Absorption lines seen in the spectra of high-z QSOs allow one to probe the cosmology, galaxy formation and its evolution. In particular the detectability
of absorption does not depend on the luminosity of the absorbing source, it provides a luminosity unbiased tracer of the universe. In addition to probing cosmological evolution one will be able to place stringent constraints on the variations of fundamental
constants. In this talk the author will review the present status of the field and summarize future programmes with TMT.
Prof. Srianand, senior professor at IUCAA, is involved in various observational and theoretical projects: 1) probing the physical conditions in the high-z proto-galaxies traced by DLAs, 2) probing the time and space variation of fundamental constants using very high resolution spectra of QSOs, 3) tracing the redshift evolution of the CMB temperature using fine-structure lines and 4) semi-analytic modelling of formation and evolution of IGM, reionization and redshift evolution of global star formation rate density in the universe.