I have read this question:
The equations that govern quantum mechanics predict that the angular momentum (that is, spinning or orbiting) in a system can't take on any value, but instead come in lumps. The "reduced Planck constant" ℏ=h/2π is the size of a lump of angular momentum. An electron orbiting a nucleus can do so with zero angular momentum, with angular momentum ℏ, with angular momentum 2ℏ, and so on, but for a hypothetical value like 1.37ℏ there are no solutions to the electron's equations of motion. Light also carries angular momentum: any process that emits or absorbs a photon must involve an angular momentum change of ℏ (or a larger integer multiple of ℏ).
Now as far as I understand it, spacetime is continuous. When cosmological redshift affects light's wavelength, that is, the wavelength is stretched, the photons building it up will lose energy. But since spacetime is continuous, this energy loss can come in arbitrarily small amounts, and yes, smaller then Planck's constant.
So cosmological redshift can transform theoretically a photon to certain energy levels that cannot be expressed by the multiples of Planck's constant.
So basically, if spacetime expansion is continuous (not-quantized), then it (through cosmological redshift) can transform photons into energy levels that cannot be expressed as multiples of Planck's constant, but if spacetime expansion is quantized (so spacetime can only expand in increments, such as units of Planck length), then the wavelength can only be stretched in increments so that the energy of the photons always is expressible as multiples of Planck's constant.
- Can cosmological redshift only decrease photons' energy by increments (that correspond to Planck's constant)?