Radio-frequency limits to the abundance of intergalactic neutral hydrogen

Radio-frequency spectra were obtained in order to search for neutral hydrogen in or along the line of sight to the radio galaxies 3C 264, 3C 270, 3C 272.1, and 3C 296. New limits to the volume density, n/sub H/, of intergalactic neutral hydrogen were obtained. The respective limits for the four sources are n/sub H/<2.6 x 10-6 cm-3, n/sub H/<9.2 x 10-7 cm-3, n/sub H/<5.6 x 10-7 cm-3, and n/sub H/<1.7 x 10-6 cm-3. Here the upper limits are considered to be at better than the 90 percent confidence level (better than 2sigma). Because the number density of neutral hydrogen needed to close the Universe is n/sub H/=2.0 x 10-6 cm-3, these observations provide additional support for the conclusion that there is insufficient intergalactic hydrogen to stop the expansion of the Universe