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 = 1, then such a z corresponds to a distance of about 20 000 Mpc (= 20 Gpc).
Of course, the absence of a measurable value of Breg does not rule out the presence of a possible random magnetic field Bran throughout the Universe. Searches for such a Bran component have been reviewed by Kronberg (1994), and an upper limit of 10-9 Gauss is also indicated out to a redshift of z ~ 3.5.