Observatories of the
The picture on the right above shows the spectrum of one such QSO (1422+231) at redshift 3.62, when the universe was only 3 Giga-years old, about 20 % of its present age. The many ripples on top of the smoothly varying continuum are individual absorption lines caused by the neutral hydrogen Lyman alpha transition. There are hundreds of individually distinguishable absorption lines, all caused by the same transition at 1215.67 Angstrom. We are seeing this absorption line repeated all over the place because the universe is expanding so the gas clouds are receding from us, each cloud at a slightly different velocity, which causes a Doppler-shift in the wavelength of the absorption line. The clouds recede the faster the further they are away from us, according to Hubble's Law. This phenomenon is also known as the Lyman alpha forest. It is now known that the Lyman alpha forest is the observational manifestation of the intergalactic medium (IGM), a pervasive phase of highly ionized gas from which galaxies condense under the pull of gravity.
Over the past two decades, my colleagues and I have derived some of the basic properties of the IGM from observations and produced theoretical models that explain these findings from first principles. We have attempted to understand how galaxies acquire their matter by accretion from the IGM. Constructing the first theoretical models of metal absorbing gas based on cosmological hydro-simulations, we showed how absorption lines from common chemical elements can arise in gas flowing into galaxies, and we proposed the currently widely accepted model of damped Lyman alpha absorbers as small faint galaxies. More recently, we have been looking at the intergalactic medium in emission, conducting the deepest spectroscopic observations so far to study interactions between galaxies and their surroundings.
For more information about some of my research interests please see the links on the sidebar.