Chris Hirata

Chris Hirata

Theoretical and Observational Cosmology

Chris Hirata

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: CMB
(*) 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]