Conservation of energy and space expansion

In summary, when a photon is emitted by a distant object at a redshift of z = 2.0, its wavelength will shift towards red due to space expansion. This means that the photon will lose energy, which raises the question of where the energy goes. However, energy conservation only applies within inertial frames, and in a non-inertial frame such as the expanding universe, there is no principle of global energy conservation. This is a well-known concept in general relativity, and can be further explored in the blog post "Energy is not conserved in an expanding universe" by cosmologist Sean Carroll. The issue is not with energy itself, but with how it is defined in this context.
  • #1
Lopes1801
1
0
This had me thinking for a while. Imagine a photon emitted by a very distant object at a redshift of z = 2.0 for example. As the photon travels through space, due to space expansion the photon's wavelength will shift towards red. With an increase in the wavelength there must come a decrease in the frequency. With that, the energy of the photon also decreases. Therefore, the photon must lose energy. And by the law of conservation of energy, this amount of energy cannot disappear. So where does the energy go to?
(I don't know whether the question should be labeled as Basic or Intermediary, so I arbitrarily chose one of the options)
 
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  • #2
Energy conservation only applies within inertial frames. Within a non-inertial frame - which is what is needed for any thought experiment big enough to incorporate galactic redshift - energy is only defined locally, and there is no principle of global energy conservation.
 
  • #3
I agree with Andrew. The cosmologist Sean Carroll has a blog post about this called "Energy is not conserved in an expanding universe"
If you google the title you will, I think, get it. A lot of fairly intuitive discussion. Non conservation of energy in GR is well known, just a bit unintuitive when first encountered.
 
  • #4
The problem with energy conservation is not energy, the problem is in defining how you define energy.
 

What is conservation of energy?

Conservation of energy is the fundamental principle in physics that states that energy cannot be created or destroyed, only transferred or converted from one form to another. This means that the total amount of energy in a closed system remains constant over time.

How does conservation of energy relate to space expansion?

According to the law of conservation of energy, the total energy in the universe is constant. This includes the energy associated with the expansion of space. As the universe expands, the energy associated with this expansion is conserved.

What is the relationship between space expansion and the rate of energy transfer?

As the universe expands, the distances between objects increase, resulting in a decrease in the rate of energy transfer between them. This means that the further apart objects are, the less energy they will transfer to each other per unit time.

Is the expansion of space a form of energy?

No, the expansion of space is not a form of energy. Energy is a property of matter and space itself does not have any energy associated with it. However, the expansion of space does affect the energy of objects by changing the distances between them.

Can conservation of energy be violated by space expansion?

No, conservation of energy is a fundamental law of physics and cannot be violated. The expansion of space does not create or destroy energy, it only changes the distribution of energy within the universe.

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