**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 × 10^{8} 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/m^{2}
for any cosmological contribution was deduced, which in turn corresponds
to < 10^{-9} Gauss for a
regular cosmic magnetic field *B*_{reg} (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 *H*_{0} = 50
km/s/Mpc, *q*_{0} = 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
*B*_{reg} does not rule out the presence of a possible
random magnetic field *B*_{ran} throughout the
Universe. Searches for such a
*B*_{ran} 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.