Computational astrophysics is a central part of modern astronomical research. The conditions of astrophysical environments (stars, galaxies, the space between them) are impossible to recreate in the laboratory and are often too complex to describe with simple mathematical models. By constructing complex computer models of the essential physical processes, we can better interpret observational results and improve our understanding of the inner workings of astrophysical environments. In this sense computational astrophysics can be said to be the experimental branch of astronomical research. The ever increasing power of computers allow us to perform ever more detailed simulations of complex astronomical phenomena.

At the Department of Astronomy, development of computational tools spans research areas from solar physics to cosmology. We specialize in the development of computational tools for gas dynamics (with and without magnetic fields) and radiative transfer. Simulations using these computer models are run on parallel systems and supercomputers.


Simulated image of the death of a massive star. Image credit E. O'Connor, K.C. Pan, YT

The central engines of core-collapse supernovae are where neutron stars and black holes are formed. In these environments, all four fundamental forces play an important role.  We need complex computational models to capture all this physics together in one simulation: general relativity to determine the gravity correctly, neutrino radiation transport and detailed neutrino interactions to capture neutrino cooling and heating, three dimensional hydrodynamics over a large dynamic range to capture the turbulence and convection taking place, and detailed nuclear theory to describe the interactions between nucleons at nuclear densities and above. For this, Assistant Professor Evan O'Connor and his research group use the FLASH hydrodynamics framework and perform simulations on some of the largest supercomputers in the world. 


Clumps of orange ionized gas surrounded by blue neutral gas in a simulation of our early Universe
Simulation of the reionization of the environment of the Milky Way and the Andromeda galaxy. Orange clumps are ionized gas surrounded by blue neutral gas. Image credit: P. Ocvirk et al. (2016)

During the time of the formation of the first galaxies in the Universe, some 13 billion years ago, ionizing radiation from stars inside those galaxies changed the entire Universe from being cold and neutral to being hot and ionized. This process is known as Cosmic Reionization. Although we know it took place, we still do not know how reionization developed over time, nor which types of early galaxies were mostly responsible for it. Professor Garrelt Mellema and his research group study this process with a combination of three-dimensional cosmological n-body and radiative transfer simulations and produce predictions for observables such as the distribution of Lyman-alpha emitting galaxies and the redshifted 21-cm signal from the still neutral parts of the Universe. The latter predictions are made as part of observational projects with the European LOFAR radio telescope, as well as the future, global, Square Kilometre Array.


An image of two distorted neutron stars who are merging into a single black hole.
Simulation of merging neutron stars. Image credit : S. Rosswog

Neutron stars are extremely dense remnants of supernova explosions. Some neutron stars are in binary systems and revolve around another neutron star. Such systems continuously emit gravitational waves and therefore slowly spiral towards an extremly violent collision.

The inspiral of two neutron stars emits a "chirping" gravitational wave signal, as observed for the first time in August 2017 by the LIGO/VIRGO detectors. The merger itself  triggers enormously bright cosmic explosions called gamma-ray bursts and also ejects extremely neutron rich matter into space in which the heaviest elements of the cosmos such as gold and platinum are formed.

The modelling of such mergers requires many physical ingredients to be brought together in a supercomputer simulation: strong-field gravity, nuclear matter, neutrino interactions and nuclear reactions. The research team of Prof. Stephan Rosswog investigates the multi-messenger signatures of such encounters.

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