Malcolm Longair has described how the Cavendish Laboratory spent the 1960s practicing human sacrifice in order to determine the extragalactic radio-source background, with the following approximate result:
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to within an uncertainty of about 20% in amplitude and 0.1 in spectral
index. This background dominates over the CMB for
1 m, and is consistent
with the integrated contribution of discrete sources.
On the other hand, it is also not ruled out that a genuine continuum background
might exist at up to 10% or so of the above level. What would this mean
if it was
really so? The hope would be to learn something about diffuse
intergalactic gas, and
there are two standard emission mechanisms to which we might appeal:
synchrotron
radiation and bremsstrahlung. The parameters available are the density
of the emitting plasma, parameterized by its contribution to
(in the case of synchrotron
radiation,
the electrons would have an assumed power-law energy distribution), plus
the local
value of either the magnetic field, B or temperature T -
both of which should scale as
(1 + z)2. The resultant background can then be worked
out in the standard way (see
Longair 1978).
For synchrotron radiation, we get
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What is a plausible value for the intergalactic magnetic field? It is
worth recalling that
magnetic fields are very much a skeleton in the closet of cosmology,
since we cannot
easily rule out rather large values - which would significantly change
our ideas about
structure formation, for example. A nice review of the issue is given by
Coles (1992);
he argues that B could be as large as 10-4 nT. This
would allow observed magnetic
fields in astrophysical sources to be made via compression, rather than
dynamo effects,
and would greatly alter the progress of galaxy clustering. For such a
field, the observed background would be produced with
h ~
10-3. This is an implausibly high density
for a plasma with fully relativistic electrons, but it is perhaps
surprising that the effect
is this close to being interesting.
Turning to bremsstrahlung, one can simply try scaling old solutions for
the X-ray
background in which a `low'-energy flux of around 10-3 Jy
sr-1 is produced by models
with T 
108 K
and h2
0.1. Since bremsstrahlung emissivity
scales as T-1/2, this implies
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If we ignore the difficulty in keeping plasma at such temperatures ionized, this seems the closest that the radio background is likely to get to setting constraints on Cold Dark Matter...