Conservation of energy in an expanding universe

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I have read that conservation of energy is not a meaningful concept in an expanding universe cosmology. See here
http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
However I have also heard the if the net energy os the universe is zero then it can have a vacuum genesis without it violating energy conservation because the positive mass energy might be cancelled out by the negative gravitational energy. See
https://en.wikipedia.org/wiki/Vacuum_genesis

My question is, if energy isn't conserved in cosmology then why do cosmologists care about the energies cancelling out for a vacuum genesis model to work?
 

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  • #2
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I am not an expert in cosmology or any advance physics,but I read that blog then I did some web research on acceleration of the universe and found this article https://news.nationalgeographic.com...iverse-expansion-what-is-dark-energy-science/
According to it, in the expansion of universe the energy conservation theory is not violated because as it says,The idea is tied to quantum mechanics, which predicts that even in the vacuum of space, particles are constantly winking in and out of existence, generating energy.The trick is that no one has been able to unify the math used in quantum mechanics, which describes the physics of the very small, with the equations in general relativity, which deal with large-scale interactions.
So which theory is wrong or right,it will be great if someone clear the doubts by explaining in detail.
 
  • #3
kimbyd
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I have read that conservation of energy is not a meaningful concept in an expanding universe cosmology. See here
http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
However I have also heard the if the net energy os the universe is zero then it can have a vacuum genesis without it violating energy conservation because the positive mass energy might be cancelled out by the negative gravitational energy. See
https://en.wikipedia.org/wiki/Vacuum_genesis

My question is, if energy isn't conserved in cosmology then why do cosmologists care about the energies cancelling out for a vacuum genesis model to work?
The basic story here is that while energy isn't conserved, General Relativity does follow a different conservation law (conservation of the stress-energy tensor). It is possible to massage the variables to create an "energy" that is conserved because of the conservation of the stress-energy tensor. This is known as the Hamiltonian formalism for General Relativity. But the "energy" which results is only a sensible number in certain specific space-time geometries (it needs to have positive spatial curvature).

Usually this solution isn't used because it's specific to a particular choice of coordinates, and doesn't work for all space-times. Typically people who work with General Relativity like to do calculations in a coordinate-free way, because the choice of coordinates can introduce subtle errors that can be difficult to debug.
 
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  • #4
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It’s not a case of which theory is right or wrong, they are all approximations of the truth as we know it. Each theory works well in the area it was conceived in. However as we try to extend the theory,s reach, we find limitations that must be overcome.

General Relativity works in the very large scale whereas Quantum Mechanics works in the small. However as we try to understand black holes as an example, we find limitations in each theory that prevent us from fully understanding what’s happening in and around the black hole.

So our collections of theories are like a patchwork quilt that covers everything we know but at the seams we have trouble going from one theory to the next. Crossing the seams means jumping from one theory to the next and trying to find an answer that both theories can agree on. The ultimate dream of physics is a unified theory that works at all scales of measurement and for every kind of physical system.
 
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  • #5
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It’s not a case of which theory is right or wrong, they are all approximations of the truth as we know it. Each theory works well in the area it was conceived in. However as we try to extend the theory,s reach, we find limitations that must be overcome.

General Relativity works in the very large scale whereas as Quantum Mechanics works in the small. However as we try to understand black holes as an example, we find limitations in each theory that prevent us from fully understanding what’s happening in and around the black hole.

So our collections of theories are like a patchwork quilt that covers everything we know but at the seams we have trouble going from one theory to the next. Crossing the seams means jumping from one theory to the next and trying to find an answer that both theories can agree on. The ultimate dream of physics is a unified theory that works at all scales of measurement and for every kind of physical system.
Thanks for your response,by the way are you talking about "Quantum gravity", because I have heard in many lectures of Brian Greene, in which he says that this theory can unify both relativity and quantum mechanics.
 
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Yes, quantum gravity and string theory are both candidates for a future unified theory until we find something that they can’t explain and then we’re back to searching again.

Feynman had an example of a spinning propeller that spun very fast. Our initial understanding was that’s it’s a simple propeller but as we look more deeply we find propellers on the propeller blades that spin even faster that we couldn’t t see before due to limits in our equipment. He then went on to speculate that the progression would continue with every new propeller found that it too would have child propellers ad infinitum and so it may be true in our real theories where there will be no final unified theory of everything.

https://hackaday.com/2018/05/15/richard-feynman-a-life-of-curiosity-and-science/

I think the video at the end of the article contains the propeller story as told by Feynman himself.
 
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  • #7
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Yes, quantum gravity and string theory are both candidates for a future unified theory until we find something that they can’t explain and then we’re back to searching again.

Feynman had an example of a spinning propeller that spun very fast. Our initial understanding was that’s it’s a simple propeller but as we look more deeply we find propellers on the propeller blades that spin even faster that we couldn’t t see before due to limits in our equipment. He then went on to speculate that the progression would continue with every new propeller found that it too would have child propellers ad infinitum and so it may be true in our real theories where there will be no final unified theory of everything.

https://hackaday.com/2018/05/15/richard-feynman-a-life-of-curiosity-and-science/

I think the video at the end of the article contains the propeller story as told by Feynman himself.
Thanks again for your sincere reply.
 
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PeterDonis
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by the way are you talking about "Quantum gravity", because I have heard in many lectures of Brian Greene, in which he says that this theory can unify both relativity and quantum mechanics.
There is no single theory called "quantum gravity". I suspect Greene is referring to string theory.

quantum gravity and string theory are both candidates for a future unified theory
As above, there is no single theory called "quantum gravity". String theory is one possible candidate for a quantum gravity theory. Another is loop quantum gravity.
 
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  • #9
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There is no single theory called "quantum gravity". I suspect Greene is referring to string theory.



As above, there is no single theory called "quantum gravity". String theory is one possible candidate for a quantum gravity theory. Another is loop quantum gravity.
Yes ,Greene was talking about string theory which can be used to solve the mystery of quantum gravity,the theory that is supposed to be a marriage between quantum mechanics and relativity,
BTW,Thanks mentors for your sincere replies.
 
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There is no single theory called "quantum gravity". I suspect Greene is referring to string theory.



As above, there is no single theory called "quantum gravity". String theory is one possible candidate for a quantum gravity theory. Another is loop quantum gravity.

Yes, thanks for the clarification, I meant loop quantum gravity.
 
  • #11
kimbyd
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There is no single theory called "quantum gravity". I suspect Greene is referring to string theory.
To expand on this a little bit, quantum mechanics and General Relativity fundamentally disagree on how the world works. A quantum system can exist in a superposition of different energy states. General Relativity requires matter to be in a specific state with definite energy: it has no way of representing matter in a superposition of states. And the simple ways of making General Relativity work with a superposition simply do not work.

There are, to date, two major attempts to rectify this discrepancy: Loop Quantum Gravity and String Theory. LQG's main issue is that it still isn't clear whether or not LQG actually looks like General Relativity in the non-quantum realm. Wikipedia has a decent overview of why this is so hard to verify, though there's a lot of jargon. Basically, both GR and LQG are complex theories, and those complexities make it difficult to see if they make the same predictions for experiments already done to test General Relativity.

String Theory's main issue is that it's really hard to find evidence in favor of the theory, perhaps even impossible: simple assumptions place the detectability of String Theory far beyond any possibility of experimental detection. It's possible we'll get lucky and the specific details will make String Theory detectable, but nobody knows.
 
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