My students, postdocs and I use pencils, chalk and computers to explore the extremes of the universe: the deepest potential wells, the densest matter, the hottest plasma; i.e. black holes, neutron stars and the early universe. The goals are to understand how they work, and how they came to be.

Over the course of my career, I have worked on the theory of black hole accretion, active galactic nuclei, relativistic magnetohydrodynamic jets, galaxy mergers, stellar dynamics and multiple star interactions in globular clusters, tidal disruption and extreme mass-ratio inspirals on black holes, predictions of sources of gravitational waves at nHz, mHz and kHz frequencies, the LISA space mission (I chaired the US Mission Definition Team 1997-2001, and served on the LISA International Science Team and chaired its Sources and Data Analysis Working Group 2001-2011, and was PI of the Big Bang Observer study), X-ray binaries and binary pulsars.  I am currently on the science team of ULTRAsat, and am the Theory Lead of the UltraViolet  Explorer UVEX, a NASA mission scheduled for launch in 2030.

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Many long-standing questions about these extremes of physics can be answered by combining electromagnetic, particle and gravitational wave observations of the sources. Examples of such questions are: What is the nature of pulsar winds, and how do they affect companion stars? What happens when a star is tidally disrupted by a supermassive black hole? How do accretion disks work? When and how did the black holes in galactic nuclei form and grow? What powers gamma-ray and fast radio bursts? How do single and binary neutron stars and black holes form and merge?

My current research, with students, postdocs, and external collaborators, is  on the formation and evolution of millisecond pulsars and their companions, on magnetars and exotic supernova interactions, on the aftermath of tidal disruption of stars by black holes and the uses and sources of Fast Radio Bursts. This work combines in individual problems high-energy physics, plasma physics, magnetohydrodynamics, stellar structure and atmospheres, climate physics and gravitational physics, and has close connections to observations at wavelengths from radio through gamma-ray. As Co-PI of the Caltech-UCSB-UCB ZTF theory network, I also spend time trying to decipher the many mysterious objects being discovered by ZTF.  So many ideas and so little time mean there is always room for another good student.

Besides deciphering the extremes of the universe, I also have a hobby of figuring out simple quantitative explanations of everyday phenomena. This is fascinating, makes for good conversation and financial investments, and is valuable practice for research in astrophysics. You can learn and contribute to this in my Order of Magnitude Physics course.