Princeton has a long tradition in observational, numerical, and theoretical cosmology with research efforts in Physics, Astronomy and at the IAS.  Princeton faculty helped develop today’s standard cosmological model (Bahcall, Dunkley, Gott, J. Ostriker, Spergel, Steinhardt, Zaldarriaga) and helped introduce important concepts such as dark matter, dark energy, and inflation.  Paul Steinhardt (physics) was not only a key figure in the development of the inflationary model, but has been recently developing its most promising alternative: the ekpyrotic universe.  Princeton faculty are working on a diverse set of problems in theoretical cosmology: time travel (Gott), the topology of large-scale structure (Gott), the shape of the universe (Spergel), formation and evolution of galaxies and large-scale structure (Bahcall, J. Ostriker), clusters of galaxies and their use as cosmological tools (Bahcall, J. Ostriker), the distribution of dark matter (Bahcall, J. Ostriker), non-Gaussianities from the early universe (Spergel, Zaldarriaga), galaxy formation, and the physics of the IGM (Bahcall, J. Ostriker).

Princeton students and faculty are playing leading roles in both cosmic microwave background surveys and optical surveys, which have established our current concordance model of cosmology. Jo Dunkley, Lyman Page, Suzanne Staggs, and David Spergel are mapping the cosmic microwave background with the Atacama Cosmology Telescope (ACT), and are studying its interaction with foreground galaxies and gas. Michael Strauss, Jenny Greene, Jim Gunn, and Robert Lupton are carrying out a large area imaging survey with the Hyper Suprime-Cam (HSC) on the Subaru 8.2-m telescope, using gravitational lensing to map the distribution of dark matter. They are also part of an international consortium building the Subaru Prime Focus Spectrograph (PFS), which will measure the redshifts of millions of z>1 galaxies. Alexandra Amon is playing a key role in weak lensing analyses with the Dark Energy Survey.  Strauss and Lupton are involved in all aspects of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), the pre-eminent ground-based survey telescope of the 2020's. Gunn continues his leadership role in the Sloan Digital Sky Survey. Princeton is also playing a leading role in NASA’s WFIRST mission: David Spergel is a co-chair of the Science Working Group. Adam Burrows, Jenny Greene, and Robert Lupton are members of WFIRST Science Investigation teams.

J. Ostriker and his students have helped develop numerical cosmology.  They have developed hydrodynamical simulation codes that have helped shape our understanding of the Lyman alpha forest, the formation of galaxies, and the Warm Hot Intergalactic Medium. They work closely with Jim Stone, E. Ostriker, and others to model the small-scale ("sub-grid") physics that determines the physical properties of galaxies, and with Strauss, Greene, and others to compare the results of their simulations with observations. Indeed, computational astrophysics is a major focus of the department.