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

[bill alsept]

Marcus, I believe I'm interested in the subject of your last three postings but it’s a little beyond me. Do you have the patience to explain it one more time in a simpler way? Thanks

 

[marcus]

Bill,
I can try to help but I don't know very much about the things Sindoni talks about in his Review paper on Emergent Gravity. Also I don't know much about what is covered in the Finazzi Liberati Sindoni paper. I'm impressed, but it's new stuff to me.
Sindoni http://arxiv.org/abs/1110.0686
FLS http://arxiv.org/abs/1103.4841

I come at this from the perspective of the paper mentioned in the initial post of this thread:
Bianchi Rovelli "Why all these prejudices...?" http://arxiv.org/abs/1002.3966
That, by contrast, is an easy paper to read, very down to earth---you could start there.

They basically say that Lambda (a small constant curvature---or reciprocal area constant) belongs naturally in the Einstein GR equation on the lefthand side because it is allowed by the symmetry of the theory--covariance.
There is no need to think of it as an energy. No need to drag it over the the righthand side where the energy and matter terms are.
No need to get it confused with the QFT Vacuum Energy. That is QFT's problem, they calculate something in a fixed flat geometry context (quite alien to GR) and it comes out ridiculously wrong. So they should deal with it.
Lambda on the lefthandside of the GR equation is a tiny constant curvature determined observationally.

"Dark energy" is a phony idea. "Dark energy problem" is hype. Case closed. So that's simple enough.

Tom Stoer has a good discussion at the beginning of the thread.

Now what you express interest in here is different. I was talking about two new papers that I don't understand and wish someone here would explain to me. THEY PRESENT AN IDEA OF HOW GR COULD EMERGE FROM SOMETHING DEEPER (pre-geometry?) and even HOW LAMBDA MIGHT COME TO BE what it is.

So one thing they do is strengthen the case that we should not think of Lambda as some kind of "dark energy" field. And they say this explicitly. It is a feature that emerges along with the rest of GR, in their scheme, from some more fundamental degrees of freedom.

Today I have been spending time offline trying to read the Sindoni paper. I am woefully unprepared to explain it, or help you.
Same with the Finazzi Liberati Sindoni (FLS). I was struggling with that yesterday. I guess I should get back to it now.

 

[bill alsept]

Thanks Marcus I appreciate any help. It is the two papers that interest me. I think I’ll try reading through them a few more times.

 

[marcus]

Me too, maybe we can help each other out.
There are other papers in this cluster, by Sindoni et al, that appeared earlier this year. They may help us understand.

 

[PatrickPowers]

I have read that Albert Einstein declared his introduction of the the cosmological constant greatest blunder of his life.

 

[marcus]

I have read that Albert Einstein declared his introduction of the the cosmological constant greatest blunder of his life.

Patrick,
there's a subtle point here that is significant but often missed. A very readable discussion, that does not short-cut the facts, is on page 2 of the "Why all these prejudices...?" paper http://arxiv.org/abs/1002.3966:
You might be interested in having a look at the halfpage of discussion leading up to this conclusion, which I quote.

Einstein had in his hands a theory that predicted the cosmic expansion (or contraction) without cosmological constant, with a generic value of the cosmological constant, and even, because of the instability, with a fine-tuned value of the cosmological constant. But he nevertheless chose to believe in the fine-tuned value, goofed-out on the instability, and wrote a paper claiming that his equations were compatible with a static universe! These are facts. No surprise that later he referred to all this as his “greatest blunder”: he had a spectacular prediction in his notebook and contrived his own theory to the point of making a mistake about stability, just to avoid making ... a correct prediction! Even a total genius can be silly, at times.
Why is this relevant for the debate about the cosmological constant? Because short-cutting this story into reporting that Einstein added the cosmological term and then declared this his “greatest blunder” is to charge the cosmological term with a negative judgment that Einstein certainly never meant.
In fact, it may not even be true that Einstein introduced the λ term because of cosmology. He probably knew about this term in the gravitational equations much earlier than his cosmological work. This can be deduced from a footnote of his 1916 main work on general relativity [9] (the footnote is on page 180 of the English version). Einstein derives the gravitational field equations from a list of physical requirements. In the footnote, he notices that the field equations he writes are not the most general possible ones, because there are other possible terms. The cosmological term is one of these (the notation “λ” already appears in this footnote)...​

 

[Haelfix]

"Dark energy" is a phony idea. "Dark energy problem" is hype. Case closed. So that's simple enough.
"No need to get it confused with the QFT Vacuum Energy. That is QFT's problem, they calculate something in a fixed flat geometry context (quite alien to GR) and it comes out ridiculously wrong. So they should deal with it."

I apologize, but this is ridiculous and completely missing the point. It does not suffice to invent a new theory of quantum gravity, and explain why the contributions to the cosmological constant are smaller then expected at that energy scale. There are hundreds of papers out there with ideas like that, the analog gravity paper is no exception, and the reason none of them has convinced anybody, is because they are only answering the first step in what is a much bigger universality problem.

The real problem is that we know experimentally that at least certain quantum contributions at our normal energy scales do in fact gravitate. Every time you step on a scale, approximately 90% of your weight lies in this magic (I am of course talking about virtual gluon contributions to the mass of nucleons). If this did not gravitate, it would instantly show up in violent departures from the equivalence principle.

Now, let us for simplicity restrict to a world which only includes gravity and QED, since we know a lot about the latter up to at least energy scales of 100 GEV where it is very precisely described by an effective field theory that is weakly coupled, and pointlike.

Now I can't draw it here, but there is a diagram that contributes to the famous Lamb shift, but this time weakly coupled to gravity (so it looks like a tadpole). If we take the cutoff scale as 100GEV, the vacuum energy of this diagram's contribution to the ZPE of the electron is still approximately 10^55 larger than experiment. So, the statement of the problem is now the following:

Why does *this* contribution to the zero point energy of the electron in vacuum vanish (or is tuned or is canceled by some unknown mechanism) but the analogous diagram, in the environment of atoms that represents the shift in the atomic mass arising from ZPE fluctuations does not (and very accurately gravitates by tests of the equivalence principle).

Now, it gets worse... If you think you have an answer to the above problem, you have to explain another puzzle. Why does the vacuum contributions vanish in the real world (with a mix of complicated matter fields all contributing in various ways), but not in the far more symmetric electroweak vacuum state arising from SU(2)*U(1)? It cannot vanish in both, since the mass of the electron vanishes in the unbroken phase and it is precisely this mass which contributes to both subleading contributions to the aforementioned electron ZPE (some ~10^53 too large) as well as to the classical value of the Higgs potential (its really top quark loops that dominate here, but the electron also does contribute).

The point being, you cannot answer the question by simply begging the question like Rovelli does. Everyone agrees that if you could actually SHOW explicitly that the ZPE vanishes, then you have at least partially solved the problem, but then he doesn't, which is why it is a complete nonanswer.

Anyway, you can be sure that the answer to this puzzle sends whoever solves it straight to Stockholm. So I assure you, it is not 'hype'! Instead it is a problem that has to have a solution, and its just the case that no one has figured one out yet b/c it is very difficult.

(Addendum: If someone do not understand what I am writing above, or the exact details it is probably best to start at the beginning with a classical review paper at least stating the problem clearly)

For cosmologists, Sean Carrol has written a fairly elementary treatment here:

http://relativity.livingreviews.org/Articles/lrr-2001-1/

as well as his CERN course video (highly recommended):
http://www.youtube.com/watch?feature=player_embedded&v=cYVj2RhXxeU

Once you have understood and digested the above, the more theoretically rigorous review is given by Weinberg's classic paper

http://www-itp.particle.uni-karlsruhe.de/~sahlmann/gr+c_seminarII/pdfs/T3.pdf

 

[marcus]

It looks to me as if Haelfix is just saying stuff that is irrelevant but obvious, stuff everybody knows that does not connect with topic. Earlier I quoted the FLS paper in hope someone might comment. No FLS-relevant comment so far.

The Finazzi-Liberati-Sindoni (FLS) paper could be something of a game-changer, so I want to back up and reconsider what I was saying. Here is an excerpt from their conclusions...

==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. Second, the energy scale of Λ can be several orders of magnitude smaller than all the other energy scales for the presence of a very small number, nonperturbative in origin, which cannot be computed within the framework of an effective field theory dealing only with the emergent degrees of freedom (i.e. semiclassical gravity).

The model discussed in this Letter shows all this explicitly. Furthermore, it strongly supports a picture where gravity is a collective phenomenon in a pregeometric theory. In fact, the cosmological constant puzzle is elegantly solved in those scenarios. From an emergent gravity approach, the low energy effective action (and its renormalization group flow) is obviously computed within a framework that has nothing to do with quantum field theories in curved spacetime. Indeed, if we interpreted the cosmological constant as a coupling constant controlling some self-interaction of the gravitational field, rather than as a vacuum energy, it would straightforwardly follow that the explanation of its value (and of its properties under renormalization) would naturally sit outside the domain of semiclassical gravity.

For instance, in a group field theory scenario (a generalization to higher dimensions of matrix models for two dimensional quantum gravity [19]), it is transparent that the origin of the gravitational coupling constants has nothing to do with ideas like “vacuum energy” or statements like “energy gravitates”, because energy itself is an emergent concept. Rather, the value of Λ is determined by the microphysics, and, most importantly, by the procedure to approach the continuum semiclassical limit. 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. To date, little is known about the macroscopic regime of models like group field theories, even though some preliminary steps have been recently done [20]. Nonetheless, analogue models elucidate in simple ways what is expected to happen and can suggest how to further develop investigations in quantum gravity models. In this respect, 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]

To throw in an another stick, I pondered about and inferencial interpreration of the cosmological constant in this in this old thread https://www.physicsforums.com/showthread.php?t=239414 with the purpose of stimulating some thinking

Of course my argument appeals to general forms of any action, that is understood as -log P where P is a transition probability.

I just compared the FORM of the E-H action, with the FORM of an action I get from a particular construction.

The conclusion is that a kind of integrated "cosmological constant term" appears generically in any action of that form, and it's interpreted as having to do with the observers truncation of confidence. If the maximum probability was 100% then the cosmological constant should approach zero. When the maximum probability is large but finite, due to limited inferrability capacity of the finite obsever, the terms is bound to be non-zero - but finite.

This is not very specific, but it illustrates a alterantive logic, that MIGHT be able to work out specifically.

I think to connect this to the specifici cosmo constant in 4D spacetime, one needs to construct spacetime by inference from the microsctructure of information. Where dimensionality is bound to be emergen as well, perhaps a little bit like a truncated principal component analysis for dimensional regulation, where the truncation is forced upon the inference due to the observers incompleteness.

Then to the point is that all QFT thinking, seems to picture the observer at infinite or in some background - which then effectively has infinite mass - thus the cosmo constant "should be zero" if they only could find out how to cancel the summation properly... but tis perspective fails for gravity, when the observer is inside, so it seems reasonable that the cosmo constant that's effectiuvely inferred by earht based cosmo observations are not expected to be zero. Here the "massive background" does not exist. I think this is at least conceptually related to this issue.

But to get from here to explicit solutions seems hard indeed since it involves the entire chain of complexities, such as mass generation, theory scaling and evolution etc.

/Fredrik

 

[marcus]

Some readers may have overlooked the points in the Finazzi Liberati Sindoni paper that I'm asking for comment on, so I will boil down and highlight

==quote FLS http://arxiv.org/abs/1103.4841 ==
... 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.
... it strongly supports a picture where gravity is a collective phenomenon in a pregeometric theory. In fact, the cosmological constant puzzle is elegantly solved in those scenarios. From an emergent gravity approach, the low energy effective action (and its renormalization group flow) is obviously computed within a framework that has nothing to do with quantum field theories in curved spacetime. Indeed, if we interpreted the cosmological constant as a coupling constant controlling some self-interaction of the gravitational field, rather than as a vacuum energy, it would straightforwardly follow that the explanation of its value (and of its properties under renormalization) would naturally sit outside the domain of semiclassical gravity.

For instance, in a group field theory scenario (a generalization to higher dimensions of matrix models for two dimensional quantum gravity [19]), it is transparent that the origin of the gravitational coupling constants has nothing to do with ideas like “vacuum energy” or statements like “energy gravitates”, because energy itself is an emergent concept. Rather, the value of Λ is determined by the microphysics, and, most importantly, by the procedure to approach the continuum semiclassical limit. 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. ... In this respect, 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==

Since we are considering a matter of critical judgment here, I might mention that Liberati has well over 3000 citations
http://inspirehep.net/search?ln=en&rm=citation&jrec=1&p=a+Liberati
The guy is a world-class cosmologist/phenomenologist. Still fairly young (40-year-old) and turning out top-cited papers.
His PhD adviser was Dennis Sciama, if the name means anything to you.
In case anyone might be confused about this, Liberati is not part of the Loop QG community. He has never attended the biannual Loops conference. He's more the outsider phenomenologist type---long-time interest in cosmological/astrophysical tests.

No Fra, the FLS paper is not "Rovelli-style"

 

[Fra]

I didn't read the paper to see what they mean but in my thinking the "cosmological term" in the general action I associated it when it comes to specifically 4D spacetime, can only be understood beyond the "just another parameter" level if we also understand how 4D spacetime emerges, because that is what would somehow factor out that term.

Probably the differences lies in what is meant by emergence. Each time before when I've read Rovelli style papers it was clear that there are different meanings of the concept. I don't believe in any fundamental DOFs. The alterantive concept of emergence is just interacting effective theories, since there is no master theory (since in my view theory attaches to observer machinery), there are no fundamental DOFs. The task thus becomes how to even make something constructure without referring to fundamental DOFs.

The difference for emergence would be in like "emergence FROM something else", or "emergence in terms of just evolution relative to the prior state" where there are no fixed background context at all.

This is even the constructing principle behind the association in my old post. The idea is that the possible future can only be rated in terms of an action measure in terms of a specific reduced time history. This is why P_max < 1, and thus us why the normally unbounded information divergence IS bounded in this case. And this was also the keys that allows the expectation that the term is small, but strictly non-zero as inferred by a finite observer.

So maybe if we focus on the "emergence of spacetime" the cosmo constant problem will be solved automatically, if done the right way. IMO the emergence of spacetime, is an inference, and it's hosted by an observer. Ie. an extension of the essence that Smolin et all mention in the relative locality idea that spacetime is simply a result of an inference from actual data! Now that data needs to be stored and processed by something with finite capacity. This is where a lot of things are missing...

/Fredrik

 

[marcus]

Bianchi and Rovelli are not saying anything new, are they? Take eg. this 2007 review

http://arxiv.org/abs/0705.2533
"The observational and theoretical features described above suggests that one should consider cosmological constant as the most natural candidate for dark energy. Though it leads to well known problems, it is also the most economical [just one number] and simplest explanation for all the observations. Once we invoke the cosmological constant, classical gravity will be described by the three constants G, c and Lambda"

Thanks for pointing that out! I think you are right. It has been a fairly commonplace view among cosmologists all along. That is, Lambda is not some kind of exotic energy field with possibly varying density and equation of state parameters.

For a while after 1998 people naturally wanted to make sure that they were right---no variation was observed, so there is increasing confidence in the standard Lamda-CDM cosmic model (which treats Lambda as a constant curvature built into spacetime.)

So I don't see that Bianchi Rovelli are saying anything new. They are just puncturing a bubble of hype. Pointing out that the "dark energy" Emperor is walking down the street buck naked

What I do see as new is what Liberati et al are saying. They look deeper into the quantum origin of this classical curvature constant. Why, when spacetime emerges from pregeometry d.o.f., does it emerge with this curvature?
They illustrate with a "what-if" group field theory (GFT) example.

BTW Atyy in a sense you chose the perfect example (that 2007 review) to point out that Bianchi Rovelli's message is mainstream. It was an invited review for a special issue of GRG edited by Herman Nicolai, Roy Maartens, and George Ellis---than which there is no whicher

 

[Harv]

Does anyone here doubt that quantum vacuum energy exists?

 

[marcus]

Does anyone here doubt that quantum vacuum energy exists?

Of course not! How about this: read the Bianchi Rovelli paper. They discuss quantum vacuum energy at length, and the difficulties with calculating it accurately.
But by all means read the paper. Anyone who wants to join in the discussion should. It is a fairly non-technical easy read.

I already gave the link. But I will again:

http://arxiv.org/abs/1002.3966
Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
9 pages, 4 figures
(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."

There is also a short (4-page) paper by Stefano Liberati et al, which is both relevant and fascinating. It considers where that Lambda constant in classical spacetime geometry might be coming from in an emergent spacetime picture. Classically Lambda is a curvature naturally occurring on the lefthand side of the Einstein field equation, whose value is measured to be about
1.16 x 10^-35 second^-2
You might want to take a look at the final page of the Liberati paper where they state their conclusions:

http://arxiv.org/abs/1103.4841
The cosmological constant: a lesson from Bose-Einstein condensates
Stefano Finazzi, Stefano Liberati, Lorenzo Sindoni
(Submitted on 24 Mar 2011)
...Here we directly compute this term and confront it with the other energy scales of the system. On the gravity side of the analogy, this model suggests that in emergent gravity scenarios it is natural for the cosmological constant to be much smaller than its naif value computed as the zero-point energy of the emergent effective field theory. The striking outcome of our investigation is that the value of this constant cannot be easily predicted by just looking at the ground state energy of the microscopic system from which spacetime and its dynamics should emerge. A proper computation would require the knowledge of both the full microscopic quantum theory and a detailed understanding about how Einstein equations emerge from such a fundamental theory. In this light, the cosmological constant appears even more a decisive test bench for any quantum/emergent gravity scenario.

=============================

The tendency in observational cosmology in recent years has been to confirm and accept that Lambda is in fact simply a constant and not necessarily connected with the naive QFT calculation of vacuum energy (which is after all based on a non-quantum static flat Minkowski geometry.) To some extent this is a matter of one's background and opinions---I'm not talking about diehard QFT-ers, this is the trend I see in observational cosmology. Here are some illustrative linksere is one I found by Paolo Serra et al (2009)
http://arxiv.org/abs/0908.3186
No Evidence for Dark Energy Dynamics from a Global Analysis of Cosmological Data
Paolo Serra (UC Irvine), Asantha Cooray (UC Irvine), Daniel E. Holz (Los Alamos National Laboratory), Alessandro Melchiorri (University of Rome), Stefania Pandolfi (University of Rome), Devdeep Sarkar (UC Irvine, University of Michigan)
Physical Review D

From the Serra et al conclusions [their italics ]:
"We find no evidence for a temporal evolution of dark energy—the data is completely consistent with a cosmological constant. This agrees with most previous results, but significantly improves the overall constraints [13, 14, 19, 20]."

Here is another by Tamara Davis et al (2007)
http://inspirehep.net/record/742618
Scrutinizing Exotic Cosmological Models Using ESSENCE Supernova Data Combined with Other Cosmological Probes
Astrophysical Journal

One by Wood-Vasey et al (2007)
http://inspirehep.net/record/741585?ln=en
Observational Constraints on the Nature of the Dark Energy: First Cosmological Results from the ESSENCE Supernova Survey
Astrophysical Journal

There is also the "WMAP7" report of Komatsu et al. which appeared in 2010.
This was part of a NASA series of papers presenting the full 7-year data from the WMAP mission.
Here is the link. http://arxiv.org/abs/1001.4538
Page 24 has some constraints on the equation of state number w which in case Lambda is simply a constant would be exactly w = -1. Indeed that is about what you get combining latest WMAP+BAO+SN data. (The high-z supernova data SN is the most effective at constraining w. The BAO data is based on galaxy counts and is also good---they combined all the best.)
For example on page 24 in section 5.1 you see:
"The high-z supernova data provide the most stringent limit on w. Using WMAP+BAO+SN, we find w = −0.980±0.053 (68% CL)..."

That is really really close to -1. As time goes on the constraints seem to tighten and I hear less and less about Lambda considered as an actual "energy". We may be getting closer to accepting it simply as a small constant amount of curvature.

 

[Haelfix]

The tendency in observational cosmology in recent years has been to confirm and accept that Lambda is in fact simply a constant and not necessarily connected with the naive QFT calculation of vacuum energy (which is after all based on a non-quantum static flat Minkowski geometry.) To some extent this is a matter of one's background and opinions---I'm not talking about diehard QFT-ers, this is the trend I see in observational cosmology. Here are some illustrative links:

That is really really close to -1. As time goes on the constraints seem to tighten and I hear less and less about Lambda considered as an actual "energy". We may be getting closer to accepting it simply as a small constant amount of curvature.

Hi Marcus, you are mixing up two things here. W = -1 is very much what the simple QFT prescription is about. It is interpreted as arising from a cosmological constant, with units of energy density (g/cm^3). You are also free to think of it as a sort of negative pressure in the context of the FRW lambda dust solution.

What you are confusing this with is the case for w < -1, which is what is commonly known as quintessence (which is a scalar field that mimics the observed cosmological constant in our epoch and introduces an explicit time dependence). That latter involves very exotic physics, and is decidedly NOT predicted by the standard QFT calculation..

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!

So consider an empty box and physics that contains a huge positive cosmological constant term. You can think of weighing the box if you put it on a scale, or alternatively you can think of the geometry that this induces (an expansion scalefactor term that looks like A(t) ~ E^(Ht)) but the problem of having a quantum vacuum density 10^120 orders of magnitude too big is still wrong on physical grounds, no matter what side of the original field equations you put it on in order to solve the equation. An empty box solution simply does not weigh that much, and it does not induce curvature of that magnitude in the real world!

What saves the prediction (but also what defines the problem) is that the quantum vacuum is not the only contribution to the total cosmological constant, we are instructed to cancel it with a classical contribution. This latter is typically arbitrary, and we are thus left with the finetuning problem. Why does 2 apparently different physical quantities, cancel to fantastic accuracy?

So this is the problem! It is not that we have a theory that gives a wrong prediction. We can make our theories give the right value. The problem is that this value is wildly different then what you might consider natural!

One possible resolution is to just say that the quantum ZPE sums to identically zero in a more refined theory. And that is FINE and of course would partially solve the problem! For instance with exact supersymmetry you can show that this is indeed what happens!

However, the solution MUST exist at all scales, not just up at the Planck scale. And so the solution must be transparent within the low energy physics based formalism defined up to 100 GEV. So for instance in the context of supersymmetry, one can see explicitly that the thing that cancels the electron tadpole diagrams, is the analogous selectron diagrams!

So the point is you have to actually SHOW this mechanism explicitly.

To give an analogy it would be like arguing for the clay millenium prize regarding QCD. You can't simply say, 'well we don't observe free quarks in nature, therefore QCD is confining -QED'. The whole point is in *showing* this, mathematically!

 

[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 ..."