Is energy conserved in cosmology according to the laws of thermodynamics?

[TrickyDicky]

I've just read the FAQ about this and IMO it is "not even wrong" to say that energy conservation doesn't apply to Cosmology. The fact is energy conservation is clearly stated in the EFE by the vanishing divergence of the stress-energy tensor and since GR is the theory we currently use in Cosmology, cerrtainly energy is conserved in cosmology.
There seems to be some silly confusion in that FAQ answer, the fact that currently the global energy can't be defined doesn't imply "energy is not conserved in cosmology", just like we don't say that since there is no "standard" way to define the total entropy of the universe it follows that the 2nd law of thermodynamics does't apply in cosmology.
It can be said that since we can't get out of the system called "universe", there is no easy way to define the universe energy unless we introduce a time-symmetry (Noether theorem). But that is a feature of the way we define energy. It doesn't have anything to do with GR.

 

[WannabeNewton]

\bigtriangledown _{\nu }T^{\mu \nu } = 0 is only local though so how could it justify global energy conservation under the friedmann metric which doesn't have \frac{\partial }{\partial t} as a killing field?

 

[pervect]

Where are the relativity FAQ's located? I thought the should be at https://www.physicsforums.com/forumdisplay.php?f=210 , but I don't see the FAQ in question (on energy conservation in GR)
there.

I'll defer detailed comments until I re-read the FAQ. I'm pretty sure I've read it before,and I doubt there is anything that a minor tweak wouldn't fix, but I'd like to re-read it again more carefully.

 

[WannabeNewton]

Where are the relativity FAQ's located? I thought the should be at https://www.physicsforums.com/forumdisplay.php?f=210 , but I don't see the FAQ in question (on energy conservation in GR)
there.

I'll defer detailed comments until I re-read the FAQ. I'm pretty sure I've read it before,and I doubt there is anything that a minor tweak wouldn't fix, but I'd like to re-read it again more carefully.

https://www.physicsforums.com/showthread.php?t=506985

 

[TrickyDicky]

\bigtriangledown _{\nu }T^{\mu \nu } = 0 is only local though so how could it justify global energy conservation under the friedmann metric which doesn't have \frac{\partial }{\partial t} as a killing field?

That \bigtriangledown _{\nu }T^{\mu \nu } = 0 is only local is debatable, chiefly when locally means as big a system as one can define, surely it assures local conservation, and by being in covariant form it opens the way to global conservation by introduction of symmetries.
I'm not trying to justify anything, my point was that energy conservation applies to cosmology if we strictly use the GR equations without further assumptions.

 

[TrickyDicky]

Maybe it would be more correct if it specified that the lack of energy conservation is a feature of our current cosmological model rather than something intrinsic to the general relativity equations.

 

[pervect]

Let's compare it to the sci.physics.faq- which addresses a similar question.

In special cases, yes. In general — it depends on what you mean by "energy", and what you mean by "conserved".

In flat spacetime (the backdrop for special relativity) you can phrase energy conservation in two ways: as a differential equation, or as an equation involving integrals (gory details below). The two formulations are mathematically equivalent. But when you try to generalize this to curved spacetimes (the arena for general relativity) this equivalence breaks down. The differential form extends with nary a hiccup; not so the integral form.

I've always thought "It depends on what you mean by energy and what you mean by conserved" was a bit weasel-worded. The bit about the differential and integral formulations is very helpful to the advanced reader, but it's not so helpful to the less advanced reader.

Note that the sci.phsics FAQ is concerned with whether or not energy is conserved in general realtivity, while the FAQ in question is in the cosmology section and applies to cosmology.

While we might want to mention somewhere in the FAQ that we have definitions of energy that work in certain situations, it's accurate and IMO helpful to state right-up-front that none of these situations appear to apply to our universe - by which I mean the universe as a whole, not some infinitesimally small piece of it.

 

[WannabeNewton]

Maybe it would be more correct if it specified that the lack of energy conservation is a feature of our current cosmological model rather than something intrinsic to the general relativity equations.

Right ok, I think I see what you're getting at. So you're not disputing anything, your're just saying this isn't something that should be uniquely attributed to GR?

 

[TrickyDicky]

I really thing it would pay off to clarify that we know from 1917 thanks to Noether theorem that energy conservation is just the result of time symmetry, and therefore the only reason we state energy is not conserved in the universe as a whole is the introduction of time asymmetry by the FRW metric, not something in the EFE or due to the way we define the total energy of the universe.

 

[TrickyDicky]

Right ok, I think I see what you're getting at. So you're not disputing anything, your're just saying this isn't something that should be uniquely attributed to GR?

Yes, and also that the phrase "energy conservation doesn't apply to cosmology" is misleading and should be rephrased.

 

[Q-reeus]

Given the claim coe holds 'locally', are we talking about first or higher order curvature terms re 'global' departure? Is the lack of a gravitational contribution to the stress-energy tensor seen as the problem here? Shouldn't there be some relatively simple thought experiment capable of showing any violation if it exists? One would expect specific scenarios have been worked through over the last 95+ years, to leave no doubt! :zzz:

 

[Dale]

Shouldn't there be some relatively simple thought experiment capable of showing any violation if it exists? One would expect specific scenarios have been worked through over the last 95+ years, to leave no doubt! :zzz:

The CMBR is a good example that leaves no doubt. The CMBR photons that we receive now have lost a lot of energy from when they were last scattered.

 

[Q-reeus]

The CMBR is a good example that leaves no doubt. The CMBR photons that we receive now have lost a lot of energy from when they were last scattered.

No doubting the fact of enormous CMBR redshift and thus reduction in net radiant energy, but is that a sufficient proof? Why could that not be a case of energy transfer - from CMBR to expansion rate for instance (ie matter gets an added 'kick')? As many are aware the prevailing view amongst cosmologists is that the total energy of the universe (inclusive of radiation, matter, and 'dark energy') is an invariant zero; from beginning to end. As I recall it this assumption strictly relies on an overall zero 4-curvature (flat spacetime), which studies have apparently shown to either be true or a good approximation.
If on the other hand the claim of a global failure of coe is valid, this cannot magically only appear on the scale of the visible universe. One should be able to model some perfectly finite 'average' volume containing the requisite uniform distribution of radiation, matter, and maybe DE, and show that it's time evolution leads to an excess or deficit of total energy. Certainly not my province, but would be extremely surprised such has not been done. Anyone aware of such studies, and how that then tallies or not with the zero energy universe concept?

 

[Dale]

As many are aware the prevailing view amongst cosmologists is that the total energy of the universe (inclusive of radiation, matter, and 'dark energy') is an invariant zero; from beginning to end.

Do you have any reference for this? I was not aware that the FLRW spacetime admitted a globally conserved energy.

 

[TrickyDicky]

As many are aware the prevailing view amongst cosmologists is that the total energy of the universe (inclusive of radiation, matter, and 'dark energy') is an invariant zero; from beginning to end. As I recall it this assumption strictly relies on an overall zero 4-curvature (flat spacetime), which studies have apparently shown to either be true or a good approximation.

Hi, Q-reeus.
AFAIK, that is not the prevailing view among cosmologists, might you be referring to zero spatial 3-curvature? Zero 4-curvature usually refers to the SR model.

 

[Q-reeus]

Do you have any reference for this? I was not aware that the FLRW spacetime admitted a globally conserved energy.

Sur - here's one I had saved: http://arxiv.org/abs/astro-ph/0212574 I tend to look at the conclusions part.

 

[TrickyDicky]

Sur - here's one I had saved: http://arxiv.org/abs/astro-ph/0212574 I tend to look at the conclusions part.

Ok, I had never heard of this, I'll take a look at it. However I still wouldn't say this is the prevailing view.

 

[Q-reeus]

Hi, Q-reeus.
AFAIK, that is not the prevailing view among cosmologists, might you be referring to zero spatial 3-curvature? Zero 4-curvature usually refers to the SR model.

TrickyDicky - I may be taking a bit of liberty claiming it is for sure the majority view, but the flat spacetime bit is correct as the article linked in #16 explains. Lawrence Krauss makes a big thing of zero universe in an entertaining (~ 1 hr long) Youtube presentation:
http://www.freelecturevideos.com/ph...lawrence-krauss-aai-2009-video_243dcb1d9.html Here's another link re flat spacetime and zeu http://arxiv.org/abs/gr-qc/0605063

 

[TrickyDicky]

TrickyDicky - I may be taking a bit of liberty claiming it is for sure the majority view, but the flat spacetime bit is correct as the article linked in #16 explains. Lawrence Krauss makes a big thing of zero universe in an entertaining (~ 1 hr long) Youtube presentation:
http://www.freelecturevideos.com/ph...lawrence-krauss-aai-2009-video_243dcb1d9.html Here's another link re flat spacetime and zeu http://arxiv.org/abs/gr-qc/0605063

I just read the paper and find it very interesting. And it further supports my criticism of the energy conservation FAQ, I really think that as it stands it is not only misleading but wrong and should be modified.

 

[Dale]

Sur - here's one I had saved: http://arxiv.org/abs/astro-ph/0212574 I tend to look at the conclusions part.

Interesting. I need to read this in more detail.

 

[Q-reeus]

I just read the paper and find it very interesting. And it further supports my criticism of the energy conservation FAQ, I really think that as it stands it is not only misleading but wrong and should be modified.

In the article of that last link I gave, Berman in part 3 makes the point that working in the wrong coordinate system can introduce fictitious forces then wrongly interpreted as gravitational, and that this throws out the calculations. I'll leave it for you experts to debate!

 

[TrickyDicky]

Certainly it makes sense if you look at it like this: at every infinitesimal point (aka "locally") in the universe the energy is conserved (according to both SR and GR), and at the same time at every infinitesimal point the total energy is zero. Integrating for the total of points in the manifold volume it leads to consider that in the universe the energy is conserved and it totals zero energy.
This only seems to need for the Equivalence principle, SR, and the Bianchi identities to hold true in order to be a viable conclusion.

 

[Q-reeus]

...at every infinitesimal point (aka "locally") in the universe the energy is conserved (according to both SR and GR), and at the same time at every infinitesimal point the total energy is zero...

I take it you will agree this bit can only correspond to a perfectly homogenized universe. Obviously in our real 'lumpy' universe local energy content will have big ups and downs, but averaged out on the universal scale, corresponds to the homogenized 'point' result.

 

[TrickyDicky]

I take it you will agree this bit can only correspond to a perfectly homogenized universe. Obviously in our real 'lumpy' universe local energy content will have big ups and downs, but averaged out on the universal scale, corresponds to the homogenized 'point' result.

Those big ups and downs always involve finite dimensions.
A different story is if you want to include singularity points, but since singularities (i.e. BB at t=0 or BH singularities) by definition are out of the realm of known physics and I don't want to speculate I left them out.

 

[Q-reeus]

Those big ups and downs always involve finite dimensions.
A different story is if you want to include singularity points, but since singularities (i.e. BB at t=0 or BH singularities) by definition are out of the realm of known physics and I don't want to speculate I left them out.

Ahh, my bad - was interpreting that previously quoted bit to mean zero energy density rather than just energy (ie you were just saying infinitesimal volume -> zero net energy in the limit as v -> 0). OK so you were actually making a connection argument there, not that energy is everywhere zero just because it is overall. Should have known better.

 

[TrickyDicky]

Ahh, my bad - was interpreting that previously quoted bit to mean zero energy density rather than just energy (ie you were just saying infinitesimal volume -> zero net energy in the limit as v -> 0). OK so you were actually making a connection argument there, not that energy is everywhere zero just because it is overall. Should have known better.

No sweat. I posted my connection argument so it can be torn apart anyway

 

[pervect]

Lets say you have a perfect fluid, and you try to define the energy of the fluid as the integral of T_00 in the rest frame of the fluid.

In curved space-time, it's my understanding that you can have \nabla \dot T_{ij} = 0[/itex], and have the above energy vary with time. For instance, you could have X number of watts per m^3. If you take the limit to zero volume, the "energy" creation rate is zero. But for any finite volume, the energy creation rate is finite.

Example: in a FRW space-time, there's a natural background frame for the "cosmological fluid" as defined above. But you'll find that the integral of T_00 in this frame is NOT constant with time, even though the FRW solution obeys Einstein's eqauations. If I get more time, I'll post the relevant section from MTW about this.

 

[PAllen]

The following reviews the 'mainstream' range of opinion on bridging the gap from local, differential statements (where strong conclusions are possible) to statements at infinity in asymptotically flat spacetime (strong statements possible). The focus is on definition and properties of so called quasi-local energy, momentum, and angular momentum. Not much is said about properties at infinity for non- asymptotically flat spacetimes.

http://relativity.livingreviews.org/Articles/lrr-2009-4/

 

[Dale]

This is an interesting take.

The authors are specifically considering universes where the energy becomes infinitely dilute, i.e. approaching Minkowski metric for sufficiently large time. I wonder if this is, in a sense, an asymptotically flat spacetime where energy is indeed conserved. I never though of going asymptotically in the time dimension, but it makes a certain amount of intuitive sense to me.

Of course, my intuition has been wrong in GR many times

 

[Q-reeus]

Wow - very first sentence in the Introduction to http://relativity.livingreviews.org/Articles/lrr-2009-4/ (link given by PAllen in #28) is a bit of a bombshell: "Over the last 30 years, one of the greatest achievements in classical general relativity has certainly been the proof of the positivity of the total gravitational energy, both at spatial and null infinity."
Right, well if so then that throws into limbo the apparently secure findings I linked to in #16 & #18 re invariant zero energy universe. Sure as hell there's no way to add up a net positive gravitational energy and a necessarily positive matter+radiation+DE and get zero! So one school of thought is dead wrong here, and I'm not game to put money on either. What that review article makes clear is that after nearly 100 years of intensive effort, important things are still not clear. I depart this thread!