Redshifts of galaxies and the expansion of the universe

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Discussion Overview

The discussion revolves around the concept of redshifts of photons emitted from supernovae and their relationship to the expansion of the universe. Participants explore the implications of energy loss associated with redshift and the conservation of energy within the framework of General Relativity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the energy lost by photons as they redshift contributes to the expansion of the universe.
  • Another participant asserts that the energy is simply lost, not contributing to the expansion.
  • A subsequent participant raises the issue of conservation of energy in the context of General Relativity.
  • It is noted that energy is not always conserved in General Relativity, with references provided for further reading.
  • A participant expresses confusion about the fate of the lost energy, asking where it goes.
  • Another participant clarifies that the lost energy goes "nowhere" and emphasizes that conservation laws depend on symmetries, which are absent in an expanding universe.
  • A later reply indicates understanding of this explanation after the clarification.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of energy loss due to redshift, with some asserting it is simply lost while others question the conservation of energy in this context. The discussion remains unresolved regarding the broader implications of these ideas.

Contextual Notes

Participants reference the lack of time-translation symmetry in an expanding universe as a reason for the non-conservation of energy, but the implications of this are not fully explored or agreed upon.

kent davidge
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(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?
 
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kent davidge said:
If so, is that energy contributing to the expansion of the universe?
No, it's just lost.
 
Bandersnatch said:
No, it's just lost.
What about the conservation of energy?
 
Bandersnatch said:
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.
 
kent davidge said:
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?
 
Bandersnatch said:
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.
 

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