Absorption Measurement of Hydrogen Molecules in the Early Universe

We investigate the observability of hydrogen molecules in absorption. The absorption efficiency of hydrogen molecules becomes comparable with, or larger than, that of the dust grains in the metal-poor condition expected in the early Universe. If we can use bright infrared continuum sources behind the molecular-gas clouds, the absorption measurement of the hydrogen molecules could be an important technique to explore the primordial gas clouds that are contracting into first-generation objects.