The theoretical astrophysics group at Caltech undertakes investigations of a wide range of theoretical topics in contemporary astrophysics and gravitation physics. This group currently comprises four professors, about ten postdoctoral fellows, several senior visitors, and ten to twelve graduate students. Physical facilities include an interaction room in which a library and preprint collection are maintained, and a network of about 30 unix workstations (HP, IBM and Linux), plus graphics facilities. Members carry out larger computations (e.g. 3-D MHD, cosmological n-body, and gravitational-wave data analysis) on Caltech's Intel Paragon and HP Exemplar massively parallel computers and at national supercomputer centers.
The goal of the research 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, at JPL, and at the Observatories of the Carnegie Institution of Washington, and with the Caltech/MIT group building the Laser Interferometer Gravitational-Wave Observatory (LIGO).
Roger Blandford and his students have collaborated with radio, infrared and optical astronomers who have recently found several examples of multiply imaged galaxies and quasars and made successful models of the optical arrangement. In a few instances it is hoped that it will be possible to measure the time delay in the variation of the different images and thereby determine the Hubble constant. In a collaboration with former postdocs Tereasa Brainerd and Ian Smail he was able to detect statistically the weak distortion of the images of distant galaxies due to intervening galaxies and so to estimate their masses. This work is continuing with postdoc Tomislav Kundic and graduate student David Hogg using superior data acquired by Hubble Space Telescope.
A quite different research interest concerns the theory of Active Galactic Nuclei. In series of papers written with former postdoc Amir Levinson, Blandford devised models of the inner parts of jets that were able to account for the recently discovered intense gamma-ray emission. Currently, graduate student Ruben Krasnopolsky is implementing three dimensional magnetohydrodynamic codes on parallel computers with the intent of simulating these spectacular outflows. It is hoped that these simulations will also help us to understand the flows associated with Broad Absorption Quasars, a research speciality of postdoc Nahum Arav and also of former graduate student Hee-Won Lee.
A third research area is the mechanisms by which pulsars emit coherent radio emission. Graduate student Maxim Lyutikov is investigating a particularly promising mechanism involving cyclotron maser emission in the outer magnetosphere and hopes to be able to account for some of the rich and well-documented phenomenology of the observed pulses.
Marc Kamionkowski and collaborators are studying how measurements of temperature anisotropies and polarization in the CMB can be used to address a number of fundamental questions about the Universe. For example, what is the geometry of the Universe? Is there a cosmological constant? How much dark matter is there? Did the Universe begin with a period of inflation? If so, what was the new physics responsible for inflation? The all-sky image at the top of this page is a simulation of the predicted fluctuations of the microwave sky, with temperature coded by color, and polarization by black vectors.
Another research area is the nature of the dark matter that provides most of the mass of the Milky Way. One suggestion is that the mass is in supersymmetric particles postulated to explain certain conundrums in the standard model of particle interactions. Kamionkowski and collaborators are computing the properties and cross-sections of these particles in support of experiments aimed at detecting these particles directly in the laboratory or indirectly via observation of energetic neutrinos from particle annihilation in the Sun or anomalous cosmic-ray antiprotons, positrons, or gamma-rays from particle annihilation in the halo.
There is now compelling evidence that galaxies and clusters of galaxies grew through gravitational infall from tiny primordial inhomogeneities in a very smooth early universe. Still, it is not clear precisely how the diversity of spiral, elliptical, dwarf, and irregular galaxies arose. The recent plethora of direct observations of early galaxies and observations of diffuse backgrounds from the earliest epochs of star and galaxy formation is inspiring Kamionkowski and collaborators to model the processes of galaxy formation.
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 and the properties of the pulsar binaries. Phinney and graduate student Steinn Sigurdsson 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.
Postdoctoral fellow Dong Lai showed that residuals in the timing of the binary pulsar J0045-7319 are due to spin-orbit coupling, and that the spin of the B-star companion is misaligned with the orbital plane. Postdoctoral fellow Andrew Melatos is investigating particle acceleration in the strong electromagnetic waves and winds of pulsars. Graduate student Brad Hansen and Phinney have investigated the formation of planets from accretion disks around pulsars (such as PSR 1257+12), and developed the most precise models for the cooling of the helium white dwarf companions to pulsars. These allow recent observations of pulsar companions with Keck telescope and HST to be interpreted, and show that the spin-ages of pulsars are not good estimates of the true time since recycling of millisecond pulsars. Graduate student Glenn Soberman and Phinney have developed models for accreting X-ray binaries in which the mass transfer does not conserve mass and angular momentum, and also studied electron-positron pair creation in the magnetospheres of millisecond neutron stars, to determine the conditions in which they will be observable as radio pulsars. Other recent activities of Phinney's group include studies of the origin of magnetic fields and ionizing photons in the early universe, star formation and stellar collisions in galactic nuclei, and the surface physics of neutron stars and irradiated neutron star companions.
Goldreich and former 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 white dwarf oscillations, and the evolution of rotation and magnetic fields in degenerate dwarfs.
Alan Wiseman (postdoc), Ben Owen (grad student) and collaborators have computed from general relativity the gravitational waveforms emitted by neutron-star and black-hole binaries in the last few minutes of their inspiraling lives. Owen has shown how to convert those waveforms into an optimized discrete family of templates, for use in a LIGO matched-filter search for such waves. Seven present and former members of our group are currently carrying out such a search using data from the LIGO 40meter prototype interferometer and using Caltech's Intel Paragon computer. If one of the binary's objects is a massive black hole and the other a far less massive, inspiraling object, then one can construct from the observed waves a map of the hole's spacetime curvature, according to an analysis by grad student Fintan Ryan. When the objects are black holes of comparable mass, waves from their final merger will reveal the behavior of spacetime curvature when it is highly nonlinear and dynamical; for such mergers, Scott Hughes (grad student) and Eanna Flanagan (a former grad student in our group; now an assistant professor at Cornell) have conceived and explored data analysis algorithms and have identified and quantified the information about the mergers that numerical relativity needs to provide as a foundation for the data analysis. Patrick Brady and Jolien Creighton (postdocs) and Kip Thorne are devising and exploring techniques in numerical relativity for computing the early phases of the merger waves.
Thorne and Flanagan have computed the noise produced by scattered laser light in the LIGO beam tubes and have designed a set of baffles that are now being installed in the tubes to control the scattered light and its noise. Within 10 years LIGO interferometers are expected to reach sensitivities constrained by the half-widths of the quantum mechanical wave functions of their 10kg test masses. To circumvent these constraints, grad student Yuri Levin, senior visitor Vladimir Braginsky, and Thorne are trying to invent practical "quantum nondemolition" (QND) readout systems that will manipulate and narrow the test-mass wave functions in the process of measuring their motions. As a first step, Levin has shown that two somewhat idealized readout systems devised by Braginsky are capable of narrow-band QND behavior.
Other areas of present or recent research by members of CaRT are the structure and evolution of spacetime singularities in the cores of black holes, phase transitions and critical behavior in gravitational collapses that almost form black holes, mechanisms by which the laws of physics might prevent the formation of closed timelike curves ("time machines"), and the physics of wormholes. The image above right shows a view of the universe seen from 15 mouth radii down the throat of a wormhole, as computed by graduate student Scott Hughes.
The Contents contains links to the other Physics departments.
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PMA Home Page: http://www.pma.caltech.edu
Caltech Home Page: http://www.caltech.edu