Probing the Diffuse Baryons Using Quasar Absorption Line Spectroscopy

 More than 90% of the cosmic baryons reside outside of galaxies in a diffuse phase that is too tenuous to detect in emission. Census of baryons at low redshift universe revealed that as many as 30% of the cosmic baryons are not yet accounted for, which is known as the "cosmic missing baryons problem". Baryons are also found to be missing from  individual halos, which is known as the "halo missing baryons problem". Therefore, probing the diffuse gas outside of the luminous part of galaxies, i.e., in the circumgalactic medium (CGM) and in the intergalactic medium (IGM), is extremely important to understand the baryon budget and the evolution of cosmic structures. Quasar absorption line spectroscopy is a proven technique to probe the elusive gas in the CGM/IGM. In the talk, I will present an overview of how quasar absorption line spectroscopy, using Hubble Space Telescope (HST), can be used to obtain physical conditions and chemical compositions of the elusive circumgalactic gas traced by Lyman-Werner band absorption lines of molecular hydrogen (H2) and by weak low-ionization metal lines. The initial results from our HST pilot program on probing the outskirts of galaxy clusters will also be discussed briefly.