cosmological redshift and energy conservation

[Rathnakumar]

As a photon travels to us from a distant object, space expands and so
its wavelength increases. Hence its energy should decrease according
to the formula hc/lambda. How do we reconcile this with energy
conservation?

[marcofuics]

...the whole universe has enlarged, so maybe if you reintegrate over
all space the energy remains constant.

[avijeet prasad]

The total number of photons may not be constant in the expanding universe.
So even though the energy of an individual photon may decrease due to red shift,
the increase in the number of photons compensates for it and the total energy is conserved.
The following article may be of interest as to how these "post big bang" photons are created :
www.wbabin.net/physics/rothenstein16.pdf