Absorption measurement of hydrogen molecules in the early universe

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