  Most of the Universe's populations of low energy cosmic ray electrons in the
energy range of 1-100 MeV still manage to elude from detection by our
instruments, since their synchrotron emission is at too low frequencies. We
investigate a mechanism which can lead to observable emission of such electron
populations: synchrotron-self Comptonization (SSC). The inverse Compton (IC)
scattering can shift an otherwise unobservable low-frequency 10 kHz-10 MHz
photons into observable radio, infrared (IR) or even more energetic wave-bands.
The resulting emission should be polarized. We also consider IC scattering of
the cosmic microwave background (CMB) and the cosmic radio background (CRB).
Electron spectral aging due to synchrotron, IC and adiabatic losses or gains
influences the resulting spectrum. The predicted radiation spectrum is a strong
function of the history of the source, of the low energy spectrum of
relativistic electrons, and of redshift. It has typically two maxima, and a
decrement in between at CMB frequencies. Detection will give a sensitive probe
of the environment of radio galaxies. We demonstrate that the fossil remnants
of powerful radio galaxies are promising detection candidates, especially when
they are embedded in a dense intra-cluster medium (ICM). GHz peaked sources
(GPS) have very low SSC luminosities, which may, however, extend into the X-ray
or even the gamma ray regime. Clusters of galaxies with relativistic electron
populations may be another detectable sources. Fossil radio plasma released by
our own Galaxy could be revealed by its large angular scale SSC flux. We
discuss the expected detectability of these sources with new and upcoming
instruments as LOFAR, GMRT, EVLA, ATA, ALMA, PLANCK, and HERSCHEL.
