Computational Astrophysics

On these pages you will find an selection of the wide range of computational challenges tackled by KIPAC researchers. Our mission is to bridge theoretical and experimental physics communities to bring their combined strength to bear on some of the most challenging and fascinating problems in particle astrophysics and cosmology.

Computing is important to nearly all the scientific activities at KIPAC.  This includes theoretical calculations as well as calculations relevant observations and experiments --- from simulating telescopes and their data to processing the enormous amounts of data being collected by the instruments.  In astrophysics, computing is our only laboratory.  If we want to understand what happens when a star gets consumed by a black hole or what happens when galaxies collide it is computing that allows us to experiment. It would take hundreds of millions of years to study the outcome of a galaxy collision. However, programming our supercomputers with the equations describing the physics of these amazing events allows us to see what happens on a human timescale. One has to include how gravity tugs on things, how gas pressure and magnetic fields push against it, how dark matter behaves differently from regular matter, how light emitted from gas depletes its thermal energy,  etc. We are able to do this by carefully devising accurate and stable algorithms that capture the enormous length and time scales of these astronomical and cosmological processes.

In fact KIPAC researchers are following how the entire large scale structure of the Universe evolved over all of cosmic time.
These computations are compared with observed data, which provides input to the models,  and refines our understanding of how the Universe came to be the way it is.

Simulations: Ji-hoon Kim & Tom Abel
Visualization: using partiview by Ji-hoon Kim & Tom Abel

In fact KIPAC researchers are following how the entire large scale structure of the Universe evolved over all of cosmic time and are involved in comparing it with actual observed data ever more refining our understanding of how the Universe came to be the way it is.

A simulated Universe on the left compared with the observed one on the right half.

A recent description of some work in visualization and understanding dark matter and galaxy formation is described in this seven minute video:

Episode 5: Dark Matters from Science Bytes on Vimeo.

Over the years KIPAC researchers have led many studies also related how black holes grow, stars come about, pulsars (fast spinning neutron stars) work, where Gamma Ray bursts may come from and many more.

Recent observations of blazar jets require researchers to look deeper into whether current theories about jet formation and motion require refinement. This simulation, courtesy of Jonathan McKinney (KIPAC), shows a black hole pulling in nearby matter (yellow) and spraying energy back out into the universe in a jet (blue and red) that is held together by magnetic field lines (green).

Blazar Jet Video

Visualization of a simulated vertical slice through an accreting disk of matter around a black hole.  Mass of varying density (blue low to red high) is pushing through the magnetic field (black lines) into the hole.

Accreting black hole disk video

The cosmos contains particle accelerators that are much more powerful than our biggest machines. The mechanism for them is a black hole paired with matter around it (called an accretion disk) that emits jets of particles and radiation as it swirls around, and into, the hole. The system may be in the middle of a galaxy or isolated in deep space, but if the emitted jet happens to point toward earth they are detected as ultra-high energy cosmic rays. The commonly accepted theory for the development of the jets is called the Blandford-Znajek Mechanism, named for Roger Blandford, director of the Kavli Institute for Astroparticle Physics and Cosmology (KIPAC) and Roman Znajek. In 1977, they suggested that the rotational energy of the hole is extracted to accelerate the particles. There have been many details to learn about the process, however.

New computer simulations digitally model these problems by establishing initial conditions and applying the relevant forces to evolve the system. Because the gravitational conditions are extreme, Einstein's General Relativity is an important factor. The physics of high-energy charged particles in strong magnetic fields is described by magnetohydrodynamics. The two combined theories are known as GRMHD. In a recent paper, KIPAC postdoc Jonathan McKinney and Alexander Tchekhovskoy of Princeton University, along with Blandford, reported on encouraging results that reproduced observed properties, and they solved some problems with previous models.

The simulations show that the energy flowing out of the hole is higher than the energy contained in the matter that falls in. This is possible if the hole's rotational energy is being extracted in the process due to enormous magnetic forces that are optimized by the Blandford-Znajek mechanism.  Additional effects seen in the modeling are helping to resolve some of the other outstanding issues related to the shape and intensities of the magnetic fields and energy jets.

The GRMHD simulations are yet another example of the profound interdependence of astrophysics and high energy physics, as they combine the study of relativistic plasmas, cutting- edge computing, gravity at extreme energies, nature's largest particle accelerators, and some of astronomy's most spectacular and enigmatic phenomena that pervade the universe.

The quasi-periodic oscillations (QPO) found in the simulations depend upon the BH properties such as its rotation rate (its spin), which may allow a measure of BH spin and may ultimately help test the validity of Einstein's GR because the QPO probes the strong gravity regime.