5.4. Cosmological scales 5.4.1. Foreground Cosmological Screen
(~ 20 Gigaparsecs) The physical conditions of the gas outside of
clusters of galaxies are not well known. Forman et al. (1984)
found values near n ~ 2 × 10-8 cm-3 and T ~ 3 × 108 K, from
X-ray data and analysis. Somewhat higher n values have been used
elsewhere. On the scale of the Universe, a study of RM of 309
distant quasars (and galaxies), with both a measured redshift z
and an observed rotation measure RM, led to a search for a
possible increase of RM with z. No such increase was found up to
a redshift z = 3.5, e.g., Figure 2 in Vallée (1990c). An
observed upper limit of extragalactic RM = 2 rad/m2 for any
cosmological contribution was deduced, which in turn corresponds
to < 10-9 Gauss for a regular cosmic magnetic field Breg
(outside clusters of galaxies), and a mean particle density of
10-7 cm-3 (or < 10-10 Gauss for 10-6 cm-3). Thus there is no
cosmological magnetized foreground screen, out to a redshift z =
3.5. For H0 = 50 km/s/Mpc, q0 =