Why all these prejudices against a constant? ( dark energy is a fake probem)

[mitchell porter]

It is not that we have a theory that gives a wrong prediction. We can make our theories give the right value.

i.e. You can cancel the QFT vacuum energy, and account for the observed dark energy, by supposing that the cosmological constant = "dark energy - QFT vacuum energy".

But doesn't the QFT vacuum energy depend on the high-energy cutoff? (except when it's always exactly zero at all scales). In which case, the value of the cosmological constant required by the strategy above, will depend on the cutoff.

I can see two ways around this.

First, you say that there is an objective cutoff, due to new physics. This approach has two further subdivisions, a philosophical approach and a concrete approach.

The philosophical approach applies when you don't know what this objective cutoff is, or what the objective vacuum energy is, so you can't say what the actual value of your finetuned cosmological constant is supposed to be; but you just suppose that its value is such as to cancel whatever the objective vacuum energy is.

The concrete approach would apply if you had a theory which intrinsically exhibits a concrete cutoff, e.g. an energy above which ordinary QFT no longer applies. This implies that you have a quantitative framework in which there is a known objective vacuum energy, and in which you can visibly finetune the cosmological constant to a specific value in order to cancel the objective vacuum energy.

The other primary option would be to work with renormalization somehow. In other words, the vacuum energy is treated as "infinity", the cosmological constant as "finite constant - infinity", and all calculations are performed in a framework where you always actually use a cutoff (and get a resulting dark energy equal to "finite constant"), but this is also a framework where you can show mathematically that the cancellation works at any energy scale.

This "renormalization approach" is sort of halfway between what I called, above, the philosophical approach and the concrete approach. And as I understand it, it resembles how the vacuum energy cancellations for exact supersymmetry work, except that there's no nonzero finite constant left over.

I think AdS/CFT must provide examples of a framework in which the "renormalization approach" applies, because in any given instance of the duality, the bulk space (the AdS space) has a known, nonarbitrary, nonzero cosmological constant, and yet everything fits into the framework of QFT (on the CFT side of the duality). So it would be of interest to understand how AdS/CFT deals with vacuum energy in the bulk, on the way to obtaining a negative cosmological constant.

edit: See http://arxiv.org/abs/1106.3556" ).

 

[Fra]

But doesn't the QFT vacuum energy depend on the high-energy cutoff? (except when it's always exactly zero at all scales). In which case, the value of the cosmological constant required by the strategy above, will depend on the cutoff.

I can see two ways around this.

If the cutoff is physically motivated, rather than just seen as an arbitrary energy scale, maybe no way around it is needed.

All we need to answer to our earthbased inferences. And I think it's quite fair to say that any Earth based observation is necessarily cutoff, at least by the order of ~ m_{earth} c^2. We've have to consume the Earth to even make sense out of higher energy probing - which brings me to the point - apart from the issue of BH formation, we could have to use all our own memory and information in that measurement, but then there is no place to encode the feedback. So there has to be a balance.

This may seem like a silly point, but who knows if an "electron would agree on the size of lambda"? I am not so sure about that. Maybe the fact that two observing systmems does NOT agree upon lambda, explains some other interactions taking place in theory space (which then also have a cutoff). Ie. ANY theory has a cutoff originating from the complexity of the encoding system.

Isn't that a third option here? The only tihnk I can imagine is that this _might_ fit vaugely into some of the string dualities and landscape evolutions?

/Fredrik

 

[marcus]

Let's be clear as to the main issues in this thread. Here is how the Bianchi Rovelli paper lays them out in introduction:

==quote 1002.3966 page 1==
...What we say here does not mean that there is no interest in exploring theoretical explanations of the acceleration alternative to the ΛCDM model. Good science demands us to be a priori skeptical of any theory, even when it works well, and always explore alternatives. Even less are our observations criticisms to the observational work aiming at testing the ΛCDM scenario. Exploring alternative theoretical explanations, and pushing the empirical test of all the theories we have, is obviously good science.

But what we say does mean that it is misleading to talk about “a mystery” (not to mention “the greatest mystery of humanity”), for a phenomenon that has a current simple and well-understood explanation within current physical theories. It is especially wrong to talk about a mysterious “substance” to denote dark energy. The expression “substance” is inappropriate and misleading. It is like saying that the centrifugal force that pushes out from a merry-go-round is the “effect of a mysterious substance”.

There are three stories (of very different kind) that are routinely told in presenting the difficulties of the cosmological constant scenario. These are:

i. The alleged historical rejection of the cosmological constant by Einstein, and then by the general-relativity community.

ii. The coincidence problem.

iii. The enormous difference between the small value of the cosmological constant revealed by the cosmic acceleration and the large value that can be derived from quantum field theory.

We believe that there is confusion, either historical or conceptual, in each one of these three stories, as commonly presented, and we discuss them below.

There is probably nothing very original in this note. The points we make here can be heard in discussions among physicists. However, for some reason they do not have much space in the dark-energy literature. We though it appropriate to make them available in writing.

==endquote==

I think we are mainly concerned with point iii here. A person steeped in QFT viewpoint may view Lambda as a classical fudge or lifeline, to correct for the stupendous ZPE calculated from non-QGR-based QFT.
That is he may think of the embarrassing 120-order-of-magnitude QFT vacuum energy discrepancy as in some sense "correct" but just needing to be "canceled" by some Lambda lifepreserver that the other people are responsible for.

As this points out there is another possible perspective on the embarrassing QFT discrepancy. That is: it is a QFT problem---probably showing that QFT needs some foundational work. One might for example speculate that the embarrassing vacuum energy might go away if QFT would simply stop using Minkowski geometry, and ground itself in quantum relativistic geometry.

Be that as it may, I think it would be a good idea if people who want to discuss in this thread would simply READ the relevant section of the paper.
http://arxiv.org/pdf/1002.3966
It starts on page 5. The relevant section is:
IV. THE VACUUM ENERGY IN QUANTUM FIELD THEORY

Perhaps it would help focus discussion if I were to paste some excerpts in. Then those who have read section IV (relevant to our discussion) could refer to some immediately visible text.

 

[Fra]

Be that as it may, I think it would be a good idea if people who want to discuss in this thread would simply READ the relevant section of the paper.
http://arxiv.org/pdf/1002.3966
It starts on page 5. The relevant section is:
IV. THE VACUUM ENERGY IN QUANTUM FIELD THEORY

Perhaps it would help focus discussion if I were to paste some excerpts in. Then those who have read section IV (relevant to our discussion) could refer to some immediately visible text.

FWIW, here is my as always oddball perspective:

I did read the paper and some key questions is where the raise this:

Does this large energy exist for real? That is, does it have observable eects? In particular: does it act as a source for the gravitational eld, as all forms of energy are known to do? Does it have a gravitational mass (and therefore an inertial mass)?
...
In fact, simple physical arguments indicate that the vacuum energy, by itself, cannot be \real" in the sense of gravitating: if it did, any empty box containing a quantum eld would have a huge mass, and we could not move it with a force, since gravitational mass is also inertial mass.
...
On physical grounds, vacuum energy does not gravitate. A shift in vacuum energy does gravitate.

I see a confusion and/or debate about the "nature" of the zero point energy.
Ie. is it a property of the observED system or a property of the observING system?

The confusion is there only if you think that the zero point energy is a proprety of the observED system. But this is IMO the same mistake as when people thinkg that the collapse fundamentally applies to the observED system rather than to the observING systems state.

If one for a second acknowledges that zero point energy is merely an EXPECTATION the observING system has on the observED system, it seems reasonable to attribute that huge about of INFORMATION (read the large zero point energy) to the environtment of hte observED system, not the system itsel, becaues that's where it's ENCODED, and it's this CODE that has inertia (at least in my addmittedly odd view).

Ie. it's the INTERTIA of the environment that should be HUGE - and indeed it IS! In fact in the observable = scattering matrix picture it's infinite. But my point always was this makes no sense, unless you actually HAVE infinitely massive observers; which you don't.

So I think the observable effects of this would - in principle that is (it's not yet worked out) - is not huge inertia of the empty box of quantum fields; it's the inertia of the observING systems that encodfes the expectations; and in principle I'm sure TWO such internacting observING systems should exhibit a gravitational attraction. Or at least that is the conceptually the principle idea behind the interacting observers - the INERTIA is attributed to the inertia of information updates! and these are not attributes of the observed systems, but rather of observers.

So the universality of "gravity" relies in my hypothesis that ANY two communicating information processing systems, are facing an attracing in terms of a drive to decrease their information divergence. (The technical challanges is still certainly to work this out; in particular to work out explicittly the known 4D metrics and their dynamical equations from the deeper more abstraction information measures)

In particular does it make no sense to consider gravitationa between two infinitely massive systems. This is IMO the main conceptual reason why I think that QFT as the theory of inference as it stands will never quite merge with gravity without ending up with other pathologies.Because QFT, as it's constructued relies on an infinitely massive observer. This is of course realted to, but a distinct point, to the asymptotical backgrounds.

All this would mean "foundational rework" of QFT indeed. But it would ALSO mean fundamental rework of gravity. Classical GR actions or classical geometry can't be in the starting points.

In particular does these ideas mean that I think the IDEA that the effective theory bound by some energy, is constructed by AVERAGING or integrating out the high energy modes are flawed logic as I see it.

/Fredrik

 

[Haelfix]

i.e. You can cancel the QFT vacuum energy, and account for the observed dark energy, by supposing that the cosmological constant = "dark energy - QFT vacuum energy".

But doesn't the QFT vacuum energy depend on the high-energy cutoff? (except when it's always exactly zero at all scales). In which case, the value of the cosmological constant required by the strategy above, will depend on the cutoff.

Sure. In fact it will go like O(M^4) + O (M^2 Me ^2) + ... Where M is the cutoff and Me is the mass of some fermion. Simply taking M -- > infinity, yields a divergent answer.

Of course in the language of effective field theory, we assume that there is new physics and thus a physical cutoff, where the new physics enters to soften the divergence. This is probably up at the GUT scale or Planck scale, but for illustrative purposes, we simply take it to be the absolute minimum that is consistent with experiment. To wit, the electroweak scale. (Incidentally, the classical contribution to the cosmological constant receives heavy contributions here due to SSB, quark condensates and the like)

Again, this is a regime where the standard model + GR works to fantastic accuracy. And since the problem is already acute it serves to make the point.

In short, properly understood, the cosmological constant problem is essentially an *infrared* problem, not an ultraviolet one. It is another example of a hierarchy problem in physics, except this time the relevant scales are the difference in size between the Hubble scale and particle physics (as opposed to particle physics and the Planck scale).

Asking the question in AdS/CFT is interesting, and trying to tame the problem by trying to soften the scaling into the renormalization group is definitely one of the popular methods that people have tried, however I think the current feeling is that the solution probably won't be found in quantum gravity, but rather is cosmological in origin. Also the renormalization group ideas are a little adhoc and typically reintroduce finetuning elsewhere (Weinberg mentions several such ideas in his review)..

 

[smoit]

In short, properly understood, the cosmological constant problem is essentially an *infrared* problem, not an ultraviolet one. It is another example of a hierarchy problem in physics, except this time the relevant scales are the difference in size between the Hubble scale and particle physics (as opposed to particle physics and the Planck scale)...

I'd say that the CC problem is definitely a UV problem because we are dealing with a highly relevant operator so you cannot ignore all the extra degrees of freedom arising in the UV. Curiously, in the SO(16)XSO(16) heterotic string with broken SUSY (non-tachyonic) and an infinite tower of stringy states contributing, one gets a finite answer for the CC but unlike the N=1 D=4 SUGRA, where the first non-vanishing term is quadratic in the cutoff, the first non-vanishing contribution to the 10D vacuum energy comes at order Str(M⁸_SUSY)M²_string, where M_SUSY is the scale of SUSY breaking.
My personal hunch is that to compute the CC, at least the quantum piece, one needs to figure out the string spectrum, which at low energies would reproduce some effective N=1 D=4 SUGRA with spontaneously broken SUSY (e.g. by some F-term), and then just compute the one-loop partition function using that string spectrum. I bet that the naive supergravity result would no longer hold and that the supertrace would also vanish at some higher order like in the example I highlighted here.

 

[marcus]

To remind folks of the logical context in which the discussion here takes place.

...

iii. The enormous difference between the small value of the cosmological constant revealed by the cosmic acceleration and the large value that can be derived from quantum field theory.
​

I think we are mainly concerned with point iii here. A person steeped in QFT viewpoint may view Lambda as a classical fudge or lifeline, to correct for the stupendous ZPE calculated from non-QGR-based QFT.
That is he may think of the embarrassing 120-order-of-magnitude QFT vacuum energy discrepancy as in some sense "correct" but just needing to be "canceled" by some Lambda lifepreserver that the other people are responsible for.

As this points out, there is another possible perspective on the embarrassing QFT discrepancy. That is: it is a QFT problem---probably showing that QFT needs some foundational work. One might for example speculate that the embarrassing vacuum energy might go away if QFT would simply stop using Minkowski geometry, and ground itself in quantum relativistic geometry.

Be that as it may, I think it would be a good idea if people who want to discuss in this thread would simply READ the relevant section of the paper.
http://arxiv.org/pdf/1002.3966
It starts on page 5. The relevant section is:
IV. THE VACUUM ENERGY IN QUANTUM FIELD THEORY

Perhaps it would help focus discussion if I were to paste some excerpts in. Then those who have read section IV (relevant to our discussion) could refer to some immediately visible text.

For starters here's a clarifying passage from page 6.
==1002.3966==
But what has all this to do with the question whether in (very) low-energy physics the physical value of the cosmological constant is zero or is small?

The question of whether or not there is a cosmological term λ in the low-energy classical Einstein equations, is independent from the question of what is the mechanism that protects this term (zero or small) from being scaled-up to a high scale by radiative corrections. The first question pertains to low-energy gravitational physics; the second pertains to high-energy particle physics. The two are independent in the sense that the second question exists independently from the answer to the first. The first has been already answered by observation, as it should: the cosmological term in the Einstein equations does not vanish. The second is open, and has not been changed much by the observations that λ ≠0. It is just one of the numerous open problems in high-energy physics.

We think that the origin of the confusion is that there are two distinct ways of viewing the cosmological term in the action. The first is to assume that this term is nothing else than the effect of the quantum fluctuations of the vacuum. Namely that λ = 0 in (21) and the observed acceleration is entirely due to the radiative corrections Λ (in the above notation). The second view is that there is a term λ in the bare gravitational lagrangian, which might (or might not) be renormalized by radiative corrections. The two points of view are physically different. We think that the common emphasis on the first point of view is wrong.

In other words, it is a mistake to identify the cosmological constant λ with the zero point energy Λ of a QFT, for the same reason one should not a priori identify the charge of the electron with its radiative corrections.
===endquote===

Anytime anyone wants to pull up a PDF of the Bianchi Rovelli article, just google "constant prejudices" ---it is what the article is about and what it is critical of. You will get the arxiv link on first or second hit.

 

[fzero]

We think that the origin of the confusion is that there are two distinct ways of viewing the cosmological term in the action. The first is to assume that this term is nothing else than the effect of the quantum fluctuations of the vacuum. Namely that λ = 0 in (21) and the observed acceleration is entirely due to the radiative corrections Λ (in the above notation). The second view is that there is a term λ in the bare gravitational lagrangian, which might (or might not) be renormalized by radiative corrections. The two points of view are physically different. We think that the common emphasis on the first point of view is wrong.

In other words, it is a mistake to identify the cosmological constant λ with the zero point energy Λ of a QFT, for the same reason one should not a priori identify the charge of the electron with its radiative corrections.

From this quote, this seems to me to be very much a strawman argument. As Haelfix has already said in an informed post:

Now the separate confusion is that there is absolutely no problem whatsoever in moving the cosmological constant term from the left side to the right side of the Einstein field equations in general. You can always do that!

That does not change the predictions or physics in any way, in particular whether the term is renormalized or not!

Specifically, the argument that a quantum field theorist who takes the Einstein theory seriously would treat the bare cosmological term as vanishing is incorrect. Rather, one would include the bare term and then field theory background and radiative corrections would lead to the renormalized cosmological term that is observed. The only reason that this is not done more often in practice is the perturbative nonrenormalizabilty of the Einstein theory, which makes such an exercise rather futile for most purposes. Doing QFT in a curved background will not make these problems go away. However, as far as the classical physics of the Einstein equation goes (observational cosmology is insensitive to quantum fluctuations around the vacuum), it makes no difference where the contributions to the cosmological term arise.

The real prejudice at work here is whether or not the Einstein equation should be considered as a fundamental part of the UV physics or whether it is an IR result derivable from more fundamental physics. In the former case, one obviously needs to include a bare cosmological term from the outset. In the latter case, it is not clear that such a bare term even has an objective meaning in the fundamental theory, so the cosmological term might be entirely due to radiative effects. In either case, the proper treatment of the cosmological constant is entirely dependent on the framework and any simplifying assumptions that are being made.

 

[marcus]

You apparently don't quite get it. H. remark is irrelevant to the argument. Of course you can move Lambda to the other side

The main thing is yes the GR equation is IR. B&R even say "(very) low energy". And QFT is completely out of there. They ought to solve their own preposterous ZPE problem. QFT cannot be considered fundamental because it is built on Minkowski space. Its ZPE arises in a complete other regime from Lambda. Read what Liberati et al has to say about the emergence of Lambda. I quoted some in post #34 https://www.physicsforums.com/showthread.php?p=3503823#post3503823 earlier in this thread.

Here, it will make it clearer if I quote some more Bianchi Rovelli, still page 6, continuing where I left off a couple of posts back:

In other words, it is a mistake to identify the cosmological constant λ with the zero point energy Λ of a QFT, for the same reason one should not a priori identify the charge of the electron with its radiative corrections.

If we get confused about this, we make a funny logical mistake. We have an observed physical phenomenon (the accelerated expansion). A simple physical theory explains it (general relativity with nonvanishing λ). However, particle physics claims that it can provide an independent understanding of the phenomenon (a cosmological term entirely produced by vacuum fluctuation). So we discard the simple explanation. But the new understanding goes wrong quantitatively (by 120 orders of magnitude). Now, every reasonable person would conclude that there is something missing in the particle-physics argument; especially knowing that the argument is already known to be wrong in flat space. But this is not the conclusion that is usually defended. Rather, it is claimed that what is unacceptable, and needs to be changed is the first simple explanation of the phenomenon!

There is no known natural way to derive the tiny cosmological constant that plays a role in cosmology from particle physics. And there is no understanding of why this constant is not renormalized to a high value. But this does not mean that there is something mysterious in the cosmological constant itself: it means that there is something we do not understand yet in particle physics. What could this be?​

 

[mitchell porter]

smoit #56, are you saying that for the 10-dimensional SO(16) x SO(16) heterotic string, you can get the observed cosmological constant by assuming a physically reasonable supersymmetry scale? If so, could you then look for a way to compactify six dimensions without adding to the vacuum energy?

 

[marcus]

smoit #56, are you saying that for the 10-dimensional SO(16) x SO(16) heterotic string, you can get the observed cosmological constant by assuming a physically reasonable supersymmetry scale? If so, could you then look for a way to compactify six dimensions without adding to the vacuum energy?

Heh heh, yes Smoit. Will you now explain the value of the observed cosmological constant by assuming a 10D string theory?

So far no one has responded to what I quoted from Liberati et al. He is a highly respected QG phenomenologist, not specifically associated with anyone approach Loop or other. I quoted from the FLS paper (Finazzi, Liberati, Sindoni) in post #34
https://www.physicsforums.com/showthread.php?p=3503823#post3503823
Anybody have any direct response to FLS points?

As a reminder, here are excerpts from their conclusions---please go back to #34 to see the full passage:

==quote FLS http://arxiv.org/abs/1103.4841 ==
...The implications for gravity are twofold. First, there could be no a priori reason why the cosmological constant should be computed as the zero-point energy of the system. More properly, its computation must inevitably pass through the derivation of Einstein equations emerging from the underlying microscopic system. ...

... In this respect, it is conceivable that the very notion of cosmological constant as a form of energy intrinsic to the vacuum is ultimately misleading. ... the reasoning of this Letter sheds a totally different light on the cosmological constant problem, turning it from a failure of effective field theory to a question about the emergence of the spacetime.
==endquote==

 

[Fra]

There is no known natural way to derive the tiny cosmological constant that plays a role in cosmology from particle physics
...
it means that there is something we do not understand yet in particle physics. What could this be?[/INDENT]

I share this stance.

I'll just want to add that just because there is something we do not yet understand about particles physics (which btw, I think is that QFT formalism simply isn't a ther cosmoloical measurement theory we need due to referencing infinitely massive observers) doesn't exclude there is ALSO sometihng we do not yet understand about gravity.

This seems very much rooted in the konwn issue of observables. I agree with Marcus that to expect a "QFT explanation" of cosmological expansion with QFT as it stands makes not sense IMO. All of QM/QFT is devised as a measurement theory - against a fixed context. This context is either classical reality, or some boundary at infinity where one collects S-matrix data. Of course in classical reality, the background metric is attached to the observer frame. The problem is that all of that makes sense only in special cases. Not in the most general QG domain since all the qualifiers break down.

I think the challange is to find the new framework that extends measurementtheory to cosmological scenarios; first THEN does it make snse to try to see how gravity fits in the corrected picture. This as I see it certainly must included serious reworking on QM&QFT foundations.

/Fredrik

 

[smoit]

smoit #56, are you saying that for the 10-dimensional SO(16) x SO(16) heterotic string, you can get the observed cosmological constant by assuming a physically reasonable supersymmetry scale? If so, could you then look for a way to compactify six dimensions without adding to the vacuum energy?

I don't know! Indeed, the value one gets is a mere coincidence since it's a value for the vacuum energy in 10D. Indeed, assuming M_SUSY~(TeV)~10^-15M_Plank and M_string~M_Planck one gets Str(M_SUSY⁸)M_string²~10^-120M_Planck¹⁰.

As I said before in #56, if you want to compute the CC in a realistic compactification you first need to compute the string spectrum in such a background, i.e. a background that reduces to some N=1 D=4 SUGRA with spontaneously broken SUSY, and then compute the partition function. Again, you'll have not only the zero modes (SUGRA modes) but also an infinite tower of stringy modes, both momentum and winding, plus an infinite tower of various KK modes all contributing to the CC. It would be interesting if one could do this even for a simple, say orbifold, compactification. All I was saying was that quoting the SUGRA result where the first non-vanishing supertrace contribution is quadratic in the cutoff means nothing as this is just a computation in an effective 4D QFT, which is missing an infinite number of contributions, which may alter the result completely. The point is that at such short distances the theory effectively becomes 10 dimensional and no longer just a QFT and the CC computation is UV-sensitive so I'm raising a speculation that this may ultimately address the perturbative quantum part of the problem. There may also be various non-perturbative contributions as well as tree-level pieces, and that's what makes the whole problem so tricky.

 

[smoit]

In other words, it is a mistake to identify the cosmological constant λ with the zero point energy Λ of a QFT, for the same reason one should not a priori identify the charge of the electron with its radiative corrections​

No sane particle theorist makes such an identification, Markus! Read the Polchinski reference and you'll see that nowhere does he identify the cosmological constant only with the zero point energy. On the contrary, as people have repeatedly said here, the CC receives all kinds of tree-level, perturbative and non-perturbative contributions and the observed tiny value includes of all of them.

 

[marcus]

Good, so you agree with Bianchi Rovelli on that point! You quoted part of what they said on page 6 although it looks like you attributed it to me.

They say it is a mistake to identify the cosmo constant with the QFT zero point energy, and you obviously agree since you claim that no sensible particle theorist would confuse the two.

So now we can go on to the next step in their argument, which continues on page 7. They start by pointing out that vacuum energy by itself does not gravitate, only shifts/differences do, not the zeropoint itself. We all know this--I'm sure you agree with the next passage, however simply for completeness I recap:

An effect commonly put forward to support the “reality” of such a vacuum energy is the Casimir effect. But the Casimir effect does not reveal the existence of a vacuum energy: it reveals the effect of a “change” in vacuum energy, and it says nothing about where the zero point value of this energy is. In fact, simple physical arguments indicate that the vacuum energy, by itself, cannot be “real” in the sense of gravitating: if it were, any empty box containing a quantum field would have a huge mass, and we could not move it with a force, since gravitational mass is also inertial mass. On physical grounds, vacuum energy does not gravitate. A shift in vacuum energy does gravitate. This is nicely illustrated by an example discussed by Polchinski in [3]:...​

There is the Polchinski reference you mentioned! I am glad to see you are reading ahead, Smoit. Now we come to the next step in their argument. Let's consider it together, maybe you will find a flaw and point it out to me. Now we are on page 7.

Why does standard QFT have so much trouble adjusting to this straightforward physical fact? We do not know the answer, but there is a general consideration that may be
relevant: in which theoretical context is formulated the argument for large radiative corrections to λ? If it is in a context in which we disregard gravity, then a large vacuum energy is physically irrelevant, because the λ term in the action (14) couples only to the gravitational field g, and is invisible if we disregard gravity. The next option is...

...But then there is a catch: if λ is different from zero, then (φ ,η) is not a solution...​

And so they go down the list of ways to address QFT's problem. Trying different theoretical contexts. This I think is the heart of their argument. See what you think.

 

[smoit]

They say it is a mistake to identify the cosmo constant with the QFT zero point energy, and you obviously agree since you claim that no sensible particle theorist would confuse the two.

What audience are they addressing in their paper? Undegraduates who just had a quantum mechaniscs class and learned about the zero-point energy? What is it that's new in their paper that particle theorists did not know already?

What I and many people have already said here is that apart from the perturbative piece, the CC contains several other types of contributions - tree-level and non-perturbative. Why is this so hard to grasp?

 

[smoit]

An effect commonly put forward to support the “reality” of such a vacuum energy is the Casimir effect. But the Casimir effect does not reveal the existence of a vacuum energy: it reveals the effect of a “change” in vacuum energy, and it says nothing about where the zero point value of this energy is. In fact, simple physical arguments indicate that the vacuum energy, by itself, cannot be “real” in the sense of gravitating: if it were, any empty box containing a quantum field would have a huge mass, and we could not move it with a force, since gravitational mass is also inertial mass. On physical grounds, vacuum energy does not gravitate. A shift in vacuum energy does gravitate. This is nicely illustrated by an example discussed by Polchinski in [3]:...​

This "simple argument" is obviously flawed since an empty box containing all quantum fields in our vacuum already contains ALL contributions - tree-level and quantum, which all add up to the tiny value. There is no experiment that I'm aware of where one can separate the total tree-level contribution to the CC from the total quantum contribution to the CC. Only if someone could magically switch off the tree-level piece and the mass of the "empty" box would still be tiny, would one be able to claim that the zero-point energy does not contribute much to the inertial mass.

The Casimir effect clearly shows that the quantum contributions, which we can compute and measure do, in fact, gravitate and I definitely agree with Polchinski. Every quantum field contribution produces an upward (for bosons) or downward (for fermions) shift in the vacuum energy. The Casimir effect clearly indicates that such individual quantum contributions do gravitate and once they are all added up the total zero-point energy should still gravitate, unless one has exact supersymmetry and they all precisely cancel.

 

[marcus]

One might say this to a QFT'er. You have a 120 order of magnitude problem in YOUR theory. It is not GR's problem. If you think you can fix it with some of the dodges Smoit mentioned, go for it and good luck to you! So far we don't see it getting fixed by those means, however.

On the other hand if you really want a quantum gravity fix, then be clear about it. You are going to have to move QFT out of the Minkowski context, and you will ultimately have to rebuild QFT on a quantum geometry basis, e.g. LQG.

 

[smoit]

So far we don't see it getting fixed by those means, however.

We? Meaning all the retired mathematicians who post on the physics forum?

 

[marcus]

We? Meaning all the retired mathematicians who post on the physics forum?

Yes Mr. Smolin-and-Woit and despite a bt of sarcastic snarling and grumbling we and our like are legion.

But in fact when I said "we" I was interpreting from the passage from Bianchi and Rovelli that I quoted in post #65:

"Why does standard QFT have so much trouble adjusting to this straightforward physical fact? We do not know the answer, but ..."

 

[marcus]

Since we are discussing the argument on page 7 of the Bianchi Rovelli paper, I should give the link again:
http://arxiv.org/abs/1002.3966
Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
(Submitted on 21 Feb 2010)
The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.
9 pages, 4 figures

An easy way to get the paper is simply to google "constant prejudices"
The arxiv link should turn up as the first or second hit.
Anyone coming in new would be well-advised to read the paper. It is easy to understand and puts the discussion here in a clearer light.

 

[smoit]

Since we are discussing the argument on page 7 of the Bianchi Rovelli paper, I should give the link again:
http://arxiv.org/abs/1002.3966
Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
(Submitted on 21 Feb 2010)

FYI, after almost 2 years, the authors have so far failed to publish it in a refereed journal.

 

[Harv]

The Casimir effect clearly shows that the quantum contributions, which we can compute and measure do, in fact, gravitate

But we can't verify the coupling of gravity directly to the individual loops involved in these quantum contributions.

 

[smoit]

But we can't verify the coupling of gravity directly to the individual loops involved in these quantum contributions.

The shift in the electrostatic energy due to vacuum polarization (experimentally measured in the Lamb shift) contributes an amount large enough that, in case if it did not gravitate, would violate the equivalence principle to a precision of one part in a million. However, we can experimentally verify the equivalence principle to a precision of one part in 10¹² and therefore these loops must couple to gravity.

 

[Fra]

The shift in the electrostatic energy due to vacuum polarization (experimentally measured in the Lamb shift) contributes an amount large enough that, in case if it did not gravitate, would violate the equivalence principle to a precision of one part in a million. However, we can experimentally verify the equivalence principle to a precision of one part in 10¹² and therefore these loops must couple to gravity.

I'm not sure if you refer to some new experiments I'm unaware of but I think it's still important to note which domains to theory space where certain "principles" are tested. Ingoring that is one of the things I find most disturbing, extrapolations of "evidence" into new domains are often made without much argument. This is exactly what people also do with things that are well tested for PARTICLE physics which usually means the observer is in a classical laboratory and the system is a very small subsystem. Inferences from such situations just don't generalize to cosmological scenarios. This fact is often ignored on grounds that "such extrapolations worked in the past".

I suspect the 10^12 test you refer to is the classical mechanics test of the torsion pendulum, right?

If there really experiments made that verifies the equivalence principle for actual lamb shift, that would be news for me. I think it's in principle testable, but the problem is as far a I know that the gravitational field on Earth is too weak to yield much of a significant possibility to test it here?

But I do not follow all new experiemtns, if someone konws of an actual test of the equivalence principle for lamb shifted systems I would be interested to read about how the experiment was conducted. I have seen some old papers where it was "in principle testable" was devicded, but the conclusion was that in practice it wasn't becaues hte gravity on Earth is so weak.

I'm not sure if some astronomic observations of lambshifted systems near more massive bodies is possible? I'm not sure how that would be done.

/Fredrik

 

[smoit]

@Fra, Here is a reference that is the most referred in the literature but you'd have to go to your library to actually read it: http://slac.stanford.edu/spires/find/hep/www?irn=6818293

Please, read very carefully what I said in my previous comment b/c your response indicated that you had misunderstood it.

 

[Harv]

The shift in the electrostatic energy due to vacuum polarization (experimentally measured in the Lamb shift) contributes an amount large enough that, in case if it did not gravitate, would violate the equivalence principle to a precision of one part in a million. However, we can experimentally verify the equivalence principle to a precision of one part in 10¹² and therefore these loops must couple to gravity.

I looked at Polchinski's paper and understand now why you're right. Thanks.

 

[smoit]

@Harv, no problem! Joe's article is good and pretty much sums up the current situation. I saw his talk on this topic at KITP back in 2006, you can find it here if you're curious: http://online.itp.ucsb.edu/online/strings_c06/polchinski/

 

[Haelfix]

I'd say that the CC problem is definitely a UV problem because we are dealing with a highly relevant operator so you cannot ignore all the extra degrees of freedom arising in the UV.

Yea tis true, that's why I qualified my statement. For the nonexperts, what this means in practise is that you need to know all the fields and matter all the way up to the Planck scale, as they will all contribute (naively increasing in magnitude, not necessarily in sign) contributions to the total constant. You can't ignore them. This is also why no first principles solution to the problem exists and probably never will exist (absent the discovery of some highly constraining mechanism or symmetry).

However I think I am correct in pointing out that the apparent magnitude of the problem occurs b/c of the scales mismatch, which is essentially set by the infrared physics.

where the first non-vanishing term is quadratic in the cutoff, the first non-vanishing contribution to the 10D vacuum energy comes at order Str(M8SUSY)M2string, where MSUSY is the scale of SUSY breaking.

Yep that's interesting, but I don't quite understand these constructions. What is MSusy specifically here (is it arbitrary)? Also, why wouldn't the mechanism that generates these nonperturbative corrections at the low energy scale (in the language of the effective field theory) not also generate unwanted and observable KK states?

As far as numerology goes. I like the following two observations as well. If you take the cutoff to be the mass of the lightest neutrino, the scales match. Another weird coincidence... The supersymmetry breaking scale seems to be exactly halfway (on a logarithmic scale) between the vacuum energy scale and the Planck scale. Why?

 

[smoit]

Yep that's interesting, but I don't quite understand these constructions. What is MSusy specifically here (is it arbitrary)? Also, why wouldn't the mechanism that generates these nonperturbative corrections at the low energy scale (in the language of the effective field theory) not also generate unwanted and observable KK states?

By M_SUSY I just denoted a generic scale of level spacing. Its value should be related to the string scale, I think, since SUSY is broken at the string scale in this construction.
That's why I did not really want to identify M_SUSY with the scale obtained in some 4D EFT from the soft breaking but the numerology looks cute
You probably mean perturbative instead on non-perturbative, right? This computation is in 10D, so there are no KK states involved. That said, at such extremely short distances even in a compactified 4D vacuum all the KK modes become light and the theory does effectively become 10 or maybe 11 dimensional. I think that what one really needs here for a realistic computation is to translate soft SUSY breaking in some 4D EFT into the splittings in the entire string spectrum and then compute the one-loop partition function. My hunch is that the scale of the boson-fermion splitting in the string levels would be related to the gravitino mass scale instead of the string scale and the result of the computation may actually give the correct order of magnitude. What was really neat for me to learn was that the finiteness of the CC in a non-tachyonic non-SUSY string theory is guaranteed by the modular invariance, despite the presence of an infinite tower of contributions in the UV. Here is a nice reference where you can read about these ideas: http://arxiv.org/PS_cache/hep-th/pdf/9503/9503055v2.pdf

 

[Fra]

@Fra, Here is a reference that is the most referred in the literature but you'd have to go to your library to actually read it: http://slac.stanford.edu/spires/find/hep/www?irn=6818293

Please, read very carefully what I said in my previous comment b/c your response indicated that you had misunderstood it.

Smoit thanks for the link. I haven't read it but that indeed looks like the classical paper I also thought you meant. It could well be that I didn't get your point at all, in that case I'm sorry.

Anyway my point was this: That paper tests WEP to one part in 10^12, but it's all classical mechanics (torsion balance) and relatively speaking macroscopic classical systems (which is dominated by baryonic mass) with classical measurements.

Thus I question the validity of that test when applied to situations where the classical mechanics framework just don't hold. Also just as a ballpark number it seems the contribution of lamb shift to the classical level mass is the order of 1 in 10^15 or so? Which seems to be beyond hte level os current experimental tests?

So I didn't quite get how that classical mechanics test of WEP for 1 part in 10^12 gives any information about the the nature loop corrections in general (which then of course goes outside classical mechanics)?

Perhasp I'm missing something, could you explain?

/Fredrik

 

[Fra]

Trying to make sure I understand the logic, let me know if I get it wrong:

such individual quantum contributions do gravitate and once they are all added up the total zero-point energy should still gravitate

Are you suggesting that (when considering the origin of mass) since the idea is that the actual classical masses (such as those in the 1971 torsion balance experiment) are largely made up of confined energy such as confined virtual gluons etc, therefore the conclusion is that all such "virtual energies" as infered by all observers(?) must contribute equally to both inertial and gravitatonal mass?

/Fredrik

 

[smoit]

Also just as a ballpark number it seems the contribution of lamb shift to the classical level mass is the order of 1 in 10^15 or so? Which seems to be beyond hte level os current experimental tests?

I'm not sure where you obtained this ballpark number. I suggest you read page 3 in "arxiv.org/PS_cache/hep-th/pdf/0603/0603249v2.pdf"[/URL].

[quote="Fra, post: 3620485"]
So I didn't quite get how that classical mechanics test of WEP for 1 part in 10^12 gives any information about the the nature loop corrections in general (which then of course goes outside classical mechanics)?

Perhasp I'm missing something, could you explain?

/Fredrik[/QUOTE]

If you are questioning how one can measure quantum effects by classical means than you should read a book or take a class in quantum mechanics. The whole reason for inventing quantum mechanics in the first place was the experimental results which could not be explained by classical physics, e.g. the discrete atomic spectra, etc. It seems as though you are questioning the ability to measure quantum effects by classical instruments and I suggest that you simply create a separate thread with the appropriate title to carry on the discussion there.

 

[smoit]

Trying to make sure I understand the logic, let me know if I get it wrong:


Are you suggesting that (when considering the origin of mass) since the idea is that the actual classical masses (such as those in the 1971 torsion balance experiment) are largely made up of confined energy such as confined virtual gluons etc, therefore the conclusion is that all such "virtual energies" as infered by all observers(?) must contribute equally to both inertial and gravitatonal mass?

/Fredrik

I said nothing about virtual gluons and their contributions to the rest mass of baryons b/c I didn't want to get into discussing lattice QCD results for hardon masses but yes, virtual gluon loops do contribute a significant portion of a baryon's inertial mass and such effects are much more significant than the virtual photon loops Polchinski talks about.

 

[RUTA]

I read the Bianchi & Rovelli paper as well as Chap VIII.2 “The Cosmological Constant Problem and the Cosmic Coincidence Problem” in Zee, Quantum Field Theory in a Nutshell, Princeton Univ Press, 2003 (p 434). I do not find any disagreement regarding the facts, only their reactions thereto. For example:

B&R write, “But to claim that dark energy represents a profound mystery, is, in our opinion, nonsense.” In contrast, Zee introduces this subject by saying, “I now come to the most egregious paradox of present day physics.”

Regarding the coincidence problem, B&R write, “it is quite reasonable that humans exist during those 10 or so billions years (sic) when \Omega_b and \Omega_\Lambda are within a few orders of magnitude from each other.” In contrast, Zee writes, “the epoch when \rho_M \sim \Lambda happens to be when galaxy formation has been largely completed. Very bizarre!”

Both agree that (per B&R) “There is no known way to derive the tiny cosmological constant that plays a role in cosmology from particle physics. And there is no understanding of why this constant is not renormalized to a high value.” B&R’s reaction to this fact is, “But this does not means (sic) that there is something mysterious in the cosmological constant itself: it means that there is something we do not understand yet in particle physics.” While Zee writes, “But Nature has a big surprise for us. While theorists racked their brains trying to come up with a convincing argument that \Lambda = 0 observational cosmologists steadily refined their measurements and recently changed their upper bound to an approximate equality \Lambda \sim (10^{-3} \mbox{ev})^4! The cosmological constant paradox deepens.”

In short, the B&R paper merely argues for a particular emotional reaction to the situation regarding the cosmological constant in physics and cosmology. I would be surprised to see this paper published in a physics journal, since it does not expand upon our knowledge of physics. However, the paper’s use of a timely topic to highlight one aspect of our failure to unify the Standard Model with gravity is not without value. I could see this included in the proceedings for a philosophy of science symposium, for example. It would also be appropriate for a pedagogical journal in physics. It certainly motivated me to look more deeply into the issue. Thanks for posting this, marcus.

Edit: I'm trying to figure out how to use TeX in PF. Obviously, I haven't found an "in line" tex command yet.

 

[juanrga]

The cosmological constant becomes a mistery as soon as you do not write it on the left hand = "the gravity" side of the equations

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4}T_{\mu\nu}

but it you write it on the right hand = "the matter" side.

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R = \frac{8\pi G}{c^4}T_{\mu\nu} - \Lambda g_{\mu\nu}

In vacuum (with T=0) you still have some kind of "matter" which affects spacetime:

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R = - \Lambda g_{\mu\nu}

If you leave this term on the left hand side, the question where it comes from and why it is there, is still open, but it is not a qustion about matter, dark energy or something like that; it is a question about gravity.

Sorry but a mystery does not disappear by moving a term, from the right to the left, on the same equation.

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R = - \Lambda g_{\mu\nu}

is just so problematic as

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = 0

 

[marcus]

Sorry but a mystery does not disappear by moving a term, from the right to the left, on the same equation.

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R = - \Lambda g_{\mu\nu}

is just so problematic as

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = 0

Wait Juan I don't think you grasped Tom's point! It is quite a valid one if you are familiar with the custom in General Relativity of writing the equation geometry (e.g. curvature terms) on the left and matter terms on the right.

Lambda is a curvature constant and occurs naturally and unsurprisingly in the geometry LHS
(since as Einstein observed early on, it is allowed by the symmetries of the theory).

You only make a puzzle out of it if you consider this natural curvature term to be "matter", and symbolize this by moving it to the RHS of the equation.

If you make this mistake then you baffle your self with asking "Now what could this matter be?!"

As Tom pointed out the constant curvature term Lambda is analogous to a constant of integration---that you are taught in beginning Calculus class to put in the answer when you integrate. It must be there because it is allowed by the conditions of the problem.

 

[marcus]

...
In short, the B&R paper merely argues for a particular emotional reaction to the situation regarding the cosmological constant in physics and cosmology. I would be surprised to see this paper published in a physics journal, since it does not expand upon our knowledge of physics. However, the paper’s use of a timely topic to highlight one aspect of our failure to unify the Standard Model with gravity is not without value. I could see this included in the proceedings for a philosophy of science symposium, for example. It would also be appropriate for a pedagogical journal in physics. It certainly motivated me to look more deeply into the issue. Thanks for posting this, marcus.

I think B&R state clearly at the outset that their purpose is to debunk the hype. The paper is aimed at fellow physicists who describe the "cosmological constant problem" in exaggerated language.

They give some examples of this near-hysterical rhetoric right the start of the paper. That is clearly the target.

I think that too much hype tends to damage the prestige and credibility of physics. Physicists have been shouting "wolf" or "fire" or "recreating the big bang" and "theory of everything!" so much that the educated audience has gotten into the habit of discounting what they say as attention-getting rubbish.

Since the intended audience of the paper is other physicists, particle physicists primarily I would say, I can't imagine why they would want to publish it in a philosophical or pedagogical journal. It is a warning to tone down the exaggerated rhetoric.

As such, the arxiv is a good place to post it. Or else possibly in the opinion section of a magazine like Physics Today. It's not a research or review article, after all. But why bother, since arxiv is already a perfect outlet?

I was amused by Smoit pointing out that the article had not been published in a peer-review journal, as if this were a criticism. Rovelli has over 14,000 cites to his over 200 professional articles. He hardly needs to try to peer-publish everything he writes to bolster his trackrecord. Since this piece is primarily advice to fellow physicists to sober-up and cut the hype for the good of the field, I actually doubt it has been submitted anywhere. Arxiv is the perfect place to reach those who are able to get the message.

 

[RUTA]

Zee's response to the facts presented in his text and B&R's paper is that they constitute "the most egregious paradox of present day physics." Why would B&R expect to change that reaction by simply rehashing what is known? Therefore, I would say this paper can only expect to find a sympathetic audience among those not familiar with the technical aspects of the issue, i.e., it's a pedagogical piece.

 

[simplicial]

Some other comments on the dark energy problem.

http://www.nature.com/nature/journal/v466/n7304/full/466321a.html

NATURE | NEWS AND VIEWS
Cosmology forum: Is dark energy really a mystery?
Bianchi, Rovelli, Kolb

Nature 466, 321–322 (15 July 2010)
doi:10.1038/466321a

The Universe is expanding. And the expansion seems to be speeding up. To account for that acceleration, a mysterious factor, 'dark energy', is often invoked. A contrary opinion — that this factor isn't at all mysterious — is here given voice, along with counter-arguments against that view.

 

[juanrga]

Sorry but a mystery does not disappear by moving a term, from the right to the left, on the same equation.

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R = - \Lambda g_{\mu\nu}

is just so problematic as

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = 0

Wait Juan I don't think you grasped Tom's point! It is quite a valid one if you are familiar with the custom in General Relativity of writing the equation geometry (e.g. curvature terms) on the left and matter terms on the right.

Lambda is a curvature constant and occurs naturally and unsurprisingly in the geometry LHS (since as Einstein observed early on, it is allowed by the symmetries of the theory).

You only make a puzzle out of it if you consider this natural curvature term to be "matter", and symbolize this by moving it to the RHS of the equation.

If you make this mistake then you baffle your self with asking "Now what could this matter be?!"

As Tom pointed out the constant curvature term Lambda is analogous to a constant of integration---that you are taught in beginning Calculus class to put in the answer when you integrate. It must be there because it is allowed by the conditions of the problem.

Sorry guys, but both equations of above are the same. The reason for the which the expression

-\Lambda g_{\mu\nu}

can be written as

\frac{8\pi G}{c^4}T_{\mu\nu}^{DE}

is related to the nature of the vacuum in quantum field theory. Or said in another way, the correct equation is

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4}T_{\mu\nu} + \frac{8\pi G}{c^4}T_{\mu\nu}^{DE}

and setting T=0 for vacuum, as tom did, gives

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4}T_{\mu\nu}^{DE}

instead of his equation.

The problem is that the difference between the geometry and the matter is of 120 orders of magnitude. This is the CC problem. and this problem is not solved by moving a term from the left of an equation to the right (evidently the discrepancy only moves, it does not disappear )

 

[marcus]

Some other comments on the dark energy problem.

http://www.nature.com/nature/journal/v466/n7304/full/466321a.html

NATURE | NEWS AND VIEWS
Cosmology forum: Is dark energy really a mystery?
Bianchi, Rovelli, Kolb

Nature 466, 321–322 (15 July 2010)
doi:10.1038/466321a

The Universe is expanding. And the expansion seems to be speeding up. To account for that acceleration, a mysterious factor, 'dark energy', is often invoked. A contrary opinion — that this factor isn't at all mysterious — is here given voice, along with counter-arguments against that view.

Great! I didn't know about this view getting into print in Nature. Did they have a debate then? What position did Rocky Kolb take? He is a distinguished guy at the U Chicago Astro department---one of the top astrophysics and cosmology departments in the Usa.
http://astro.uchicago.edu/people/edward-rocky-w-kolb.shtml[/URL]

Ha! I found a free link to the News and Views feature called "Is dark energy really a mystery?" [url]http://www.astro.uu.nl/~vinkj/LSS/Nature_2010_Bianchi.pdf[/url]

Bianchi & Rovelli say No it isn't and give a halfpage summary of their reasons.
Kolb says Yes it is, and gives his own halfpage argument.

 

[marcus]

Here is an excerpt from the condensed version that Bianchi and Rovelli published in Nature journal "News and Views" section, the 15 July issue. They had already disposed of two other arguments and were moving on to the third.

==quote B&R's piece in Nature==
The third objection concerns ‘vacuum energy’. Quantum field theory (QFT) seems to predict a vacuum energy that adds to the cosmological force due to Λ — just as radiative corrections affect the charge of the electron. But this hypothetical contribution to Λ is much larger than the observed Λ. The discrepancy is an open puzzle in QFT in the presence of gravity ^6,7. But it is a conceptual mistake to confuse Λ with QFT’s vacuum energy. Λ cannot be reduced to the ill-understood effect of QFT’s vacuum energy — or that of any other mysterious substance. Λ is a sort of ‘zero-point curvature’; it is a repulsive force caused by the intrinsic dynamics of space-time.

Tests on the ΛCDM model must continue and alternative ideas must be explored. But it is our opinion — and that of many relativists — that saying dark energy is a ‘great mystery’, for a force explained by current theory, is misleading. It is especially wrong to talk about a ‘substance’. It is like attributing the force that pushes us out of a turning merry-go-round to a ‘mysterious substance’.
...
==endquote==
For the full Nature article see:
http://www.astro.uu.nl/~vinkj/LSS/Nature_2010_Bianchi.pdf
The Bianchi, Rovelli, Kolb piece has a link to B&R's Arxiv article
"Why all these prejudices against a constant?"
http://arxiv.org/abs/1002.3966

This "constant prejudices" article is the topic of this thread, and just to be clear about the purpose and focus of the article it opens by quoting the first sentence of an article in Physics World co-authored by cosmologist Ofer Lahav (prof Astro. at University College, London). This is the kind of hype they are targeting:
==quote Calder and Lahav in Physics World 23 (June 2010), 32–37 ==

“Arguably the greatest mystery of humanity today is the prospect that 75% of the universe is made up of a substance known as ‘dark energy’ about which we have almost no knowledge at all.”

==endquote==
Full article "Dark Energy: how the paradigm shifted"
www.tiptop.iop.org/full/pwa-pdf/23/01/phwv23i01a33.pdf[/URL]

 

[juanrga]

Sorry guys, but both equations of above are the same. The reason for the which the expression

-\Lambda g_{\mu\nu}

can be written as

\frac{8\pi G}{c^4}T_{\mu\nu}^{DE}

is related to the nature of the vacuum in quantum field theory. Or said in another way, the correct equation is

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4}T_{\mu\nu} + \frac{8\pi G}{c^4}T_{\mu\nu}^{DE}

and setting T=0 for vacuum, as tom did, gives

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4}T_{\mu\nu}^{DE}

instead of his equation.

The problem is that the difference between the geometry and the matter is of 120 orders of magnitude. This is the CC problem. and this problem is not solved by moving a term from the left of an equation to the right (evidently the discrepancy only moves, it does not disappear )

The above two last equations are incorrect and would be written as

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4}T_{\mu\nu}

equivalent to

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R = \frac{8\pi G}{c^4}T_{\mu\nu} + \frac{8\pi G}{c^4}T_{\mu\nu}^{DE}

For vacuum

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R + \Lambda g_{\mu\nu} = 0

or (equivalent)

R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R = \frac{8\pi G}{c^4}T_{\mu\nu}^{DE}

Another correction. That -\Lambda g_{\mu\nu} can be written as \frac{8\pi G}{c^4}T_{\mu\nu}^{DE} is independent of the nature of the vacuum in quantum field theory. It is a definition. The problem is when T_{\mu\nu}^{QFT-vacuum} is used to compute T_{\mu\nu}^{DE}

 

[atyy]

The Casimir effect clearly shows that the quantum contributions, which we can compute and measure do, in fact, gravitate and I definitely agree with Polchinski. Every quantum field contribution produces an upward (for bosons) or downward (for fermions) shift in the vacuum energy. The Casimir effect clearly indicates that such individual quantum contributions do gravitate and once they are all added up the total zero-point energy should still gravitate, unless one has exact supersymmetry and they all precisely cancel.

I agree in general that there is a fine tuning problem with the cc coming from quantum effects. But I thought the Casimir effect isn't evidence of this since it can be calculated without using zero-energy, like in http://arxiv.org/abs/hep-th/0503158?

I think AdS/CFT must provide examples of a framework in which the "renormalization approach" applies, because in any given instance of the duality, the bulk space (the AdS space) has a known, nonarbitrary, nonzero cosmological constant, and yet everything fits into the framework of QFT (on the CFT side of the duality). So it would be of interest to understand how AdS/CFT deals with vacuum energy in the bulk, on the way to obtaining a negative cosmological constant.

edit: See http://arxiv.org/abs/1106.3556" ).

Physics Monkey https://www.physicsforums.com/showthread.php?t=548726" that looks at this in a 1+1 Ising model and its gravity dual.

 

[RUTA]

I like Kolb's response to B&R (thanks for providing that article, marcus). He defines "mystery" per Webster's, i.e., “Something not understood or beyond understanding,” then points out that Lambda is not understood. Only way to beat that is provide another definition of "mystery" or provide an origin for Lambda. Since B&R can't do the latter, I'd be interested in hearing their definition of "mystery."

 

[marcus]

B&R "constant prejudices" paper which is the topic of this thread opens by quoting the first sentence of an article in Physics World co-authored by cosmologist Ofer Lahav (prof Astro. at University College, London). This is the kind of hype B&R are targeting:
==quote Calder and Lahav in Physics World 23 (June 2010), 32–37 ==
“Arguably the greatest mystery of humanity today is the prospect that 75% of the universe is made up of a substance known as ‘dark energy’ about which we have almost no knowledge at all.”
==endquotewww.tiptop.iop.org/full/pwa-pdf/23/01/phwv23i01a33.pdf[/URL]==

Earlier I quoted an excerpt from the version that Bianchi and Rovelli published in Nature journal "News and Views" section, the 15 July issue.

Anyone who has read the piece in Nature carefully will realize that the operative word is "substance". They argue that it is misleading to talk about Λ (a small constant curvature) as a "substance".
==quote B&R's piece in Nature==
But [B]it is a conceptual mistake to confuse Λ with QFT’s vacuum energy[/B]. Λ cannot be reduced to the ill-understood effect of QFT’s vacuum energy — or that of any other mysterious [I]substance[/I]. Λ is a sort of ‘zero-point curvature’; it is a repulsive force caused by the [B] intrinsic dynamics of space-time.[/B]
===endquote===
Efforts are under way to understand how this "zero point curvature" arises from the underlying quantum dynamics of space-time.
As quantum relativists the authors are naturally interested in how the zero point curvature relates to QG degrees of freedom: "the intrinsic [quantum] dynamics of space-time". There have been several articles about this. For a recent examples see page 41 of the 2010 paper by Meusburger and Fairbairn--also the paper by Han (a member of the Marseille group who has co-authored with B&R.)

==continuing the B&R excerpt==
Tests on the ΛCDM model must continue and alternative ideas must be explored. But it is our opinion — and that of many relativists — that saying dark energy is a ‘great mystery’, for a force explained by current theory, is misleading. It is especially wrong to talk about a ‘substance’. It is like attributing the force that pushes us out of a turning merry-go-round to a ‘mysterious substance’...
==endquote==
For the full Nature article see:
[url]http://www.astro.uu.nl/~vinkj/LSS/Nature_2010_Bianchi.pdf[/url]
The Bianchi, Rovelli, Kolb piece has a link to B&R's Arxiv article
"Why all these prejudices against a constant?"
[url]http://arxiv.org/abs/1002.3966[/url]

 

[marcus]

As quantum relativist one wants to understand how the Einstein equation (with its zero-point curvature constant Λ) arises.
And specifically, in connection with the cosmological constant, one presumably wants to understand a LENGTH. The length scale of this small ubiquitous constant curvature.
What underlies this length is not understood, but there are some intriguing ideas.

BTW the length in question is easy to calculate from standard estimates of cosmological parameters and is 9.3 billion ly. Same order of magnitude as several other length scales basic to cosmology.

I mentioned Meusburger and Fairbairn's paper where this length plays a role. Also Han's paper.
BTW B&R themselves have a simple 2-page paper about the physical meaning of this length, and of the quantum group deformation parameter (essentially an exponential form of the length)---I'll get that link too, it might be of interest.
http://arxiv.org/abs/1105.1898

Here is the link for Han's paper:
http://arxiv.org/abs/1105.2212
Cosmological Constant in LQG Vertex Amplitude
Muxin Han
(Submitted on 11 May 2011 (v1), last revised 12 Jun 2011 (this version, v2))
A new q-deformation of the Euclidean EPRL/FK vertex amplitude is proposed by using the evaluation of the Vassiliev invariant associated with a 4-simplex graph (related to two copies of quantum SU(2) group at different roots of unity) embedded in a 3-sphere. We show that the large-j asymptotics of the q-deformed vertex amplitude gives the Regge action with a cosmological constant. In the end we also discuss its relation with a Chern-Simons theory on the boundary of 4-simplex.
6 pages, 5 figures

 

[Fra]

I'm not sure where you obtained this ballpark number. I suggest you read page 3 in "arxiv.org/PS_cache/hep-th/pdf/0603/0603249v2.pdf"[/URL].[/QUOTE]

Right, my mistake, sorry! Anyway this was just a side note, the major point was below.

[quote="smoit, post: 3620935"]If you are questioning how one can measure quantum effects by classical means than you should read a book or take a class in quantum mechanics. The whole reason for inventing quantum mechanics in the first place was the experimental results which could not be explained by classical physics, e.g. the discrete atomic spectra, etc. It seems as though you are questioning the ability to measure quantum effects by classical instruments and I suggest that you simply create a separate thread with the appropriate title to carry on the discussion there.[/QUOTE]

No, my point was rather the realism implicit in classical mechanics vs the "measurement" with infinitely massive observerrs implicit in QFT, and how this compares to finite observers making measurements on their environment. Sometimes which I think is forced once we move in theory space.

But I read the Polchinski's paper and he raises hte question, why the gravitation apparently difference between what's confined within an atom vs what's outside the atom.

In my view the difference is that if you study an atom, you have a virtually infinitely massive classical observer that via scattering experiments in principle studies a small subsystem.

This is assymetric to the case where a small finite observer looks into it's own environment.

In the latter case there is a natural cutoff, due to the observers mass. In the former case there is no natural cutoff, which is why the cutoff is introduced ad hoc. When comparing QFT framework and classical mechanics without respecting this, I think one is missing something important. After all, "mass in classical mechanics" is just a parameter, whose measurement is also classical.

I think QG takes us into the domain of non-classical observers and quantum systems.
QFT is more like classical observers and quantum systems.
classical mechanics is classical observes and classical systems.

This influences what types of observeables we get. But this is related to questions raise in the paper too. I just put it differently since I am neither into strings nor LQG. My perspective is that of inference, where interactions are explained in terms of observer observer inferences. Here the complexity of the observers is paramount as it constrains the whole picture.

/Fredrik

 

[RUTA]

Anyone who has read the piece in Nature carefully will realize that the operative word is "substance". They argue that it is misleading to talk about Λ (a small constant curvature) as a "substance".

The title of the Nature article is, "Is dark energy really a mystery?" The "abstract" reads,

The Universe is expanding. And the expansion seems to be speeding up. To account for that acceleration, a mysterious factor, ‘dark energy’, is often invoked. A contrary opinion — that this factor isn’t at all mysterious — is here given voice, along with counter-arguments against that view.

Nowhere does Kolb use the word "substance" in his response.

This article presents, as advertised, arguments "that this factor isn't at all mysterious ... along with counter-arguments against that view." Thus, Kolb showed that Lambda is "mysterious" per Webster's definition. B&R need to likewise show that Lambda is not "mysterious" per some non-idiosyncratic definition. They fail to do so. It's that simple.