While the PGI is primarily a theory initiative, progress on many of the research aims requires a successful interface with experiment and observation. Fortunately, Princeton is a leader in these fields, and the PGI will provide an opportunity to reinforce existing efforts and reach out more robustly to others, such as LIGO, which are mainly hosted by other institutions. Here we survey some of the related experimental and observational efforts we are interested in.

ACT is a ground-based telescope aimed at measuring detailed properties of the cosmic microwave background (CMB). The Simons Observatory is a next-generation ground-based effort with even better sensitivity to primordial gravitational waves. SPIDER is a balloon-based experiment that also aims to measure the CMB, including its polarization spectrum. The CMB is an exquisitely sensitive probe of the early universe. Data from the CMB are the best known observable for studying general relativity on both the quantum and cosmic scale simultaneously. Particular interest attaches to using the CMB to probe gravitational dynamics, including the possibility of primordial gravitational waves which would leave their imprint on the polarization spectrum of the CMB. The experimental gravity group in the Princeton Physics Department has a strong presence in ACT, the Simons Observatory, SPIDER, and also SuperBIT, a wide-field sub-arcsecond imager sensitive to the gravitational lensing caused by galaxy clusters.

The Event Horizon Telescope (EHT) is expected to image for the first time the event horizon of the supermassive black hole at the center of our own galaxy, and also the one at the center of the galaxy M87. These observations will detect radio emission produced by diffuse plasma accreting inwards. HATPI is a new all-sky survey telescope that will detect optical variability from bright sources. The Large Synoptic Survey Telescope (LSST) is a much larger instrument that will detect variability from much fainter (and therefore more distant) objects. Both of these instruments represent an investment in the growing field of time-domain astronomy. CMB-S4 is a further anticipated evolution of CMB measurement, aimed at an extremely fine angular resolution so as to put constraints on dark energy and to test general relativity at large scales. The Hyper-Suprime Camera (HSC) and the Prime Focus Spectrograph (PFS) on the Subaru Telescope, as well as the Euclid and WFIRST satellites, will provide new tests of general relativity and the nature of dark energy. The Princeton Astrophysics department has a strong presence in HATPI, LSST, CMB-S4, HSC, PFS, and WFIRST.

The PGI will support as a secondary goal the analysis of cosmological and astrophysical data from the experiments and observatories named above. The partner faculty involved in these efforts include Jo Dunkley, Jeremy Goodman, Jenny Greene, Bill Jones, Lyman Page, David Spergel, and Suzanne Staggs.