How light redshifts in the expanding Universe

Simulation by Robert Knop, illustrating the redshifting of light due to cosmological expansion. As the Universe expands, the wavelength of the light is stretched as it travels through the expanding spacetime of the Universe. Also note that the distance the light travels is equal neither to the distance between the galaxies at the time the photon was emitted nor to the distance between the galaxies at the time of the photon’s arrival.