Galaxy redshift and thermodynamics

In summary, the conversation revolves around the idea that the redshift of light in the universe is not just a consequence of an expanding universe, but is required to comply with the laws of thermodynamics. This is supported by the reasoning that if there was more blueshifting than redshifting in the universe, it would violate the laws of thermodynamics. The conversation also touches on the relationship between universe size and entropy, as well as the implications for an expanding or contracting universe. There is also discussion about the difficulties in reconciling cosmological redshift with the laws of thermodynamics and quantum mechanics. Overall, the conversation highlights the complex and interconnected nature of the laws of thermodynamics and their potential implications for the universe.
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I just finished rereading the great "A Brief History of Time". To me, what stands out the most in this book, is its ability to keep raising questions while you read it. This thought came up. It's been stuck in my mind for days, so I will humbly post it here to get some feedback. Please forgive my poor (technical) English and the assumptions and simplifications I made. The hypothesis is as follows:

Hypothesis:
Predominant galaxy redshift is not just a consequence of an expanding universe. It is in fact required to comply with the Laws of Thermodynamics. In other words, as long as thermodynamics laws hold and entropy can only grow, the universe can only expand.

Reasoning:
(Especially here, please forbid my poor technical English, I just hope that the idea comes through though).
- When light redshifts, it loses part of its capacity to do work due to that light losing temperature. In other words, if that light was to be used after redshifting in a Carnot Cycle, its temperature would be lower, the temperature gradient would be lower, and less work could be extracted.
- On the other hand when light blueshifts it increases its capacity to do work due to that light gaining temperature.
If, on a macro scale, there was more blueshifting than redshifting in the Universe, light would be on average increasing its capacity to do work, which seems a violation of the Laws of Thermodynamics.
Therefore, to avoid an overall increase in the capacity of light to do work, there has to be more redshift than blueshift, and the Universe can do nothing but expand for as long as the laws of thermodynamics hold.

Self assessment of this reasoning:
I'm well aware that this reasoning assumes a simplified Universe and that it doesn't take into account a lot of aspects and interactions, but so far I haven't found a strong reasoning against.

For example, light doesn't increase its capacity to do work while travelling, as my wording might suggest, since the redshift of blueshift happens due to the different velocities of source and observation point. Hence I have considered that my reasoning could be moot and easily explainable through looking at the problem from the point of view of different frames of reference. But I don't think that it renders my hypothesis moot since it's not about a pair of frames of reference, but as the Universe as a whole (supposedly thermodynamically) closed system with all the frames of reference in it.

Also, I would like to point out that my feeling is that the hypothesis and reasoning would be better expressed in terms of entropy gradients, yet I don't have the academic skill to do so.

Possible consequences
The above is as far as I would go making a logical statement. The following are just ideas that could be (or not) consequences of the above. Don't take them very seriously.
1. Should the above hold true, it doesn't by itself prove that the Universe will keep expanding forever. But it ties Thermodynamic Laws to universe expansion and it would hint that, should the universe start contracting (and blueshift become predominant), thermodynamic laws would need to be reversed.
2. It would at the same time hint to there being a relationship between Universe size and entropy (a relationship that already appears to be there when looking at the Universe through the lens of the holographic principle). It would hint that the Big Bang would be the moment with the lowest entropy and that as size grows entropy grows. Similarly, should the universe start contracting, entropy would start to decrease down to the lowest amount which would be at the Big Crunch.

I would really appreciate if you came back with your ideas. It may well be that this, or equivalent perspectives, have already been analyzed by the scientific community. As said, I would appreciate your comments, especially if you could include links or references to materials that I could use to increase my knowledge in this area.

Many humble thanks in advance.
 
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  • #3
Thanks, great reading! My question is complementary. Do the laws of thermodynamics imply an expanding universe?
 
  • #4
Bolhuso said:
Thanks, great reading! My question is complementary. Do the laws of thermodynamics imply an expanding universe?
If the [simple phrasing of the] first law is inconsistent with an expanding universe, that would be a problem for the idea that the [simply phrased] laws of thermodynamics demand an expanding universe.

We account for the large scale expansion of the universe using the theory of general relativity.
 
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  • #5
jbriggs444 said:
If the [simple phrasing of the] first law is inconsistent with an expanding universe, that would be a problem for the idea that the [simply phrased] laws of thermodynamics demand an expanding universe.

We account for the large scale expansion of the universe using the theory of general relativity.

The difficulty around thermodynamics and cosmological redshift is that any redshift occurring after the light has been emitted means that the photon lost energy without any discernible work being done. The BBT offers no sink into which a photon can lose energy in the interval between emission and absorption. You can contrast this with Doppler shifts, which occur at the moment of emission. This appears to clearly violate the first and, perhaps, second laws depending on how they are considered. It also has problems in terms of quantum mechanics, since photons are emitted with fixed energies.
 
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  • #6
JohnDee said:
The difficulty around thermodynamics and cosmological redshift is that any redshift occurring after the light has been emitted means that the photon lost energy without any discernible work being done.
That isn't a problem: energy is frame dependent even in Galilean relativity. That does not imply a loss of energy.
 
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  • #7
russ_watters said:
That isn't a problem: energy is frame dependent even in Galilean relativity. That does not imply a loss of energy.
I'm very out of my depth here but I would like to understand more.

As far as I know, absolute energy is frame dependent in Galilean relativity but the amount of work done isn't. For example, during a plastic collision between two particles, the absolute kinetic energy of each of the particles depends on the reference frame. However, the energy lost as heat during the collision will remain constant no matter the speed of the inertial frame.

If redshift does indeed affect the capability of light to do work then I understand @JohnDee 's concern about this energy disappearing from existence as stated in his post.
JohnDee said:
The difficulty around thermodynamics and cosmological redshift is that any redshift occurring after the light has been emitted means that the photon lost energy without any discernible work being done. The BBT offers no sink into which a photon can lose energy in the interval between emission and absorption. You can contrast this with Doppler shifts, which occur at the moment of emission. This appears to clearly violate the first and, perhaps, second laws depending on how they are considered. It also has problems in terms of quantum mechanics, since photons are emitted with fixed energies.

In fact, I used to believe that's why entities such as dark energy were created. To be able to balance the equations.

Again, this is extremely far away from the topics I have studied but I'd like to understand where is the failure in the reasoning if there is one.
 
  • #8
Juanda said:
As far as I know, absolute energy is frame dependent in Galilean relativity but the amount of work done isn't. For example, during a plastic collision between two particles, the absolute kinetic energy of each of the particles depends on the reference frame. However, the energy lost as heat during the collision will remain constant no matter the speed of the inertial frame.
That's correct.
Juanda said:
If redshift does indeed affect the capability of light to do work then I understand @JohnDee 's concern about this energy disappearing from existence as stated in his post.
Here you're talking about different collisions. The amount of work done in one collision is the same for everyone, but when you compare two different collisions the amount of work done can of course be different. E.G. if a car collides with a wall and absorbs 1 gigajoule of energy, everyone agrees it absorbed one gigajoule of energy. But if it collides with a truck, it might absorb 5 gigajoules of energy. Everyone agrees with that too. Same car, different collisions so different energy.
 
  • #9
Juanda said:
I'm very out of my depth here but I would like to understand more.

As far as I know, absolute energy is frame dependent in Galilean relativity but the amount of work done isn't. For example, during a plastic collision between two particles, the absolute kinetic energy of each of the particles depends on the reference frame. However, the energy lost as heat during the collision will remain constant no matter the speed of the inertial frame.

If redshift does indeed affect the capability of light to do work then I understand @JohnDee 's concern about this energy disappearing from existence as stated in his post.In fact, I used to believe that's why entities such as dark energy were created. To be able to balance the equations.

Again, this is extremely far away from the topics I have studied but I'd like to understand where is the failure in the reasoning if there is one.
If you haven't already, you could try Sean Carroll's blog.

https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Note that, in general, energy conservation results from time invariance (via Noether's theorem). In an expanding universe, time invariance no long holds and global energy conservation is lost.

Redshift measurenents, in particular, do not demand some absolute, intrinsic change to a light signal. An idea which is often the starting point for crackpot theories! Instead, the measurements of light wavelength reflect the usual frame dependence of energy measurements. In this case, of different local frames within a globally expanding (or curved, to be precise) spacetime.
 
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  • #10
PeroK said:
If you haven't already, you could try Sean Carroll's blog.

https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Note that, in general, energy conservation results from time invariance (via Noether's theorem). In an expanding universe, time invariance no long holds and global energy conservation is lost.

Redshift measurenents, in particular, do not demand some absolute, intrinsic change to a light signal. An idea which is often the starting point for crackpot theories! Instead, the measurements of light wavelength reflect the usual frame dependence of energy measurements. In this case, of different local frames within a globally expanding (or curved, to be precise) spacetime.
That was a very interesting read.

I deeply enjoy physics but, as an engineer, my knowledge is focused on classical mechanics which is useful in most situations. Even within classical mechanics there already are so many things I cannot solve that I can't imagine myself studying more theoretical fields.
Once moving into general relativity, quantum mechanics, or such aspects of physics most of what I know just comes from light readings and videos I might watch in my free time. As proven here, applying preconceived notions from classical mechanics to such scenarios only serves to see the limits of those more primitive theories. Kind of how the Ultraviolet Catastrophe emerged from trying to explain a phenomenon with the wrong formulas so new formulas now considering the quantization of energy had to be created.

This forum doesn't cease to surprise me. There are a few active members with an amount of knowledge in their heads that is hard to grasp. You guys know about everything. I discovered this forum because I'm recovering at home after some minor surgery so I have plenty of free time these days. I'll be coming back to work soon and won't be able to expend as much time here. I'll miss you a lot.
 
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  • #11
Juanda said:
I discovered this forum because I'm recovering at home after some minor surgery so I have plenty of free time these days. I'll be coming back to work soon and won't be able to expend as much time here. I'll miss you a lot.
Sorry about the surgery, but glad to hear that your recovery is going well. In your short time here so far, your expertise and insights have been noticed. Just because you're going back to work soon doesn't mean you need to give up on PF; you just have to pace yourself now (says somebody who hasn't mastered that skill yet). :smile:
 
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