Cosmological Halos: A Search for the Ionized Intergalactic Medium
  Standard big bang nucleosynthesis predicts the average baryon density of the Universe to be a few percent of the critical density. Only about one tenth of the predicted baryons have been seen. A plausible respository for the missing baryons is in a diffuse ionized intergalactic medium (IGM). In an attempt to measure the IGM we searched for Thomson-scattered halos around strong high redshift radio sources. Observations of the radio source 1935-692 were made with the Australia Telescope Compact Array. We assumed a uniform IGM, and isotropic steady emission of 1935-692 for a duration between 10^7 - 10^8 years. A model of the expected halo visibility function was used in \chi^2 fits to place upper limits on \Omega_{IGM}. The upper limits varied depending on the methods used to characterize systematic errors in the data. The results are 2/sigma limits of \Omega_{IGM} < 0.65. While not yet at the sensitivity level to test primordial nucleosynthesis, improvements on the technique will probably allow this in future studies. 
Robert M. Geller, Robert J. Sault, Robert Antonucci, Neil E.B.   Killeen, Ron Ekers, Ketan Desai