Computational Astrophysics

KIPAC researchers tackle a wide range of computational challenges as part of a mission to bridge the theoretical and experimental physics communities, enabling them to bring their combined strength to bear on some of the most challenging and fascinating problems in particle astrophysics and cosmology.

Theory, simulation, observation

Computing is important to nearly all the scientific activities at KIPAC. This includes theoretical calculations as well as calculations that are relevant to observations and experiments—from simulating telescopes and their data to processing the enormous amounts of data being collected by instruments. In astrophysics, computers are the only laboratory. To understand what happens when a black hole swallows a star or when two galaxies collide, KIPAC's computational astrophysicists can perform experiments computationally. 

Some questions are physically impossible to answer through observation. For example, the outcome of a galaxy collision would would take hundreds of millions of years to study. However, programming supercomputers with equations describing the physics of these amazing events provides answers on a human timescale. By carefully devising accurate and stable algorithms specifying 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, and so on, KIPAC researchers can capture the enormous length and time scales of these astronomical and cosmological processes.

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.

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

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

Case study: cosmic accelerators

The cosmos contains particle accelerators that are much more powerful than our biggest machines. The mechanism for them is a black hole surrounded by an accretion disk that emits jets of particles and radiation as matter from the disk 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 it's 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, KIPAC's Founding Director, 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.

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

The next visualization shows 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.

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 (GR) is an important factor. The physics of high-energy charged particles in strong magnetic fields is described by magnetohydrodynamics (MHD). The two combined theories are known as GRMHD. In one paper, former KIPAC postdoc Jonathan McKinney and Alexander Tchekhovskoy of Princeton University, along with Blandford, reported on encouraging results that reproduced observed properties. They also 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.

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