Astrophysics: Cosmology
Faculty in this area of research:
Observational tests of the standard cosmological
model during the last decade have ushered in a new era of precision
cosmology. The standard model is specified by the size, spatial
geometry (open, closed, or flat), and composition of the universe
as well as the nature and statistical properties of the small-amplitude
initial fluctuations that seeded the formation of galaxies and large
scale structure in the universe. Astrophysical cosmology focuses
on measuring the parameters of the standard model and studying the
formation, evolution, and properties of cosmic structures. These
structures include not only galaxies and the larger structures in
which they congregate but also the supermassive black holes at their
cores and the gas between galaxies.
The cosmic microwave background radiation
offers stringent tests of the standard model through its fluctuations
in temperature and polarization. Recent measurements strongly support
a nearly spatially flat universe with small-amplitude density fluctuations
having the power spectrum and other properties predicted by the
inflationary model of Alan
Guth. The next few years will see a wealth of new data offering
much stronger tests of key parameters as well as signatures of the
phenomena associated with galaxy formation such as cosmic reionization
of hydrogen when the universe was about a tenth its present age.
Theoretical work on the cosmic microwave background radiation is
pursued by Edmund
Bertschinger.
Accreting supermassive black holes, or quasars,
provide a second probe of cosmological parameters and the physics
of structure formation. Quasars serve as powerful light sources
for studying intergalactic gas by absorption-line spectroscopy.
Scott
Burles is a leader in using this technique to study the primordial
deuterium abundance and, by applying the theory of big bang nucleosynthesis,
to measure the baryon abundance in the universe. As a member of
the Sloan Digital Sky Survey, Burles has access to thousands of
new quasars that will be followed up using the Magellan telescopes.
Quasar absorption line spectroscopy provides a way to study many
other phenomena occurring after cosmic reionization. The Magellan
telescopes, with superb seeing and an adaptive optics program led
by Paul
Schechter, allow MIT astronomers to study objects at what is,
effectively, the edge of the universe.
Gravitational lensing offers a third way
to probe the fundamental cosmological parameters. In the last few
years, astronomers using supernovae as standard candles have deduced
that the expansion rate is currently increasing. Within the standard
cosmological model, this surprising result implies that most of
the energy density in the universe has a large negative pressure.
Using both radio (Jacqueline
Hewitt) and optical (Paul
Schechter) techniques, MIT astronomers are measuring time delays
in the multiple images of gravitationally lensed quasars to refine
the measurement of the cosmic expansion history and thereby to deduce
the equation of state on large scales in the universe.
MIT faculty are also active in the study
of cosmic structure formation including galaxy formation and clustering.
These studies involve theoretical modeling as well as observations
spanning the electromagnetic spectrum from radio frequencies to
gamma rays. Paul
Schechter is a pioneer in the study of large scale structure
through galaxy redshift surveys and Scott
Burles is extending these studies using quasars. Claude
Canizares studies the physics of X-ray emitting plasma in clusters
of galaxies and developed an instrument for high-resolution spectroscopy
on the Chandra X-ray Observatory, one of NASA's four "great" observatories
in space. Paul
Schechter and Edmund
Bertschinger have contributed to the basic theory through analytical
models and computer simulations of structure formation.
Jacqueline
Hewitt is a leader in a new international radio interferometer
project called LOFAR, which will serve as a prototype for the Square
Kilometer Array to be built about a decade from now. These instruments
will study the beginning stages of galaxy formation occurring before
the era of cosmic reionization several hundred million years after
the big bang.
We now know that gamma-ray bursts occur in
distant galaxies and, in about one minute, release a significant
fraction of a solar mass of energy in MeV photons. While the cause
of these explosions is still unclear, their occurrence at cosmological
distances makes them a potentially powerful probe of cosmic structure
formation. MIT developed the HETE satellite to rapidly identify
and localize gamma ray bursts allowing prompt followup observations
with the Magellan telescopes and other instruments.
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