Mergers of Compact Objects are a core activity. The detection of gravitational waves from merging black holes by LIGO, and the multi-messenger observation of a binary neutron star merger are expected to be only the first events in a long-anticipated sequence of discoveries. Several research groups in Astrophysics and Physics are actively engaged in modeling the gravitational wave emission from mergers and predicting the light curves expected from events involving neutron stars. Precise calculations are essential, because the gravitational wave forms of such mergers detected by LIGO encode information about the properties (e.g. radius) of the progenitors, and this, in turn, can provide important constraints on new physics, such as the equation of state for the dense nuclear matter in a neutron star.

The spatial structure and time variability of the image from the Event Horizon Telescope (EHT) will teach us much about the properties of the black holes in these systems, as well as the kinetic plasma physics in the extreme strong gravity regime near the event horizon. A variety of research groups in Astrophysics have been studying this regime of black hole accretion flows, as well as more luminous systems where radiation pressure effects become important. In the latter case, feedback from black hole accretion on the properties of the surrounding galaxies can be measured using astronomical observations and used to constrain black hole properties.

Time-domain astronomy, as represented by HATPI, LSST, and targeting neutron-star mergers, offer a novel observational window that should significantly constrain the physics of mergers of compact objects. The light curves of candidate merger events will be detected over a broad spectrum of wavelengths, and to properly understand the results we will have to make detailed calculations involving strong gravity, fluid dynamics, and radiation.

The detection or non-detection of primordial gravitational waves via polarization measurements on the cosmic microwave background will have important implications for current models of the early universe and the (still hypothetical) initial singularity or epoch that preceded the Big Bang. The presence and strength of primordial gravitational waves are one of the biggest questions in cosmology, so there is much insight to be gained from continuing to refine theoretical models in anticipation of data from the next generation of observatories. Similarly important is to improve our theoretical understanding of the expansion history and growth of structure in the universe so as to capitalize on data from HSC, PFS, Euclid, and WFIRST.

The participating faculty in Astrophysics leading these efforts are Adam Burrows, David Spergel, Anatoly Spitkovsky, and Jim Stone.