions, and used multi-wavelength datasets to constrain theoretical models.
In the present era of big data astronomy, advances are often made by new applications of statistics and machine learning. My group has worked on topics that range from deblending galaxy images with deep learning to identifying geometric features in low signal-to-noise data and fast measurement of N-point correlations. I have also developed courses on applications of statistics and machine learning with my colleague Masao Sako and organized fun data science activities for summer undergraduate students at Penn.
I am fortunate to work with a wonderful group of colleagues. At Penn, the graduate students and postdocs I co-advise are: Shivam Pandey, Tae-Hyeon Shin, Lucas Secco, Eric Baxter, Cyrille Doux, Tanvi Karwal and Marco Raveri.
Astro 006: The Solar System, Exoplanets, and Life
Astro 211: Introduction to Astrophysics I
Astro 212: Introduction to Astrophysics II
Phys 360: Statistics, Data Science and Machine Learning
Physics 503: General Relativity
Physics 533: Topics in Cosmology