The Potential Impact of High-End Capability Computing on Four Illustrative Fields of Science and Engineering (2008) / Chapter Skim
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2 The Potential Impact of HECC in Astrophysics
2 The Potential Impact of HECC in Astrophysics
Pages 15-36
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From page 15... ...
Similarly, some of the most important discoveries made by astronomical observers have resulted from the predictions of theoretical physics, such as the cosmic microwave background radiation that is a signature of the Big Bang. The rate of discovery in astronomy and astrophysics is rapid and accelerating.
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Of course, this raises the question of whether any of them harbor life. • Unambiguous evidence for the existence of black holes.
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3. How did galaxies, quasars, and supermassive black holes form from the initial conditions in the early Universe observed by WMAP and COBE, and how have they evolved since then?
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Understanding the Formation and Evolution of Galaxies, Quasars, and Supermassive Black Holes Precise measurements of anisotropies in the cosmic microwave background over the past decade have reduced uncertainties in the fundamental cosmological parameters to a few percent or less and have provided a standard model for the overall properties of the Universe. According to this model,
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From page 19... ...
Winds, outflows, and radiation from stars, black holes, and galaxies established a feedback loop, modifying the intergalactic medium and influencing the formation of subsequent generations of objects. Numerical simulation makes it possible to follow the coupled evolution of dark matter, dark energy, baryons, and radiation so that the physics of this process can be inferred and the properties of the Universe can be predicted at future epochs.
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From page 20... ...
Star formation and the growth of supermassive black holes are not well-understood, and they occur on scales that cannot be resolved in cosmological simulations. But since these processes are essential ingredients in the formation and evolution of galaxies, simplified descriptions of them are included in the mathematical model of galaxy formation, usually incorporated as subgrid-scale functions.
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From page 21... ...
For example, the self-consistent interaction between the hydrodynamics of the gas and the evolution of the radiation field must be accounted for to properly describe the formation of the first objects in the Universe and the reorganization of the intergalactic medium. Large galaxies forming later were shaped by poorly understood processes operating on smaller scales than those characterizing the global structure of galaxies -- such as, in particular, star formation, the growth of supermassive black holes, and related feedback effects -- and the physical state of the interstellar medium within galaxies, which is also poorly understood.
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From page 22... ...
For example, a global simulation of a galaxy or a pair of interacting galaxies could be grafted together with a separate code following the inflow of gas onto supermassive black holes to estimate the impact of radiative heating and radiation pressure on the gas and dust in the vicinity of the black holes, to study the impact of black hole feedback on galaxy evolution. The algorithms that are required would, ideally, solve the equations of radiation hydrodynamics, including gravitational interactions between gas and collisionless matter, and be capable of handling magnetohydrodynamical effects.
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From page 23... ...
Star Formation Although astrophysicists now understand the basic physical processes that control star formation, a predictive theory that can explain the observed star formation rates and efficiencies in different envi ronments has yet to emerge. The most basic physics that must be incorporated is the gas dynamics of the interstellar material, including the stresses imposed by magnetic fields.
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From page 24... ...
. Feedback from newly forming stars disperses the cloud and limits the efficiency of star formation.
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Interpreting the data from Spitzer requires more sophisticated theoretical and computational studies of star formation. To achieve a revolutionary step forward in our understanding of star formation, future calculations need to consider nonideal magnetohydrodynamics in a partially ionized gas, including radiation transfer, self-gravity, ionization and recombination, and cosmic-ray transport.
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From page 26... ...
Finally, an understanding of self-gravity will be needed to follow the fragmentation of the disk and of radiation transport to model the thermodynamics realistically. The requisite mathematical models and numerical algorithms are similar to those needed for the related problem of star formation: magnetohydrodynamics, self-gravity, and radiation transport on very large grids, probably with AMR.
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From page 27... ...
Understanding Supernovae and Gamma-Ray Bursts and How They Explode Our understanding of the birth, evolution, and death of stars is the foundation of much of astro physics. The most violent end points of stellar evolution are supernova explosions, resulting in the complete disruption of the star or in the collapse of the stellar core to form neutron stars or black holes, accompanied by gamma-ray bursts (GRBs)
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The implications go far beyond this important example. How stars evolve is intimately connected with how much mixing occurs in their interiors, so that all the theoretical understanding of stellar evolution and supernovae will be affected.
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From page 29... ...
Predicting the Spectrum of Gravitational Waves from Merging Black Holes and Neutron Stars A prediction of Einstein's General Theory of Relativity is that accelerating masses should produce distortions in space-time called gravitational waves. For example, two stars in tight orbit around one another will produce a spectrum of these waves with a frequency determined by their orbital period.
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From page 30... ...
This requires computing the gravitational radiation signal from merging black holes. In principle, computing this waveform simply requires solving Einstein's equations, a set of coupled partial differential equations that describe the evolution of space-time.
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From page 31... ...
The mathematical model is a set of first-order ODEs for each particle, with acceleration computed from the gravitational interaction of each particle with all the others. Integrating particle orbits requires standard methods for ODEs, with variable time stepping for close encounters.
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For the other two major challenges identified earlier (the nature of dark matter and dark energy) , the most productive mode of investigation will likely be observation, which will collect massive amounts of data.
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From page 33... ...
To a large extent, the astrophysics community writes its own codes, and many astrophysicists are knowledgeable about programming issues on modern HECC architectures. Most computation in astrophysics uses a mix of numerical methods, including grid-based methods for hydrodynamics and magnetohydrodynamics, AMR schemes, particle-based methods for N-body and plasma dynamics, and methods for radiation transport using both grids and Monte Carlo algorithms.
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From page 34... ...
better methods for nonideal magnetohydrodynamics, radiation transfer, and relativistic gas dynamics with general relativity (for a few examples)
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2003. Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century.
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