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

juanrga

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

[tex]R_{\mu\nu} - \frac{1}{2} g_{\mu\nu} R = - \Lambda g_{\mu\nu}[/tex]

is just so problematic as

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

marcus

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

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...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.

juanrga

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

[tex]-\Lambda g_{\mu\nu}[/tex]

can be written as

[tex]\frac{8\pi G}{c^4}T_{\mu\nu}^{DE}[/tex]

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

[tex]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}[/tex]

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

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

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

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.

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/Na...10_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"
http://www.tiptop.iop.org/full/pwa-p...wv23i01a33.pdf

juanrga

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

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

equivalent to

[tex]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}[/tex]

For vacuum

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

or (equivalent)

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

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

atyy

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?

Physics Monkey brought up a paper 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.”
==endquotehttp://www.tiptop.iop.org/full/pwa-p...wv23i01a33.pdf==

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 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.
===endquote===
Efforts are under way to understand how this "zero point curvature" arises from the underlying quantum dynamics of space-time.
As quantum relativists the authors are naturally interested in how the zero point curvature relates to QG degrees of freedom: "the intrinsic [quantum] dynamics of space-time". There have been several articles about this. For a recent examples see page 41 of the 2010 paper by Meusburger and Fairbairn--also the paper by Han (a member of the Marseille group who has co-authored with B&R.)

==continuing the B&R excerpt==
Tests on the ΛCDM model must continue and alternative ideas must be explored. But it is our opinion — and that of many relativists — that saying dark energy is a ‘great mystery’, for a force explained by current theory, is misleading. It is especially wrong to talk about a ‘substance’. It is like attributing the force that pushes us out of a turning merry-go-round to a ‘mysterious substance’...
==endquote==
For the full Nature article see:
http://www.astro.uu.nl/~vinkj/LSS/Na...10_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

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

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

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

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 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

smoit

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".