What is the primary physical cause of the Cosmological Shift?

Answer

The stretching of the spatial fabric between the source and observer

The Cosmological Shift is intrinsically tied to the expansion of the universe itself. The spatial fabric stretches during the light's journey, proportionally stretching the photon's wavelength.

What is the primary physical cause of the Cosmological Shift?
astronomyuniversecosmologyblue shiftred shift