I Redshifts of galaxies and the expansion of the universe

[kent davidge]

(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?

 

[Bandersnatch]

If so, is that energy contributing to the expansion of the universe?

No, it's just lost.

 

[kent davidge]

No, it's just lost.

What about the conservation of energy?

 

[Bandersnatch]

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

 

[kent davidge]

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?

 

[kent davidge]

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.