Exploring black hole accretion in unexplored regimes using GRMHD simulations

Black holes are the most massive (known) compact objects in our universe. Their extremely strong gravitational field can attract large quantities of gas which, by the conservation of angular momentum, forms an accretion disk. Rotational shear in these accretion disks excites magnetized turbulence through the magneto-rotational instability, which in turn dissipates orbital kinetic energy, allowing the gas to fall in while emitting a significant fraction of this dissipated energy in the form of radiation and as jets, electromagnetic fountains perpendicular to the accretion disk. Testing how known, fundamental, laws of plasma physics and general relativity are able to explain this phenomenology is a daunting task. One needs to incorporate the full effects of Einstein's theory of general relativity, that describes the warping of space-time close to the black hole where most of the energy is released, and combine it with fluid dynamics to describe the gas flow. General relativistic magnetohydrodynamics (GRMHD) simulations are the only method that incorporates all of these effects. In this thesis I have developed the fastest GRMHD code (called "H-AMR"), which achieves an 2-5 orders of magnitude speedup compared to all other GRMHD codes. Highlights in this thesis include the discovery of precessing jets and the solution to a 40 year old theoretical problem regarding the alignment between the inner accretion disk rotation axis and black hole spin axis. Furthermore this thesis also addresses how poloidal magnetic fields form from toroidal magnetic fields and how jets accelerate over 5 orders of magnitude in space.