One way to describe the department's diverse research activities in astrophysics is to divide them, somewhat artificially, by the wavelength of the observations, from radio waves to X-rays. This section will also describe the department's work in experimental relativity and theoretical astrophysics.
The radio astronomy group at MIT is currently studying a number of topics in astrophysics, including gravitational lenses, radio stars, quasars, all-sky surveys, and astronomical transient sources. In the study of gravitational lenses, the program's main efforts are to discover new lens systems and to use known systems to extract information of astrophysical interest. The search for gravitational lenses is based on an extensive VLA (Very Large Array, an array of 27 telescopes in the plains of New Mexico) survey, followed by further study at optical wavelengths and with very long baseline interferometry. The detailed study of known systems includes modeling aimed at reconstructing the lens mass distribution and radio monitoring with the VLA to measure time delays between the components. These efforts should provide independent new measurements of the mass distribution in galaxies, as well as of Hubble's constant.
In collaboration with a group at Haystack Observatory, the radio group is carrying out astrometric monitoring of several nearby dMe stars to determine if they have planets around them. These measurements should be able to detect the presence of substellar companions with masses as small as a Jupiter mass.
An array of radiometers, planned to operate as two geographically separated correlating arrays, is being built in a search for astronomical transient radio sources. This search, together with similar all-sky monitoring at other wavelengths, will be very helpful in understanding the nature of the enigmatic gamma-ray burst sources.
In addition to the Haystack telescope (a 120-foot microwave and millimeter wave dish located in Westford, Massachusetts), the radio group at MIT frequently uses national, international, and foreign facilities such as the VLA, the Very Long Baseline Interferometry Network, the British MERLIN array, and the Australian Parkes telescope.
Figure 1. Illustration of the idea behind the Surface brightness Fluctuation (SBF) measurement for determining distances. A star field which is unresolved still leads to measurable bumpiness from pixel to pixel. The upper right panel shows the appearance of a piece of a galaxy at 1 Mpc. It appears that some very bright starts are resolved. The lower right panel shows what the field might look like with the atmospheric blurring removed. The lower left panel zooms in on some of the central pixels, and the upper left panel shows the actual star field. What seemed to be stars were merely statistical fluctuations. However, the Poisson statistics from pixel to pixel do tell us how many starts are present and so the mean flux per star is the total flux in the pixel divided by N. Baron Frankenstein may have used this technique in deciding when to skedaddle as the torch-bearing hoards approached his castle: when the fluctuations in the glow became noticeable to his nearsighted eyes, he knew that doom was at hand.
MIT, the Carnegie Institution of Washington, Yale University, and Harvard University have been carrying out jointly a very large galaxy redshift survey, the Las Campanas Redshift Survey, mapping more than 26,000 galaxies up to distances of about two billion light years. This survey -- some five times deeper than the well-known Harvard Center for Astrophysics (CfA) redshift survey -- can reveal the size of the largest structures and voids in the universe. MIT optical astronomers are also mapping lensing galaxies, which will remove some of the uncertainties currently associated with the theoretical modeling of gravitational lenses.
As part of a collaboration with the University of Michigan and Dartmouth College, the MDM consortium, MIT operates two optical telescopes on Kitt Peak in southern Arizona. The 1.3-meter McGraw-Hill telescope is used for a wide range of programs requiring a large number of nights. MIT astrophysicists have used it to identify and obtain complementary data for x-ray sources discovered by the HEAO-1, Einstein, and X-ray satellites, and for radio sources discovered as part of the MIT-Greenbank 5 GHz survey. The telescope has also been used to study asteroids and comets in our solar system, nearby cataclysmic variable stars, distance tracers of the Milky Way's gravitational potential, X-ray sources in the nearby Andromeda galaxy, and high redshift clusters of galaxies.
The 2.4-meter Hiltner telescope is used for the study of fainter objects and (by virtue of the telescope's superb image quality) those requiring greater angular resolution. It has been used to identify quasars that are multiply imaged by intervening galaxies acting as gravitational lenses, and to determine high-precision distances to nearby galaxies by measuring brightness fluctuations resulting from the Poisson statistics of their stars. The latter program's two goals are the measurement of Hubble's constant and the measurement of deviations from the Hubble expansion induced by dark matter.
Beyond their efforts in planning and carrying out observing programs, graduate students play a crucial role in the design and development of hardware and software for telescope control and data acquisition. MIT astrophysicists are currently involved in building spectroscopic and direct-imaging instruments; new initiatives are also underway to produce a wide-field infrared imager and a system to adaptively compensate for optical distortions introduced by the Earth's atmosphere.
MIT has recently joined the Magellan consortium, which is building two 6.5-m telescopes on Cerro Las Campanas in Chile's Atacama desert. Other partners in the consortium are the Carnegie Institution of Washington, Harvard University, the University of Arizona and the University of Michigan. With first light expected in early 1998, focal plane instrumentation for the telescopes has become a major area of effort.
MIT astrophysicists have carried out pioneering investigations of x-ray sources since the very inception of x-ray astronomy. Current investigations include the study of close binary systems containing a collapsed star (neutron star, black hole, or white dwarf). For those systems in which x-ray pulsations are detected, it is often possible to measure the mass of the neutron star; to determine the properties and chemical composition of the companion's stellar wind from studies of x-ray scattering and fluorescence; and to probe tidal interactions in the binary system from changes in the orbital period.
X-ray burst sources, which result from thermonuclear flashes of accreted material on the surface of a neutron star, can be used to probe the properties of the neutron star, including its mass and radius. MIT astrophysicists also study other sources exhibiting quasiperiodic oscillations in order to understand the source of the oscillations and infer the rotation rate and magnetic field strength of the underlying neutron star. Measured Doppler shifts of companion stars in binary systems containing black hole candidates provide crucial evidence in support of the black hole hypothesis. On a larger scale, department astrophysicists study supernova remnants with high-resolution x-ray spectrographs, thus yielding the plasma properties of the shocked interstellar medium.
MIT also runs an ongoing program to identify the optical counterparts of x-ray sources discovered in various surveys, including sources found with the High Energy Astronomical Observatory, the HEAO-1 satellite, and those imaged in the Andromeda galaxy with the Rosat satellite. Active galactic nuclei (such as quasars, Seyfert galaxies, and BL Lacertae objects) thought to contain massive black holes have been investigated to learn about their temporal variability and structure. Clusters of galaxies have been studied with imaging and spectroscopic instruments to investigate the x-ray emission from the very hot gas bound within the clusters of galaxies. Searches for dark matter in clusters of galaxies are also underway.
Over the past dozen years, the x-ray astronomy group has used the SAS-3 X-ray Observatory (whose entire scientific payload was developed at MIT), the Einstein X-ray Observatory (which includes a Bragg crystal x-ray spectrometer built at MIT), the European satellite EXOSAT, the Japanese satellites Tenma and Ginga, and most recently, the German satellite ROSAT. ASCA, a Japanese x-ray astronomy mission with an imaging x-ray CCD spectrometer provided by MIT, was launched in February 1993, and will provide x-ray spectral images for some years to come. The High Energy Transient Explorer (HETE), which will monitor the celestial sphere for transient UV, x-ray, and gamma-ray sources (including the enigmatic gamma-ray burst sources), is under development at MIT for launch in the near future.
An all-sky monitor instrument was constructed at MIT for the X-ray Timing Explorer (XTE), which was launched in January 1996. A charge-coupled device (CCD) x-ray camera and transmission grating x-ray spectrometer, to fly on the Advanced X-ray Astrophysics Facility (AXAF), scheduled for launch in 1998, are under development as part of NASA's Great Observatories program, which also includes the Hubble Space Telescope, Compton Gamma Ray Observatory, and SIRTF (Space Infra-Red Telescope Facility).
A research program to develop the technology to measure gravitational radiation from astrophysical sources by laser interferometric techniques has been underway at MIT since 1970. Current estimates of the gravitational wave strain from posited astrophysical sources (such as supernova explosions, black-hole formation, merging of neutron stars, and primeval cosmic background fluctuations) require a strain sensitivity smaller than 10 to the -20 in the 10 Hz to 1 kHz band. To achieve these sensitivities, a very large baseline system is needed; the LIGO (Laser Interferometer Gravitational wave Observatory), being developed jointly by Caltech and MIT, comprises two sites separated by continental distances. Each contains Michelson interferometers in an L-shaped vacuum system with 4km-long baselines. The interferometers are operated in coincidence, so as to reject the noise from local environmental perturbations. Eventually, the LIGO may form a network with European and Asian interferometers to measure the polarization of the waves and determine source location with sufficient precision to allow identification by electromagnetic astronomies (radio, optical, x-ray, and g-ray).
Current research is dedicated to developing the instrumentation for the LIGO: sensitive interferometers, scaling of optics from meter to kilometer baselines, ground-noise isolation systems, techniques for reducing thermal noise, and the engineering of the facilities themselves. Near-term research involves the development of data analysis algorithms, environmental monitoring and veto systems, and interferometer testing and integration in the large baseline facilities.
The MIT group, in conjunction with others, first used ground and balloon observing platforms to measure the spectrum and angular distribution of the cosmic background radiation; these measurements established the thermal nature of this radiation, in addition to its remarkable isotropy. Early work also showed that precision measurements to search for small deviations from a thermal spectrum, as well as for small amplitude anisotropies -- measurements that would reveal detailed information about the early history of the universe -- would be perturbed by the emission from more local astrophysical sources, such as our own galaxy and the solar system. Improving these measurements would require full-sky and multi-spectral coverage, in order to separate the cosmological radiation from the local astrophysical background.
These observations led to the Cosmic Background Explorer (COBE) project, a NASA mission carried out by a group of scientists from the NASA Goddard Space Flight Center, Princeton, UC Berkeley, JPL, UCLA, UC Santa Barbara, and MIT. COBE, launched in 1989, has measured the spectrum to unprecedented precision. Its measurements show no deviation from thermal equilibrium in the early universe since a few minutes after the initial explosion. COBE has measured intrinsic anisotropies of the background at the level of 10 to the -5 on large angular scales. The spatial spectrum of the anisotropies is consistent with cosmological gravitational potential fluctuations predicted by inflationary models of cosmic evolution (8), suggesting that they originated as fluctuations in the fabric of space-time at the moment of creation.
COBE scientists are still analyzing the data from the mission. One of their major ongoing efforts is to find or set limits on the radiation from the first stars and galaxies that may now form a background in the 1 to 300m band. A measurement of this background would set the time scale for the first aggregations in the universe, providing a link between the primeval and current universe.
MIT made a major contribution to cosmology with the development of the first inflationary universe model, and the department is continuing its efforts to probe the very early history of the universe. The frontier of work in cosmology at later epochs includes galaxy formation, large-scale structure, and microwave background anisotropy. MIT astrophysicists have used measurements of galaxy-peculiar velocities (the deviation of velocity from Hubble flow) to reconstruct the velocity and mass density fields on large scales. Perhaps the most significant outcome of this work is an estimate of the ratio of the mean mass density of the universe to the critical density, which determines if the universe will expand forever or undergo collapse after expanding for some finite length of time.
Another active area of research in cosmology concerns large-scale gravitational N-body simulations. MIT astrophysicists are at the forefront of these developments, currently performing high-resolution galaxy formation simulations with more than two million particles on the fastest supercomputers available.
Figure 2. Projection of all the mass in a highly-evolved cosmological N-body simulation with 16.8 million particles beginning from scale independent initial fluctuations. Shades of gray represent the logarithm of the projected density.
MIT astrophysicists are actively pursuing many research problems in the evolution of stars in binary systems. Transfer of mass from one star in the binary system, typically a normal star burning nuclear fuel in the core, to its collapsed companion (a white dwarf, neutron star, or black hole) leads to a variety of very interesting phenomena, including cataclysmic variable stars, X-ray pulsars, X-ray burst sources, and dead radio pulsars spun up and brought back to life.
A typical variable star pulsates in one mode; its mean density can be determined from its period. The Sun is a variable star, but far from a typical one. About 10 million normal modes are excited in the Sun, each with very small amplitude; even the combined amplitudes of all the 10 million modes amounts to only about one part in 10,000 variation in the Sun's luminosity. The normal mode frequencies of about 3,000 modes have been measured very accurately (the fractional error is of the order 10 to the -4), which has led to the determination of temperature and rotation rate as a function of the radial distance in the solar interior. These oscillations have provided astrophysicists with a new tool to study the interior structure of the Sun, addressing some of the long-standing questions about it.
In the area of the theory of space plasmas, MIT has the Center for Theoretical Geo/Cosmo Plasma Physics, sponsored primarily by NASA and the Air Force Office of Scientific Research. Currently, a substantial program exists in the study of weak and strong plasma turbulence, stochasticity and chaos, wave generation and propagation, plasma instabilities, and particle energization processes in the space environment. These studies apply the basic kinetic theory of charged particles in magnetized space environments to the various micro-, meso-, and macro-scale phenomena commonly observed in the solar wind, planetary magnetospheres and ionospheres, and other regimes.