The expansion of space, as described by the theory of cosmic expansion in cosmology, primarily affects the large-scale structure of the universe. It causes the stretching of the space itself between galaxies, clusters of galaxies, and other cosmic structures. However, the expansion of space does not directly affect the properties of electromagnetic waves, including light waves, within the local frame of reference.
The amplitude of a light wave represents the maximum displacement of the electric and magnetic fields that comprise the wave. As light propagates through space, it does not experience a stretching or compression of its wave amplitude due to cosmic expansion. This is because the expansion of space primarily affects the metric of spacetime itself, rather than the properties of individual photons or electromagnetic waves.
In other words, the expansion of space does not alter the wavelength or amplitude of individual photons or light waves as they propagate through space. The expansion affects the overall geometry of the universe, including the distances between galaxies, but it does not directly impact the local properties of light waves.
It's worth noting that the expansion of space can have indirect effects on light waves over extremely large cosmological distances. These effects are related to the stretching of wavelengths due to the expansion of space, leading to what is known as cosmological redshift. However, these effects become significant only over extremely vast distances and do not influence the amplitude of individual light waves within the local environment.