B Was the Big Bang a quantum mechanical vacuum fluctuation?

[nikkkom]

We need dark energy to make GR work for current expansion model, dark energy is theorized but never proven to actually exist. Thats why there are many alternative theories with and without dark energy.

Dark matter is also needed to explain observed galaxies rotation speed that do not match what is predicted by GR, dark matter also has never been observed.

That does not mean GR is wrong, it means that it is not the final ultimate theory.

No. THESE particular issues do not disqualify GR from being completely correct and consistent. Dark energy is not a "bolted-on" crutch in GR, it is mathematically consistent with GR (and actually rather simple). Same with dark matter.

There are issues which disqualify GR, but they are completely different (classical theories are fundamentally not compatible with quantum physics, so we need some sort of quantum gravity theory).

 

[Deepblu]

What

No. THESE particular issues do not disqualify GR from being completely correct and consistent. Dark energy is not a "bolted-on" crutch in GR, it is mathematically consistent with GR (and actually rather simple). Same with dark matter.

There are issues which disqualify GR, but they are completely different (classical theories are fundamentally not compatible with quantum physics, so we need some sort of quantum gravity theory).

Again I never said something about disqualifying GR nor I mentioned anything about math inconsistency!

My point one more time: GR is not the full picture

 

[Deepblu]

Lets go back to conservation of energy:

No conservation of energy not only means energy can be destroyed, but also means energy can be created from nothing.

 

[PeroK]

My point one more time: GR is not the full picture

What is?

 

[Deepblu]

What is?

I believe a Quantum theory of gravity will give us a more complete picture.

 

[PeroK]

Lets go back to conservation of energy:

No conservation of energy not only means energy can be destroyed, but also means energy can be created from nothing.

In order to believe that fundamentally conservation of energy is an absolute law of physics, you need a full understanding of energy and a watertight physical model that shows why it is conserved.

For example, pre Einstein you could have said the same about mass. Remember that? Mass can be neither created or destroyed. How can mass be created out of nothing?

But, that turned out not to be a law of physics. As a result of SR, which certainly wasn't the full picture.

Then GR came along and conservation of energy was modified as a law of physics.

QM also shook previously accepted laws.

Interestingly, you seem to be willing advances in physics not to further modify what we know but to return a pre-GR status quo.

Who knows what golden rule Quantum Gravity might revise! Personally, I think it's a forlorn hope that it will restore a universal conservation of energy.

 

[weirdoguy]

Well, probably quantum gravity will not change that our Universe is not symmetric under time translations... @Deepblu you are familiar with the definition of energy and its connection with Noether theorem, aren't you?

 

[Deepblu]

Btw no conservation of energy in GR is a subject that is open to debate. See this paper:
[vixra link removed by moderator]

 

[weirdoguy]

vixra.org is NOT a reliable source. Instead of arguing with specialists and physicists you should rather think why they say what they say.

 

[Deepblu]

vixra.org is NOT a reliable source.

I expected that answer :)

Here is another paper from arxiv.org:
https://arxiv.org/abs/physics/0407077

 

[Deepblu]

I have question:

Is information conserved on universal scale?

 

[nikkkom]

>> No. THESE particular issues do not disqualify GR from being completely correct and consistent. Dark energy is not a "bolted-on" crutch in GR, it is mathematically consistent with GR (and actually rather simple). Same with dark matter.

>> There are issues which disqualify GR, but they are completely different (classical theories are fundamentally not compatible with quantum physics, so we need some sort of quantum gravity theory).

Again I never said something about disqualifying GR nor I mentioned anything about math inconsistency!

My point one more time: GR is not the full picture

I'm starting to think that discussion with you is ... unproductive. Last try. If you continue acting up, I'll stop replying. I have better things to do with my time.

You did say that GR has problems with dark energy and dark matter. Here, your words verbatim:

"""
We need dark energy to make GR work for current expansion model, dark energy is theorized but never proven to actually exist. Thats why there are many alternative theories with and without dark energy.
Dark matter is also needed to explain observed galaxies rotation speed that do not match what is predicted by GR, dark matter also has never been observed.
That does not mean GR is wrong, it means that it is not the final ultimate theory.
"""

The word "that" in last sentence refers to dark energy and dark matter "problems" allegedly plaguing GR. They aren't, so my reply was stating they are not. Now you are denying you said they are. What the actual F is going on?

 

[bhobba]

Why you do not?

See:
https://arstechnica.com/science/201...-the-course-of-physics-but-couldnt-get-a-job/

Why it is usually conserved and the situations when its not was worked out by the great Emmy Noether.

There are various ways of having it conserved in those unusual situations by a suitable choice of the definition of energy - but we do not have agreement on those. We had a recent post from a research scientist on a slight generalization of Noether he thinks resolves it - but the whole thing is still a bit controversial and debatable:
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

Thanks
Bill

 

[Deepblu]

The word "that" in last sentence refers to dark energy and dark matter "problems" allegedly plaguing GR. They aren't, so my reply was stating they are not. Now you are denying you said they are. What the actual F is going on?

Frst, I am sorry for the confusion I caused in my replies.. maybe I was not able to deliver what I mean.
Second, have patience with me, I am trying to understand.. arguing is not my purpose.

What I meant in my last reply is that I know that dark energy and dark matter are mathematically consistent with GR, but GR will give wrong results without them.

So my final question to you is: let's say that scientists suddenly announced that Dark matter does not exist, how we can explain galaxies rotation speed with GR after that?

 

[nikkkom]

So my final question to you is: let's say that scientists suddenly announced that Dark matter does not exist, how we can explain galaxies rotation speed with GR after that?

Rigorously proving negative is impossible. For example, sterile neutrinos, if they exist, are expected to interact with matter many orders of magnitude weaker than ordinary ones.

Therefore there will be no "sudden announcement" that DM does not exist.

The experiments will put more and more stringent limits on DM interaction cross-section (and other things, such as annihilation gamma-rays, if DM annihilates), while theorists, as always, will seek ways to explain observations by various new theories. Then either DM will be detected, or a new theory will explain observations without needing DM, and DM detection experiments will eventually fall out of favor.

 

[bhobba]

Btw no conservation of energy in GR is a subject that is open to debate. See this paper:
[vixra link removed by moderator]

I don't think that no conservation of energy in under debate. In GR we do not have the defining condition on what Energy is ie the conserved quantity associated with time translation invariance - since GR is space-time curvature the definition breaks down. But that does not mean anyone wants to abandon conservation of energy - simply modify the definition in such a way it exists and is conserved in GR. There are a number of ways of doing it - the debate is which one to choose. Logically you could take the view its simply not conserved or even defined in GR but I haven't come across anyone that really wants to do that - for obvious reasons - it has proven a very useful law.

Thanks
Bill

 

[Deepblu]

Therefore there will be no "sudden announcement" that DM does not exist.

There are some voices saying that Dark matter does not exits, like in Emergent Gravity Theory by Erik Verlinde:
https://arxiv.org/abs/1611.02269

In this lecture he states that there is no Dark matter but only dark energy:


So Dark Matter is still a subject of research.

 

[Deepblu]

I don't think that no conservation of energy in under debate. In GR we do not have the defining condition on what Energy is ie the conserved quantity associated with time translation invariance - since GR is space-time curvature the definition breaks down. But that does not mean anyone wants to abandon conservation of energy - simply modify the definition in such a way it exists and is conserved in GR. There are a number of ways of doing it - the debate is which one to choose. Logically you could take the view its simply not conserved or even defined in GR but I haven't come across anyone that really wants to do that - for obvious reasons - it has proven a very useful law.

Thanks
Bill

My problem of not digesting the concept that "energy is not conserved", is the idea that energy can be created from nothing. For example if we live in a contracting universe rather than expanding, then we will see an opposite effect and it will appear than energy is created from nothing.

My second problem is that this will lead to no conservation of information (I may have misconception here).

 

[bhobba]

I believe a Quantum theory of gravity will give us a more complete picture.

Well first you have to understand what the issue with gravity is. We have a perfectly good quantum theory of gravity valid to about the Plank scale:
https://arxiv.org/abs/1209.3511

These days since the work of the great Ken Wilson, who even other well respected physicists like Sidney Coleman stood in awe of - he was that good - we now think of all our theories as effective to some some scale beyond which other theories take over. We already know one famous case - at higher energy scales QED becomes the electroweak theory. I don't think any physicist believes the standard model is valid at the Plank scale. So really the issue isn't quantum gravity - its the laws at that damnable Plank scale. It was thought string theory may resolve it - but it has morphed a bit:
https://www.ias.edu/news/cole-stringtheory-quanta

Getting back to the title of the thread - yes it is possible - but nobody knows. There are all sorts of ideas about eg eternal inflation and a Google search will bring back all sorts of other weird ideas.

Thanks
Bill

 

[bhobba]

My problem of not digesting the concept that "energy is not conserved", is the idea that energy can be created from nothing.

The modern definition of energy is as I stated - its from Noether so is automatically conserved. But in GR the defining conditions of that definition break down. So all bets are off. For example as the link about Noether explained in GR you can have 'that an object could speed up as it lost energy by emitting gravity waves, whereas clearly it should slow down.'. We can define energy in a way its still conserved in GR but not in an intuitive way. In other words it may seem like its coming from nothing - but really it isn't - it's just we are getting into regimes where energy is not what we usually think it is. BTW as far as energy goes the universe from nothing idea includes the concept of the universe having 0 energy:
https://en.wikipedia.org/wiki/Zero-energy_universe

Thanks
Bill

 

[mitchell porter]

Deepblu has brought up three heterodox ideas, I thought I would mention my own responses.

First, what if there's no dark matter? I certainly think the success of MOND, a type of modified gravity, is very important; but it is not relativistic. There is a kind of superfluid dark matter theory, due to Khoury and Berezhiani, which can reproduce the features of MOND. One might also look for MOND to be a quantum gravity effect, perhaps involving extra degrees of freedom beyond the classical metric.

Second, what if there's no dark energy? This is more problematic in that QFT has vacuum energy and you expect vacuum energy to gravitate, so you actually expect there to be something like dark energy. Although then one has the problem that the expected magnitude of dark energy is vastly greater than what is actually observed. The anthropic answer is that there are positive and negative contributions to vacuum energy, and they happen to almost cancel because if they didn't, there wouldn't be galaxies, atoms, or life. More interesting is the idea that the QFT vacuum energy does cancel or almost cancel due to some symmetry, like a crypto supersymmetry. If it's only an almost cancellation, the observed dark energy can then be the residual vacuum energy. If the cancellation is exact, then the accelerating expansion has to come from somewhere else, such as a quintessence field.

Third is the heterodox idea that interests me the most precisely because I haven't thought about it: what if energy is conserved after all, in the true theory of quantum gravity? I am used to the usual line of thought, which is that you can get local conservation of energy in GR if you use pseudotensors. But how does the issue look in quantum gravity? This seems to be very little discussed. First of all, Noether's theorem already works a little differently in quantum field theory, compared to classical physics, because of the peculiarities of the quantum framework. And here is a perspective that is new for me: energy conservation is due to time translation, but the Hamiltonian of quantum gravity has no time evolution! How does that affect the attempt to reason about energy conservation? I have no idea.

 

[nikkkom]

"what if there's no dark matter?" and "what if there's no dark energy?" are not invalid, or suppressed questions, as it is sometimes portrayed. People are looking at all possible ways to develop new theories and explain observations.

 

[bhobba]

This is more problematic in that QFT has vacuum energy and you expect vacuum energy to gravitatee

This is a big issue - you know what the vacuum energy really is - a big fat zero by a process called normal ordering. That it is not zero comes from popularizations - but your post suggests you are beyond that so I am surprised you do not know it. The only popularizer that seems to discuss this and other issues correctly is Penrose - but he includes that dreaded thing - equations. I am reading one of his books right now and he explains the 1+2+3+... = -1/12 counter intuitive equation by its real reason - analytic continuation and explains correctly why that is - as well as its issues. He also explains the Casmir effect correctly. I know to explain this stuff to lay audiences is a big issue and things of dubious correctness will get through popularizations.. But Penrose shows it can be reduced considerably.

Please, please take with a grain of salt what people say in popularizations - with the possible exception of Penrose.

Again - I think you are beyond popularizations - the above was not meant for people that know the technicalities. Were you aware of normal ordering? I know some QFT books don't mention it and just say its infinity so you need to by definition set it to zero - to which I go groan - its like some textbook explanations of re-normalization I have read. I just want to cry. Then you read the BPH method and say - why did you bother with the other rot - that makes perfect sense.

Thanks
Bill

 

[haushofer]

I haven't read the whole topic, just wanted to point out a beautiful introduction paper on quantum cosmology by Atkatz called "Quantum cosmology for pedestrians". PDF can be obtained e.g. here,

https://www.researchgate.net/publication/243491493_Quantum_Cosmology_for_Pedestrians

To me it's the simplest introduction to what it means for a universe to "tunnel into existence".

Others have responded to Deepblu after his response to my post, so I won't get into energy conservation in GR.

 

[mitchell porter]

Bill - I don't know what Penrose says, but the usual lore is that normal ordering becomes problematic once you are in curved space, because the difference between negative and positive frequency (and thus between annihilation and creation) is now frame-dependent. Also that in nongravitational QFT, you only care about energy differences; but gravity couples to the absolute value of energy, and vacuum energy functions as a cosmological constant.

 

[bhobba]

Bill - I don't know what Penrose says, but the usual lore is that normal ordering becomes problematic once you are in curved space,

Thanks for the post.

Penrose doesn't mention it ie the issue with gravity and normal ordering. Normal ordering is what I have gleaned from studying QFT. I have studied a few books - the ones I mentioned that I was not happy with plus some I am eg:
https://www.amazon.com/dp/019969933X/?tag=pfamazon01-20
https://www.amazon.com/dp/0984513957/?tag=pfamazon01-20

I wanted to move onto Weinberg, I have the books, but wanted a bridge, so I got Zee. I found some parts good other parts - well not so good - but I felt at the end of the day it wasn't the book to get me to Weinberg. I now have - Srednicki - which a regular poster/science adviser here (Vanhees) who teaches it thinks could be a good bridge. Like all things - especially as you get older - it just takes time.

I wasn't aware of the issue you mentioned with GR, but my view of Quantum Gravity is a little unusual as you may have gleaned from another post I made in this thread. Need to look into what you said - sounds both complicating and interesting. Another issue with vacuum energy is they often do an arbitrary cutoff at the plank scale. Yes - I know effective field theory and all that makes it a reasonable thing - but does it really resolve the issue. For me normal ordering was the only one that did.

Thanks
Bill