Cosmology and Astrophysics -- Theory

Theoretical Astrophysics and Cosmology

Theoretical astrophysics extends into all of physical science, from elementary particles to cosmology, to surface chemistry, to hydrodynamics, and the classical theory of fields. Astronomical observations continually discover new situations, new objects, and unexplained effects that stretch the imagination and challenge the ingenuity of the physicist. It is presumed that for the most part, the basic laws of physics, discovered over the past century in the terrestrial laboratory and described by the familiar differential field equations, constitute an adequate foundation of understanding the exotic phenomena discovered by the astronomical telescope. The problem is that the equations are so general that they provide no clue to the existence of the different classes of solutions. It is for this reason that theoretical astrophysicists are so closely tied to astronomical observations. The strength of the astronomy and astrophysics program at The University of Chicago has in fact been its distinguished tradition in establishing these links between theory and observations, from cosmology and particle astrophysics to stellar structure and evolution, and astrophysical fluid dynamics.

In the past decade the interdisciplinary field of cosmology and particle physics has been one of the most active and exciting areas of research in physics and astrophysics. The University of Chicago has long been one of the most active centers for applying the results of nuclear physics to astrophysics and cosmology, and it has also become one of the centers for research at the interface of cosmology/astrophysics and particle physics. This is due to the strong groups in particle physics (theory and experiment), general relativity, and astronomy and astrophysics at Chicago. This strength in considerable part reflects not only the University itself, but also its connections to Fermilab (30 miles west of Chicago), where strong groups in theoretical astrophysics and--most recently--experimental astrophysics have been established by University faculty. Thus, the Fermilab theoretical astrophysics group is headed by Edward Kolb (who is also a professor in the Department of Astronomy & Astrophysics), and pursues research at the interface of cosmology and particle physics. The Fermilab experimental astrophysics group is headed by Richard Kron, who is also in the Chicago Department of Astronomy & Astrophysics; this group's current primary focus is the Sloan Digital Sky Survey (SDSS). The SDSS is a project being carried out by The University of Chicago in collaboration with Princeton University, Fermilab, the Institute for Advanced Studies, Johns Hopkins University, and a Japanese science consortium, to map the Universe by obtaining positions for 100 million galaxies and red shifts for one million. This vigorous level of activity reflects the University"s faculty strength in cosmology, which includes among the theorists Kolb, Frieman, Hu, Olinto, and Turner. Thus, there are abundant research opportunities for graduate students interested in both experimental/observational and theoretical cosmology, available both on campus as well as at Fermilab.

The hot big bang model has become the standard model of cosmology, and because of the concordance of the prediction of primordial nucleosynthesis with the observed abundances of D, 3He, 4He, and 7Li the model is believed to provide a reliable description of the evolution of the Universe from 0.01 seconds after "the bang" until today. Moreover, it provides a sensible framework for making speculations about the earliest history of the Universe--all the way back to the Planck epoch (kT~1019 GeV and t~10-43 sec). Sorting out the earliest history of the Universe requires an understanding of the fundamental particles and their interactions at very high energies (>>1 GeV). Progress in particle physics (the SU(3) x SU(2) x U(1) standard model, grand unification, supersymmetry, supergravity, superstrings, etc.) has led to important advances in our understanding of the earliest moments of the Universe. The inflationary Universe scenario offers the possibility of explaining the observed large-scale isotropy and homogeneity of the Universe, the near critical expansion rate, "the monopole problem", and the origin of the small primeval density inhomogeneities needed to initiate structure formation. Cosmological phase transitions associated with the spontaneous breakdown of various symmetries can lead to the production of topological defects-domain walls, cosmic strings, monopoles, and textures, which can also act as seeds for structure formation. Finally, a relic particle species (e.g., a 30 eV neutrino, a 10 GeV - 1 TeV neutralino, or 10-5 eV axion) may account for the dark matter (and hence most of the matter) in the Universe and if so, may help to elucidate the details of the formation of structure in the Universe. The Cosmology group has been actively involved in research in all of these areas.

Theories of structure formation predicated upon density inhomogeneities produced in the early Universe and relic elementary particles as the dark matter (e.g., cold dark matter) are beginning to be tested by the COBE DMR detection of anisotropy in the microwave background. These theories will be probed further by other microwave background anisotropy experiments (such as those carried out by Stephan Meyer here at Chicago), by the Sloan Digital Sky Survey, and by experiments to detect dark matter particles in our own galaxy. The study of structure formation is providing a window to the earliest history of the Universe and the underlying fundamental physics.

The cosmology/particle physics connection is one in which important information flows in both directions. The frontiers of particle physics now involve energy scales that are much higher (>>103 GeV) than the energies that are or probably ever will be available in terrestrial labs. The early Universe, with its virtually unlimited energy budget, has come to play an increasingly important role as a "heavenly laboratory" in which to study particle physics. Cosmological and astrophysical data have been used to place important constraints on the properties of particles (both known to exist and hypothetical). For example, primordial nucleosynthesis constrains the number of light neutrino species to be at most four (confirmed by experiments at CERN and Stanford). The EFI group at Chicago has played a leading role in the use of astrophysical data to constrain particle physics theories.

Our understanding of the events occurring at the beginnings of the Universe--especially those leading to the nucleosynthesis of the elements--are obscured by physical processes which become important once stars "turn on". This gap between the early Universe, and the Universe we see today, is the focus of James Truran, who has studied the synthesis of both light and heavy elements in stars. Truran has focused on element synthesis during the phases of extremely rapid stellar evolution, such as during the course of supernovae; as well as on the "chemical" evolution of matter in galaxies such as our own, starting from the earliest stages of star formation in our galaxy.

Chicago has a long tradition in astrophysical fluid dynamics, especially concerning the evolution of stars. At the present, the work of Konigl, Lamb, Parker, Rosner, and their students and research collaborators has concentrated largely on astrophysical problems in hydrodynamics and the classical theory of fields, and their application to "active" astrophysical objects (ranging from AGNs and the mysterious gamma ray bursters to stars). It can be fairly stated that the activity of stars, and the activity of galaxies, is to be understood for the most part in terms of the flow of gases in magnetic fields in the presence of gravitational fields and electromagnetic radiation, all described by a familiar set of nonlinear differential equations. The revelation of observational astronomy is that there are more effects in the basic equations than anyone ever dreamed. The revelation of theoretical physics is that these effects, once one has some idea of their nature, can often be extracted ad hoc from the general dynamical equations. It was in this way that the concepts of the hydromagnetic dynamo, the solar wind, magnetic buoyancy, convective propulsion, dislocation dissipation, etc., were discovered and understood.

Recently, members of the EFI Theoretical Astrophysics Group have concentrated on understanding the spontaneous appearance of tangential discontinuities in continuously deformed magnetic fields, as the basis for the stellar X-ray corona, and in the same context, they have studied heat transport in such a highly-magnetized medium. They have explored the hydrodynamics of the azimuthal magnetic field of a star like the sun, showing how it suppresses magnetic buoyancy and causes the continual appearance of active regions at the surface. They have studied the nature of convective mixing and the generation of global stellar oscillations. They have also investigated the interaction between magnetic fields and highly turbulent fluids, including such processes as turbulent magnetic field diffusion and ambipolar diffusion which are central to the evolution of stars from their very beginnings as protostellar nebulae. Finally, they have developed powerful new statistical techniques in order to study the nature (e.g., the spatial distribution) of the mysterious gamma ray bursters, leading to exciting new results in the controversy regarding the possible cosmological origins of these unusual astrophysical objects. Much of the work in astrophysical fluid dynamics has focused increasingly on the use of large-scale numerical simulation as a tool for developing our physical intuitions in the highly non-linear regimes in which astrophysical fluid dynamics naturally operates. As a result, Chicago now leads a consortium of universities and national research institutions (including Argonne National Laboratory, which is managed by Chicago), whose aim is to exploit the new generation of massively parallel computers in studying turbulent mixing problems in astrophysics.