What physical effect causes light from objects moving away from us to stretch to longer, redder wavelengths?

Cosmological redshift

The stretching of light waves originating from distant galaxies moving away from an observer is a direct application of relativistic physics to light, specifically termed cosmological redshift. As space expands between the source galaxy and the observer, the photons traveling across this expanding void have their wavelengths physically elongated. This stretching shifts the observed light toward the red end of the electromagnetic spectrum, meaning longer wavelengths. While it is related to the general Doppler effect, when applied specifically to light due to the expansion of the universe over cosmological distances, it is precisely defined as cosmological redshift. The other options describe unrelated physical principles: the Doppler effect applies to sound waves as well as light but is less precise here; $z=1$ is a specific measurement, not the effect itself; and the inverse square law relates intensity to distance, not wavelength shift.