Thanks to everyone for the many useful comments and references.

Joseph Lazio wrote:
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.

What happens to an e/m wave below that cuttoff frequency?
The energy must go somewhere.

[[Mod. note -- If you have some source radiation below the cutoff
frequency, the light will be absorbed by the interstellar medium.
Think "light bulb outdoors on a foggy day". -- jt]]

--
Ray Tomes
http://ray.tomes.biz/
http://www.cyclesresearchinstitute.org/