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

 Quote by tom.stoer 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

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

Recognitions:
Gold Member
 Quote by 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.

Recognitions:
Gold Member
 Quote by RUTA ... 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.

 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.
 Some other comments on the dark energy problem. http://www.nature.com/nature/journal...l/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.

Quote by marcus
 Quote by 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 )

Recognitions:
Gold Member
 Quote by simplicial Some other comments on the dark energy problem. http://www.nature.com/nature/journal...l/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/edw...y-w-kolb.shtml

Ha! I found a free link to the News and Views feature called "Is dark energy really a mystery?" http://www.astro.uu.nl/~vinkj/LSS/Na...10_Bianchi.pdf

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.

 Quote by 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 $T_{\mu\nu}^{DE}$

Recognitions:
 Quote by smoit 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?

 Quote by mitchell porter 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 this paper (and talk).
Physics Monkey brought up a paper that looks at this in a 1+1 Ising model and its gravity dual.

 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."
 Recognitions: Gold Member Science Advisor 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

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

 Quote by smoit 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.
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

 Quote by marcus 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.

 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 wich 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

 Quote by marcus 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".