The theoretical astrophysics group undertakes investigations of a wide range of theoretical topics in contemporary astrophysics and gravitation physics. This group currently comprises four professors, about twice that number of postdoctoral fellows, several senior visitors, and eight to ten graduate students. Physical facilities include an interaction room in which a library and preprint collection are maintained, and a network of unix workstations (DEC, SUN and IBM), and graphics facilities. Members carry out larger computations on the Intel Touchstone Delta at Caltech (soon to become a Paragon), and on other parallel and vector supercomputers available through Caltech consortia and at national supercomputing centers.
The goal of the group is to define the fundamental physical processes underlying observed astrophysical phenomena. Scientific interests range from formal developments in gravitation theory at one extreme, to the synthesis and modelling of observational data at the other. The phenomena studied cover the entire range of astrophysics and cosmology: the sun and planets, stars and black holes, the interstellar medium, galaxies and quasars, the intergalactic medium, and the large-scale structure and early evolution of the universe.
Members of the group often collaborate with astronomers at Caltech, JPL, and the Observatories of the Carnegie Institution of Washington, and with the Caltech/MIT group developing the Laser Interferometer Gravitational-Wave Observatory (LIGO).
A few concrete examples of recent research are the following:
Together with graduate student David Hogg, Roger Blandford devised a model for a distant radio source B1422+231 which is thought to be quadruply-imaged by an intervening galaxy acting as a gravitational lens. This model required there to be a second concentration of mass in the lens. The source was imaged by the Keck telescope during the commisioning of the Near Infrared Camera and graduate student James Larkin found two massive galaxies near the expected location. The model predicts that two of the radio features may expand with apparent speeds as large as a hundred times the speed of light. The Caltech VLBI group hopes to detect this motion.
Blandford and collaborators have computed the amount of systematic elongation of the images of cosmologically distant galaxies to be expected from weak gravitational lensing by large scale inhomogeneity in the distribution of matter. An average image ellipticity of several percent is predicted. An observational team, including graduate student Todd Small, has made very deep images of thousands of the faintest galaxies using the 5m Hale reflector at Palomar Observatory, and are trying for the first time to measure this cosmological effect. Blandford's other current research interests include interpreting the radio and gamma-ray jets emanating from active galactic nuclei, modeling the broad, ultraviolet emission lines of quasars, and the physics of neutron stars.
The discovery of pulsars in globular clusters by groups from the Caltech physics and astronomy departments inspired Sterl Phinney to develop methods to determine from the observations the birthrates of pulsars in different types of globular clusters. This showed that previous star-collision models for pulsar formation could not account for the observed birthrates. Phinney and graduate student Steinn Sigurdsson then studied collisional and exchange reactions of pre-existing binary stars in star clusters. Binary stars in dense clusters have complicated lives, changing partners and orbits many times.
The simulations predict birthrates of recycled pulsars, radial distributions within the clusters, and properties of the binaries. The good agreement with observations confirms that pulsars in low density clusters are created mainly during interactions with pre-existing binaries. Phinney and Sigurdsson showed that a pulsar binary in the dense cluster M15 (discovered by Caltech graduate student Stuart Anderson and Prof. Tom Prince) must have been created by an exchange reaction, and provides a first elegant confirmation of the theory of heating of star clusters. Phinney has also showed how measured properties of cluster pulsars can be used to test theories of the dynamical evolution of clusters, and to determine the number of massive stars present in the clusters when they formed some 15 billion years ago.
Other recent activities of Phinney's include studies of the ionization of the early universe (with former student Lin Zuo), star formation in galactic nuclei, the formation of planets around pulsars (with graduate student Brad Hansen), the evolution of the orbits of X-ray binaries, and the physics of the acceleration of magnetohydrodynamic winds around black holes. Goldreich and research fellow Sridhar, have been investigating interstellar turbulence, manifested in the scintillation of small angular diameter radio sources. Differential refraction is due to turbulent fluctuations of the electron number density. The physics is analogous to that of the twinkling of stars seen through the Earth's atmosphere. The elongation of the images of strongly scattered radio sources suggests that the turbulence is anisotropic. Goldreich and Sridhar have proved that three-mode couplings, the lowest order nonlinear interactions, vanish for incompressible MHD. Since existing theories of weak MHD turbulence are based on three-mode couplings, they must be discarded. They went on to show that weak turbulent cascades based on four-mode couplings inevitably terminate in strong turbulence. These insights led them to propose a unique spectrum for strong Alfvenic turbulence. This spectrum is quasi-two-dimensional, as originally suggested by Higdon. Moreover, it mixes electron density variations in precisely the manner needed to match the interstellar spectrum.
Goldreich's other current research projects involve the excitation and damping of the solar oscillations by turbulent convection, and the rotation, oscillation, and magnetic fields of degenerate dwarfs.
Thorne is leading a theoretical effort in support of the LIGO Project (the section Gravitational Physics: the LIGO Project in this booklet). Graduate student Eanna Flanagan has constructed the optimal data-analysis algorithm for LIGO to use in searching for a stochastic background of gravitational waves (emitted, e.g., in the early universe or by cosmic strings). With former postdoc C. Cutler, Flanagan has shown that, by cross-correlating theoretical waveforms with observed waves from the last three minutes of inspiral of a black-hole or neutron-star binary, one can deduce the binary's masses to a few per cent precision, and its distance to about 20 per cent. Cutler and Poisson have shown that such measurements will require theoretical waveforms accurate to three "post-Newtonian orders" beyond Newtonian theory---a major computational challenge. With student Haris Apostolatos, Cutler and Thorne have shown that the waveforms are modulated by orbital precession, which is caused by relativistic spin-orbit coupling. With Thorne, students Dan Kennefick and Dustin Laurence have shown how, by combining narrow-band and broad-band waveform data, it may be possible to deduce the radii as well as the masses of a binary's neutron stars, which in turn should reveal the equation of state of nuclear matter. Student Draza Markovic has devised a method for deducing the Universe's expansion rate, deceleration rate, and cosmological constant from the statistics of LIGO measurements of inspiraling binaries.
Thorne, and his group have also been studying whether and how vacuum fluctuations of quantum fields might always prevent the creation of closed timelike curves ("time machines"), and other issues at the interface of general relativity theory, quantum field theory, and astrophysics.
Other areas of recent activity by members of the theoretical astrophysics group include simulations of the growth of structure in the universe, the formation of galaxies, tidal disruption and collisions of stars, theories of nuclear bursts on accreting neutron stars, of magnetic field decay in the superconducting interiors of neutron stars, active galactic nuclei, the acceleration and propagation of jets, the physics of accretion and extraction of energy from black holes, numerical relativity and the prediction of gravitational waveforms from merging binary neutron stars and blackholes, dynamics of stars and gas in elliptical galaxies, theory of shocks and particle acceleration in magnetised plasmas, the physics of neutron star crusts and atmospheres, particle collisions and viscosity in planetary rings, and the heating of the solar chromosphere.
1. "Rings of Fire: Nuclear Burning as the Origin of sub-Hertz Noise and Weak X-Ray Bursts in Accreting Neutron Stars," L. Bildsten, Astrophys. J., 418, L21 (1993).
2. "Cosmological Applications of Gravitational Lensing," R.D. Blandford & R. Narayan, Ann. Rev. Astr. Astrophys. 30, 311 (1992).
3. "Gamma-Ray Jets from Active Galactic Nuclei," R.D. Blandford in Compton Gamma-Ray Observatory ed. M. Friedlander, N. Gehrels & D. J. Macomb New York: AIP (1993).
4. "The Redshift-Space Power Spectrum of Galaxies in a CDM Universe," T. G. Brainerd and J. V. Villumsen, Astrophys. J. Lett., 415, L67 (1993).
5. "The Last Three Minutes: Issues in Gravitational-Wave Measurements of Coalescing Compact Binaries," C. Cutler, T.A. Apostolatos, L. Bildstein, L.S. Finn, E.E. Flanagan, D. Kennefick, D.M. Markovic, A. Ori, E. Poisson, and G.J. Sussman, Phys. Rev. Lett., 70, 2984 (1993).
6. "Encounters between Binaries and Neutron Stars," M.B. Davies, W. Benz, and J.G. Hills, Astrophys. J., 411, 285 (1993).
7. "Sensitivity of the Laser Interferometer Gravitational Wave Observatory (LIGO) to a stochastic background, and its dependence on the detector orientations," E. Flanagan, Phys. Rev. D, 48, 2389 (1993).
8. "Magnetic Field Decay in Isolated Neutron Stars," P. Goldreich and A. Reisenegger, Astrophys. J., 395, 250 (1992).
9. "Electron Injection in Collisionless Shocks," A. Levinson, Astrophys. J., 401, 73 (1992).
10. "A Calculation of the Full Neutrino Phase Space in Cold+Hot Dark Matter Models," C-P. Ma and E. Bertschinger, Astrophys. J., in press (1993).
11. "Pulsars as Probes of Newtonian Dynamical Systems," E.S. Phinney, Phil Trans. Roy. Soc. Lond. A 341, 39 (1992).
12. "The Rate of Neutron Star Binary Mergers in the Universe," E.S. Phinney, Astrophys. J. Lett. 380, L17 (1991).
13. "Gravitational radiation from a particle in circular orbit around a black hole. III: Stability of circular orbits under radiation reaction," A. Apostolatos, D. Kennefick, A. Ori, & E. Poisson, Phys. Rev. D 47, 5376 (1993).
14. "Low-Ionization Broad Absorption-Lines in Quasars," G.M. Voit, R.J. Weymann, & K.T. Korista, Astrophys. J. 413 95 (1993).
15. "Closed Timelike Curves," K.S. Thorne, in General Relativity and Gravitation 1992, eds. R.J. Gleiser, C.N. Kozameh, and O.M. Moreschi (Bristol, England: Institute of Physics Publishing, 1993), p. 295.
The Contents contains links to the other Physics departments.
More information may be found at the following WWW addresses:
PMA Home Page:
http://www.pma.caltech.edu
Caltech Home Page:
http://www.caltech.edu