Cosmological Redshift: Where Has the Energy Gone?

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Discussion Overview

The discussion revolves around the concept of cosmological redshift and the implications for energy conservation in the context of an expanding universe. Participants explore the fate of energy as photons shift to longer wavelengths, particularly in relation to general relativity.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions where the energy of photons has gone as they redshift from shorter to longer wavelengths, noting that longer wavelengths correspond to lower energy.
  • Another participant asserts that energy is "gone," emphasizing that there is no global energy conservation in general relativity.
  • A participant expresses surprise at the notion that energy is simply lost and speculates about the possibility of a gravitational potential affecting photons, similar to how it affects matter.
  • It is noted that while the energy density of radiation is negligible today, it was significant in the early universe.

Areas of Agreement / Disagreement

Participants do not reach a consensus; there are competing views regarding the implications of energy conservation in the context of cosmological redshift and general relativity.

Contextual Notes

The discussion highlights the limitations of applying classical energy conservation principles to the framework of general relativity, as well as the dependence on the definitions of energy in cosmological contexts.

Glenn G
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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.
 
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It is gone.

There is no global energy conservation in general relativity.
 
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mfb said:
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.
 
Today the energy density of the radiation is negligible, but in the early universe it was relevant.
 

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