# Redshifts of galaxies and the expansion of the universe

(Sorry for my poor English.) I was watching a PBS video on expansion of the universe and the guy says the wavelength of a photon emitted in a supernova becomes larger as it travels to the Earth. Is it because the photon lose energy (to space)?
If so, is that energy contributing to the expansion of the universe?

Related Other Physics Topics News on Phys.org
Bandersnatch
If so, is that energy contributing to the expansion of the universe?
No, it's just lost.

No, it's just lost.
What about the conservation of energy?

Energy is not (always) conserved in General Relativity. The pages below have good layman-oriented explanations of the why.
http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html
Good material. But I still can't understand some things.

From http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/:
"The thing about photons is that they redshift, losing energy as space expands. If we keep track of a certain fixed number of photons, the number stays constant while the energy per photon decreases, so the total energy decreases."

Where goes this lost energy?

From http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html:

"The Cosmic Background Radiation (CBR) has red-shifted over billions of years. Each photon gets redder and redder. What happens to this energy?"

I didnt understand what really happens.

Bandersnatch
Where goes this lost energy?
Nowhere. It is just lost.

Conservation laws depend on symmetries. If there's no time-translation symmetry (as with the expanding universe), there can't be energy conservation, so it doesn't make sense to ask where it went - why would it need go somewhere if there is no conservation law?

Nowhere. It is just lost.

Conservation laws depend on symmetries. If there's no time-translation symmetry (as with the expanding universe), there can't be energy conservation, so it doesn't make sense to ask where it went - why would it need go somewhere if there is no conservation law?
I understood it now. Thank you.