Conservation of Energy and Expansion Redshift

In summary, GR does not provide an unambiguous definition of energy, so it is theoretically justified to exempt photons from the principle of energy conservation. There is, however, another way of looking at things. If you assume expansion constitutes a form of work, it would be reasonable to suspect the missing photon energy helps power it. By the same token should the universe ever collapse, it would be reasonable to expect photons to become blue shifted, thus recovering the energy loss due to expansion. In a quantum theory of gravity, a less ambiguous definition of energy may emerge, and help resolve the issue of energy conservation. So, yes, photons lose energy via redshift, but, it to conclude that it simply vanishes is not yet warranted.
  • #1
John Morrell
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Since the universe is expanding, photons emitted by distant stars are red-shifted, having their wavelengths stretched out. But, since the energy of a photon is dependent on the inverse of its wavelength, doesn't that mean that the expansion of the universe is causing photons to lose energy? How does this not violate the conservation of energy?

A similar question deals with the admittedly minuscule but still present changes in potential energy between, for example, separated charges or masses when the space between them expands. I know the effect is likely not measurable, but energy can't just be appearing, right?
 
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  • #2
@John Morrell

Just a note ... you are posting a bunch of questions that don't really belong in the general physics section
you should be posting these in the cosmology sub-section under the astronomy section :smile:I have asked for this one to be moved
Dave
 
  • #3
John Morrell said:
How does this not violate the conservation of energy?
Energy is not conserved in an expanding universe.
 
  • #4
Okay, mind slightly blown. Also thanks for telling me about that category; somehow I didn't see it when I posted.
 
  • #5
GR does not provide an unambiguous definition of energy, so it is theoretically justified to exempt photons from the principle of energy conservation. There is, however, another way of looking at things. If you assume expansion constitutes a form of work, it would be reasonable to suspect the missing photon energy helps power it. By the same token should the universe ever collapse, it would be reasonable to expect photons to become blue shifted, thus recovering the energy loss due to expansion. In a quantum theory of gravity, a less ambiguous definition of energy may emerge, and help resolve the issue of energy conservation. So, yes, photons lose energy via redshift, but, it to conclude that it simply vanishes is not yet warranted.
 
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  • #6
Chronos said:
GR does not provide an unambiguous definition of energy, so it is theoretically justified to exempt photons from the principle of energy conservation. There is, however, another way of looking at things. If you assume expansion constitutes a form of work, it would be reasonable to suspect the missing photon energy helps power it. By the same token should the universe ever collapse, it would be reasonable to expect photons to become blue shifted, thus recovering the energy loss due to expansion. In a quantum theory of gravity, a less ambiguous definition of energy may emerge, and help resolve the issue of energy conservation. So, yes, photons lose energy via redshift, but, it to conclude that it simply vanishes is not yet warranted.
I don't mean this as a personal theory, but is it not possible that the dark energy causing the expansion of the universe is what makes up the energy difference caused by the redshifting of photons?
 
  • #7
Chronos said:
GR does not provide an unambiguous definition of energy, so it is theoretically justified to exempt photons from the principle of energy conservation. There is, however, another way of looking at things. If you assume expansion constitutes a form of work, it would be reasonable to suspect the missing photon energy helps power it. By the same token should the universe ever collapse, it would be reasonable to expect photons to become blue shifted, thus recovering the energy loss due to expansion. In a quantum theory of gravity, a less ambiguous definition of energy may emerge, and help resolve the issue of energy conservation. So, yes, photons lose energy via redshift, but, it to conclude that it simply vanishes is not yet warranted.
Does the CMB have less energy now than when it was emitted, or is the same energy just spread out over a larger space and presents as redshift?

Not sure if that makes sense...
 
  • #9
Chronos said:
If you assume expansion constitutes a form of work

There is a way to make this match up with the math (by exploiting a similarity with the Newtonian equations that describe a freely falling object in a gravitational field, and thereby defining a "gravitational potential energy" for the universe), but AFAIK it only works for a closed, matter-dominated universe with zero cosmological constant/dark energy.

Chronos said:
it would be reasonable to suspect the missing photon energy helps power it

Since AFAIK the mathematical similarity I referred to above does not work for photons (more precisely, it doesn't work for a radiation dominated universe), I don't think this can be justified by the math. And even if it could be, as above, I don't think it would work for a nonzero cosmological constant/dark energy.

Comeback City said:
is it not possible that the dark energy causing the expansion of the universe is what makes up the energy difference caused by the redshifting of photons?

I don't think so. See above.
 
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1. What is the conservation of energy and how does it relate to expansion redshift?

The conservation of energy is a fundamental principle in physics that states energy cannot be created or destroyed, only transferred or transformed. In the context of expansion redshift, this means that as the universe expands, the energy of photons (light particles) also decreases. This leads to a redshift in the light's wavelength, which is observed as an increase in the wavelength of light from distant galaxies.

2. How does the expansion of the universe affect the redshift of light?

The expansion of the universe causes the space between objects (including galaxies) to stretch. As light travels through this stretched space, its wavelength increases, resulting in a redshift. This effect is known as cosmological redshift and is one of the key pieces of evidence for the expansion of the universe.

3. Can the conservation of energy be violated by the expansion of the universe?

No, the conservation of energy is a fundamental law of physics and cannot be violated. The expansion of the universe does not create or destroy energy, it only redistributes it in the form of redshifted light.

4. How does the conservation of energy and expansion redshift impact our understanding of the universe?

The conservation of energy and expansion redshift play a significant role in our understanding of the universe and its evolution. These concepts help explain various phenomena such as the redshift of light from distant galaxies, the cosmic microwave background radiation, and the overall expansion of the universe.

5. Are there any potential exceptions to the conservation of energy in the context of expansion redshift?

There are some theories that suggest the conservation of energy may not hold in certain extreme scenarios, such as the expansion of the universe accelerating at an increasing rate. However, these theories are still being studied and have not been confirmed. In general, the conservation of energy and expansion redshift are widely accepted principles in our current understanding of the universe.

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