Is Energy Fully Conserved in an Expanding Universe?

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Discussion Overview

The discussion centers around the conservation of energy in the context of an expanding universe, particularly focusing on the behavior of photons and massive particles. Participants explore the implications of redshift and energy density changes as the universe expands and contracts, raising questions about energy conservation in general relativity.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant notes that the energy density of photons decreases with the fourth power of the universe's expansion, while the energy density of baryonic matter decreases with the third power, leading to a perceived paradox regarding energy conservation.
  • Another participant acknowledges the issues with energy conservation in an expanding universe and references an online article discussing these problems.
  • Some participants suggest that the redshift phenomenon explains the dilution of photon energy as the universe expands.
  • A participant questions the concept of simultaneity between photons and massive particles during interactions, indicating a potential misunderstanding of reference frames in general relativity.
  • One participant proposes that the "lost" momentum of photons might be related to dark energy, suggesting a connection between photon energy loss and the expansion of the universe.
  • Another participant expresses confusion over the terminology used and emphasizes the need to clarify any perceived paradoxes within established physical theories.

Areas of Agreement / Disagreement

Participants express differing views on the implications of energy conservation in an expanding universe, with no consensus reached on the resolution of the paradoxes presented. Some agree on the existence of issues with energy conservation, while others challenge the interpretations and assumptions made.

Contextual Notes

Participants acknowledge the complexity of the topic, with discussions involving assumptions about reference frames, the nature of redshift, and the interactions between photons and massive particles. The discussion remains open-ended, with unresolved questions about the implications for general relativity.

hurk4
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Not able to really find an already existing thread where my question could fit in very well, I am starting this new thread which I hope is not too much confusing.

Conservation of energy of photons in an expanding vacuum part of the universe?
The wavelength of a photon traveling through the expanding universe increases linear with the expansion. The energy of the photon goes inversely with its wavelength, so the radiation energy density goes down with the fourth power of the expansion.
Now I am confronted with the following paradox.
The decrease of the energy density of a universe only containing particles (baryons) goes down with the third power since stars etc don’t take part in the expansion.
Let’s assume an “elongated” photon hits an atom and brings it to a long term exited state.
If after a while the universe starts shrinking (a possibility in an FRLW universe I suppose), then the traveling photons increase their energy density with the fourth power again and the energy density of the particles goes up with the third power again. But then at a certain moment our exited atom falls back to its original state and a photon will leave it at a frequency different from the frequency of the photons that never excited an atom. It looks like as if energy is not fully conserved. What is wrong in my thinking, is it just a bad way of energy book keeping?
 
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You are right that there are problems with conservation of energy in an expanding space. A great online reference that discusses this issue is this one: Is Energy Conserved in General Relativity?. See the section "Expansion of the universe leading to cosmological redshift".
 
Not all that difficult to resolve. As the universe expands, it dilutes the energy of the photons transversing it - usually referred to as redshift.
 
hellfire said:
You are right that there are problems with conservation of energy in an expanding space.

Dear Hellfire,
Thank you for your answer an the link. I already printed the article and I am trying to understand it.
Kind regards,
Hurk4
 
Chronos said:
Not all that difficult to resolve. As the universe expands, it dilutes the energy of the photons transversing it - usually referred to as redshift.

Dear Chronus,
Yes (if I understand your answer right?) that is what I already understood some time. But to me the problem is in the simultaniety of photons and massive particles in the universe while having interactions (with delay times)
Kind regards,
Hurk4
 
What 'simultaneity' are you talking about? Last time I checked photons had no reference frame wrt time. I'm slightly amused by your attempt to toss GR under the bus.
 
Last edited:
Could it be the "lost" momentum is really dark energy? The momentum lost by the photons is somehow transferred/regained/recovered back to its source creating a "pressure" forcing apart all objects which emit electromagnetic radiation.

Hmmm? Not sure that works?
 
Chronos said:
What 'simultaneity' are you talking about? Last time I checked photons had no reference frame wrt time.
While our universe is expanding, photons are traveling through it end are elongated if I may say so, leading to the well know redshift. This is certainly not what I need too explain yo you. Photon density in this space goes then with the inverse third power and because of stretching of its wavelength, foton energy density falls down with the fourth power. Simultaniously, as an average, the density of starclusters (which are fixed in the universe and thus do not travel) goes down with the third power.



I'm slightly amused by your attempt to toss GR under the bus.[/QUOTE]

My english might be not good enough to understand what you mean by this expression. Certainly I have great respect for GR. Maybe I did not express myself not clear enough or to stupid so that it slightly amused you. But I suppose there is only one autonomous reality. If we find paradoxes within a well established physical theory then in the first place we have to check whether it is a real paradox . But if the paradox is real then we have to review our theory. (I will study the article Hellfire indicated and see what I personnally can find. I very much doubt my own paradox.) But if you can and will show me where I went wrong I will be happy to learn.

Many thanks in advance and kind regards,
Hurk4
 

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