2010MNRAS.406..863L

The Galactic disc is opaque to radio waves from extragalactic sources with frequencies ν less than ∼ 3 MHz. However, radio waves with kHz, Hz and even lower frequencies may propagate through the intergalactic medium (IGM). I argue that the presence of these waves can be inferred by using the Universe as our detector. I discuss possible sub-MHz sources and set new non-trivial upper limits on the energy density of sub-MHz radio waves in galaxy clusters and the average cosmic background. Limits based on five effects are considered: (1) changes in the expansion of the Universe from the radiation energy density, (2) heating of the IGM by free-free absorption, (3) radiation pressure squeezing of IGM clouds by external radio waves, (4) synchrotron heating of electrons in clusters and (5) inverse Compton upscattering of sub-MHz radio photons. Any sub-MHz background must have an energy density much smaller than the cosmic microwave background at frequencies below 1MHz. The free-free absorption bounds from the Lyman α (Lyα) forest are potentially the strongest, but are highly dependent on the properties of sub-MHz radio scattering in the IGM. I estimate an upper limit of 6 x 10⁴ L_☉/Mpc3 for the emissivity within Lyα forest clouds in the frequency range of 5-200Hz. The sub-MHz energy density in the Coma cluster is constrained to be less than ∼ 10^–15 erg/cm3. At present, none of the limits is strong enough to rule out a maximal T_b= 10¹² K sub-MHz synchrotron background, but other sources may be constrained with better knowledge of sub-MHz radio propagation in the IGM.