COLD MOLECULAR GAS AND HISTORY OF THE UNIVERSE

Molecular absorption in front of radio-loud quasars is a unique tool t o probe cold gas at high redshifts, with the benefit of high angular a nd velocity resolutions (approximate to 10(-3 '') and 30 m/s). Up to 2 0 molecular lines have been detected in a single object (HCO+, HNC, HC N, N2H+, C18O, CS, H2CO, CN, etc), and four absorbing systems have bee n detected up till now. Molecular abundances, and excitation temperatu res can be studied at length in remote galaxies. The column densities sampled by this technique are between N(H-2) = 10(20) to 10(24) cm(-2) . The diffuse components allow us to obtain an upper limit on the temp erature of the cosmic background radiation as a function of redshift. Since the cross-section for absorption is very small around a galaxy, the absorbing systems are either internal, or coming from a gravitatio nal lens. The molecular measurements can then contribute to cosmologic al determination.