The expansion of space, as described by the theory of cosmic expansion, primarily affects the wavelength of light rather than its frequency. This phenomenon is known as cosmological redshift, where the wavelength of light is stretched as space expands, resulting in a shift towards longer wavelengths (redder light).
However, it's important to note that cosmological redshift is primarily relevant for very large cosmological distances, such as those encountered with distant galaxies. For everyday situations and relatively short distances, the expansion of space has a negligible effect on the observed wavelength or frequency of light.
Radio waves, being a form of electromagnetic radiation, are also subject to cosmological redshift in the same manner as light waves. As they travel across expanding space, their wavelengths are stretched, leading to a decrease in frequency. This effect is relevant in the study of cosmology and observations of distant celestial objects, where the observed radio frequencies can be shifted to lower values due to the expansion of space.
However, in practical terms, for most radio communication or everyday applications of radio waves, the effects of cosmological redshift are negligible. The expansion of space has a much more significant impact on the behavior of light over cosmological distances rather than on radio waves used in terrestrial applications.