"RT" == Ray Tomes writes:

RT Some years ago I had a debate with Ted Bunn concerning the stated
RT 411 photons/cm^3 in the universe. I maintained that the no. of
RT photons at low frequencies (ELF ULF etc) probably vastly exceeds
RT the CMBR count of 411.

RT Ted was totally dismissive until he saw the graphic at
RT http://ray.tomes.biz/ref-back.gif from "Cosmology" by Michael
RT Rown-Robinson page 100 [...]

RT The graph shows that below about 10^9 Hz the intensity of the
RT universal background again begins to rise through to 10^6 Hz where
RT the graph finishes.

RT My questions:

RT Has any observation been made below 10^6 Hz of the general
RT background spectrum?

As Bill Keel has pointed out already, there have been space-based
observations below 1 MHz. There were actually two Radio Astronomy
Explorers. Also, there have been any number of spacecraft observing
either the Sun or the planets that have made observations below 1
MHz.

[...]
RT What do other people think when they look at this graph? It rises
RT by 16 orders of magnitude as the frequency reduces by 18 orders of
RT magnitude. It looks like a general trend to me with a few bumps on
RT - CMBR, effect of dust etc. - although admittedly the CMBR is a
RT big bump.

RT Do others see the possibility that from 10^6 Hz through to 10^-17
RT Hz it could rise at the same average rate and reach 10^20 times as
RT many photons as in the CMBR.

No. Below a certain frequency, the waves cannot propagate. The
interstellar (and presumably intergalactic) medium has a plasma
frequency, given by 9 kHz*\sqrt{n_e} where n_e is the electron density
in units of cm^{-3}. In the local interstellar medium, n_e ~ 0.025
cm^{-3}, so waves with frequencies below 1 kHz just don't propagate.

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