Hydrogen reionization is the epoch when ultraviolet photons produced by the first galaxies ionized almost all of the hydrogen in the Universe. (The vast majority of the atoms in the Universe consist of hydrogen.) Astrophysicists think that this process happened when the Universe was several hundred million years old (a  few percent of its current age), but exactly when and how it happened is something that we are struggling to answer.  I have worked on modeling this epoch (the panel above is from one of my simulations of this process) as well as on developing different observational methods that can be used to study it.  For example, I have thought about how reionization affects the statistical properties of galaxies, absorption in the spectra of gamma ray bursts, and intensity fluctuations in the cosmic microwave background radiation. For a review of these different techniques, see this link.  More recently I have thought about its effect on the thermal state of the gas and whether this is observable in 1, 2, and 3 and how its heating suppresses the formation of dwarf galaxies. Click here for movies generated from reionization simulations.

I have also studied the reionization of helium, the second-most abundant element in the Universe (constituting 25% of the total baryonic mass).  We think that radiation from accreting supermassive black holes -- objects astronomers call ``quasars’’ and the brightest objects in the Universe -- doubly ionized the intergalactic helium when the Universe was 20% of its current age.  The advantage of studying helium reionization compared to hydrogen reionization is that there is far more data about the state of the IGM at the epoch we think this process occurred.  In addition, most measurements from the IGM do not take helium reionization into account, which could lead to biases in e.g. cosmological parameter estimates from the Lyman-alpha forest.   The panel on the right shows a 430 comoving Mpc slice (~300 million light years across) through a model of the Universe  from one of my simulations of this process.  I have contributed to bolstering the argument that this process was ending when the Universe was a couple billion years, at z=3.  In addition, Eric Switzer and I identified two new observables (absorption from the 584A line of neutral helium and 8.7 GHz hyperfine absorption of singly ionized 3He) that have the potential to reveal much more detail regarding this epoch than other known methods.