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

[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 ‘[COLOR="Blue"]zero-point curvature[/COLOR]’; 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 [COLOR="Red"]‘substance’[/COLOR]. 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.

 

[Fra]

IMO the core "mystery" is mainly that we don't understand how to merge two successful frameworks. In particular how to describe "empty space". Specifically I see it as boiling down to how it's _measured_. Here GR and QM/QFT just don't compare even if similar word "vacuum" is used for the two things.

I tend to see that the main issue with measurments and expectations is that it requires and observer. Ie. empty space makes no sense unless it has a boundary by which to interact with it - thus it's all about the boundary and how it confines things. Here the difference is obvious. It's almost like two complementing pictures. The observer is sitting at difference sides of the horizon.

To observe a small subsystem, or to observer a possibly open environment are so different that concepts tested in one domain don't apply. So I don't thikn it's any "mystery", it's more like an obvious "missing part" in our understanding. The two theories are disjoint in theory space. That's more a missing piece of hte puzzle more than a mystery IMO.

/Fredrik

 

[smoit]

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.

Bianchi has 21 professional articles, 17 of them published. I'm sure that as a postdoc, he likewise "hardly needs to try to peer-publish everything he writes to bolster his trackrecord".

 

[marcus]

I was nevertheless amused at how you chose to criticize the article.

To get back to serious matters, what we observe at cosmological scale is acceleration, not a possibly mythical "dark energy" substance causing the acceleration. I think it's a bad idea to use "dark energy" as a code-name for the acceleration one wants to explain. If one is actually talking about the observed acceleration, as something to be explained, one should call it acceleration. So, with that adjustment, Kolb says

==quote Kolb in the Nature piece==
Einstein’s cosmological constant Λ is the simplest explanation for [acceleration]: it adequately fits the data, and there is no reason to exclude it. But the magnitude of Λ necessary to explain the observations places it far “beyond [our] understanding”. If the cosmological constant is the explanation for [acceleration], Λ must be about (10^28 cm)−2. The length 10^28 cm is absurdly large, and cannot at present be related to any other known or expected length scale in nature. Attempts to explain this new length scale fail by many, many orders of magnitude.
==endquote==

This is the core of what Kolb has to say. This length, which I said earlier is 9.3 billion ly, is not understood. Oh, and large too.

Quantum relativists, including Bianchi and Rovelli, would certainly not dispute that. A lot of thought has been devoted to understanding how that length fits into our picture of nature.
B&R have an interesting 2-page paper about it, which Kolb obviously did not know about, in which they give references to the literature going back to the 1990s.

Their take is basically that it could have to do with an intrinsic limit on angular resolution. A limit on our ability to detect and measure angle, that is analogous (although they don't say this) to the "Planck length" limit on our ability to detect and measure length, area...etc.

There is currently no concept of "Planck angle". So in effect B&R are probing into the possibility of defining one. That is what the "zero point curvature" they mention in the Nature article is actually about, as I see it.

If there actually is a minimum detectable angle (in, say, a deSitter or asymptotically deSitter universe with an intrinsic event horizon) then in Quantum Relativity one would be forced to use the quantum group instead of SU(2). One would have to use the q-deformed SU_q(2) instead of SU(2).

This has the side-effect of making certain series in LQG convergent. So it's interesting that it has an intuitive physics meaning, as well as the math significance.

Anyway, Rocky Kolb eloquently pointed out how interesting it would be to understand this event-horizon type length 9.3 billion ly. This is right in line with B&R interests. So he fails to actually engage with their position, and actually strengthens their case.
What they are basically saying, to paraphrase, is "let's not jump the gun and attribute acceleration to some queer mythical substance, when we don't actually know what underlies this length 1/√Λ."

http://arxiv.org/abs/1105.1898
A note on the geometrical interpretation of quantum groups and non-commutative spaces in gravity
Eugenio Bianchi, Carlo Rovelli
Published in Phys.Rev. D84 (2011) 027502

 

[RUTA]

To get back to serious matters, what we observe at cosmological scale is acceleration, not a possibly mythical "dark energy" substance causing the acceleration. I think it's a bad idea to use "dark energy" as a code-name for the acceleration one wants to explain. If one is actually talking about the observed acceleration, as something to be explained, one should call it acceleration.

I agree we should replace the term "dark energy" with "accelerated expansion." Then attempts to explain the data (distance modulus vs redshift) without accelerated expansion would not trip over the meaning of "dark energy." For example, in arXiv:gr-qc/0605088v2 (appeared in QGC) Garfinkle provides a "dark energy model" without accelerated expansion.

All they are saying is "let's not jump the gun and attribute acceleration to some queer or mythical energy, when we don't actually know that some actual energy is involved."

Then the title of the article should've been something like, "Do we need an energy to explain accelerated expansion?" In which case, Kolb would have to rewrite his contribution.

 

[marcus]

I agree we should replace the term "dark energy" with "accelerated expansion."...

So glad you agree! Misleading words people use over and over again can have a really crippling effect on how we think collectively. It takes intelligence and mental independence to cut through.

Then the title of the article should've been something like, "Do we need an energy to explain accelerated expansion?" In which case, Kolb would have to rewrite his contribution.

Yes! Absolutely. When you write for a magazine feature like "News and Views" my understanding is the editor has control of things like title and lead summary. In this case the editor obviously framed the discussion as he or she understood it and thought it would appeal to the readership.

Well. That's how the real world operates It's not a big deal. Better to get your message out with a little distortion than not to reach the Nature journal audience.