Research

Introduction

My primary research focus is to attempt to better understand the universe through the study of absorption lines seen in the spectra of high redshift quasars (or QSOs).  When we pass the light from any astronomical object through a spectrograph, it is broken into its constituent colors, producing a value for the intensity of the light as a function of the wavelength of the light.  Spectroscopy is a powerful tool with which we can learn what types of atoms astronomical objects are made of, what their energies are, and how they are moving relative to us on Earth.  In the case of QSO spectroscopy, the spectrum we see contains information about both the QSO and any material which happens to also lie along the line of site.  In my research, it is the intervening material which interests me.  The basic Idea is shown below in a toy diagram created by John Webb.

The spectra I use come from a variety of telescopes around the world, and in space, and include the Keck telescopes in Hawaii, the Magellan telescopes in Chile, and the Hubble space telescope.


The Baryon Density of the Universe

In the Big Bang model of cosmology, when the universe was a few minutes old, it had expanded to the point that the temperature was low enough for protons and neutrons to bind together and make nuclei of the lightest elements on the periodic table: Hydrogen, Helium, Lithium, and their isotopes.  The relative amounts of these nuclei are a sensitive function of the total density of protons and neutrons (also known as baryons) in the universe.  Thus, if we could measure the relative amounts of any of the two light nuclei, we could constrain the total density of baryons.  In particular, I work on the measurements of the ratio of Deuterium (which is an isotope of Hydrogen) to Hydrogen which can be made in QSO spectra.  When we make this measurement, we arrive at a startling result, namely that baryons only make up 4% of the total energy density of the universe.  This means that 96% of the energy density of the universe is in a form which we do not understand!


Statistics of the Lyman Alpha Forest

Any individual high redshift quasar spectrum contains up to thousands of absorption lines. Since each absorption line probes the density fluctuations of both real and dark matter, studies of the bulk statistical properties of QSO absorption lines are powerful cosmological tools.  I am currently engaged in a variety of statistical studies of both the Lyman alpha forest (the absorption  due to clouds of Hydrgen gas) and the absorption due to heavy elements.  When these studies are combined with the results of large, precise cosmological simulations, it is my goal to better constrain a number of astrophysical and cosmological parameters.


Lyman Limit and Damped Lyman Alpha Systems

The Lyman alpha forest is not the only type of Hydrogen absorption we see toward QSOs.  When the hydrogen content approaches levels consistent with galaxies, we observe a Lyman limit (LLS), or damped Lyman alpha (DLA) absorption system, where the differences lie in the amount of hydrogen in the absorber.  These systems provide complimentary constraints to imaging and emission line spectroscopy of  galaxies and are powerful tracers of the evolution of galaxies throughout time.