Chris Hirata
Theoretical and Observational Cosmology
My research is
focused on cosmology, the study of the structure, composition, and
evolution of the Universe. There is now a standard model for the bulk
composition of the Universe and the formation of large-scale structure,
but it has many unknowns. What are the dark matter and dark energy that
make up 95% of the Universe? Were the seeds of large-scale structure
really produced during an epoch of primordial inflation? Our group tackles
these questions through
a three-pronged approach: (1) theoretical modeling of the early
Universe and the growth of large-scale structure; (2) analysis of data
on the cosmic microwave background and from galaxy surveys; and (3)
understanding the astrophysical processes that affect the
interpretation of cosmological observations.
Examples of current research include:
(*) Precision modeling of the recombination epoch, the time ~300 kyr
after the Big Bang when the gas in the Universe transitioned from
ionized to neutral. This epoch is the "surface" that we see when we
look at the cosmic microwave background, and it must be precisely
understood in order to extract information about dark energy and
inflation from observations. Our students have explored forbidden
transitions in hydrogen and complex radiative transfer effects that
occur nowhere else in the Universe.
(*) Investigations of the atomic physics and chemistry of the epoch
between recombination and the formation of the first stars of order 100
Myr later. This led to a revision in the theory of how the first
molecules were formed.
(*) The simplest models of inflation predict that the initial density
perturbations were Gaussian-distributed. We have used quasar clustering
data to set some of the tightest limits on non-Gaussianity, and
recently extended these techniques to test the statistical homogeneity
of the Universe.
(*) Modeling the spectrum of radiation from rotating interstellar dust
grains. This may be one of the major contaminating signals in cosmic
microwave background measurements, but is also a fascinating physics
problem in its own right.
[Image credits: Bob Paz; Chris Hirata]