Cosmological Redshift: Where Has the Energy Gone?

[Glenn G]

As the Universe expands and the photons stretch to longer wavelengths (say from UV photons to the CMBR now) where has the energy gone since longer wavelength photons have less energy?
Regards,
G.

 

[mfb]

It is gone.

There is no global energy conservation in general relativity.

 

[Glenn G]

It is gone.

There is no global energy conservation in general relativity.

I'm surprised it's just gone mfb.
I was wondering if there's a type of gravitational potential it goes but for photons instead of matter.
I suppose then that in total that's quite a bit of energy that's gone if all photons get stretched this way.
G.

 

[mfb]

Today the energy density of the radiation is negligible, but in the early universe it was relevant.