Theory and observation work hand-in-hand to advance astrophysics. Theorists develop hypotheses and physical models which suggest new observational tests and searches; observational discoveries confirm or refute theories and force the development of revised hypotheses and new models. In earlier decades, Caltech theorists and observers worked together to develop the theories of stellar nucleosynthesis and stellar evolution, interstellar masers, and solar and white dwarf oscillations. Caltech theorists were instrumental in developing now-standard models of black holes, pulsars, relativistic accretion disks, relativistic jets, the extraction of energy from rotating black holes and gravitational wave generation and detection, and in the development of the two premier gravitational wave detector programs: the LIGO project and the LISA mission.
Now the theoretical astrophysics and relativity group at Caltech focuses on the death and rebirth of stars as compact objects, the development of the new fields of numerical relativity and gravitational wave detection and their exploitation for physics and astrophysics, and cosmology from the earliest moments of the universe through the cosmic microwave background and the formation of galaxies.