Is Energy Fully Conserved in an Expanding Universe?

In summary, the conversation discusses the issue of conservation of energy in an expanding universe, specifically regarding the energy of photons and particles. There is a paradox where the decrease in energy density of a universe containing only particles goes down with the third power, while the energy of photons goes down with the fourth power. The conversation also mentions the concept of redshift and how it affects the energy of photons. The participants also mention the possibility of dark energy being responsible for the lost momentum of photons. The conversation ends with the suggestion to review the theory of general relativity if the paradox is proven to be real.
  • #1
hurk4
132
0
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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  • #2
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".
 
  • #3
Not all that difficult to resolve. As the universe expands, it dilutes the energy of the photons transversing it - usually referred to as redshift.
 
  • #4
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
 
  • #5
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
 
  • #6
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.
 
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  • #7
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?
 
  • #8
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 wavelenght, 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
 

What is the Energy Conservation Paradox?

The Energy Conservation Paradox refers to the seemingly contradictory phenomenon of increased energy efficiency leading to an overall increase in energy consumption.

What causes the Energy Conservation Paradox?

The Energy Conservation Paradox is caused by a combination of factors, including rebound effects, income effects, and technological advancements. Rebound effects occur when individuals or businesses use the savings from energy efficiency to increase their energy consumption in other areas. Income effects refer to the fact that as people become wealthier, they tend to consume more energy-intensive goods and services. Technological advancements can also lead to increased energy consumption, as they make energy usage more convenient and affordable.

Is the Energy Conservation Paradox a real phenomenon?

Yes, the Energy Conservation Paradox has been observed in various industries and countries around the world. It is a well-documented and studied phenomenon in the field of energy economics.

What are the consequences of the Energy Conservation Paradox?

The consequences of the Energy Conservation Paradox can include an overall increase in energy consumption, which can lead to negative environmental impacts such as increased greenhouse gas emissions and resource depletion. It can also hinder efforts to reduce energy consumption and transition to renewable energy sources.

How can the Energy Conservation Paradox be addressed?

Addressing the Energy Conservation Paradox requires a multi-faceted approach. This can include implementing policies and regulations that discourage rebound effects and promote sustainable energy use, as well as educating individuals and businesses about the importance of reducing energy consumption. Additionally, investing in renewable energy sources and technologies can help to mitigate the effects of the paradox by reducing reliance on fossil fuels.

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