As far as I know, we have not been able to reproduce this cosmological redshift from space expanding in the laboratory. The description is rather a hand-waving one, as far as I know. As the photon traverses space, tiny amounts of space show up in the middle of the photon, which increases its wavelength... And you may then ask, where does the energy go? to increase the wavelength... To this I have to say that it depends on the model you have for how space is expanding. We only know that it is expanding.

__________________
Forming opinions as we speak