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.

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

is just so problematic as

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

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

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

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

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

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

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

-\Lambda g_{\mu\nu}

can be written as

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

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

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

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

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

instead of his equation.

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

 

[marcus]

Some other comments on the dark energy problem.

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

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

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

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

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

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

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

 

[marcus]

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

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

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

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

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

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

 

[juanrga]

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

-\Lambda g_{\mu\nu}

can be written as

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

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

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

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

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

instead of his equation.

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

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

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

equivalent to

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

For vacuum

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

or (equivalent)

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

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

 

[atyy]

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

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

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

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

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

 

[RUTA]

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

 

[marcus]

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

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

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

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

 

[marcus]

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

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

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

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

 

[Fra]

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

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

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

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

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

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

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

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

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

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

/Fredrik

 

[RUTA]

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

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

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

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

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

 

[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.

 

[RUTA]

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.

Absolutely!

 

[Physics Monkey]

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 always found it interesting that Jaffe's paper does not mention what might be called the topological Casimir effect. In this case the fields are confined by the topology of compact space instead of by any "perfectly conducting sheets" and so forth. Naively, it seems that there is no coupling dependence in such a situation.

 

[atyy]

I always found it interesting that Jaffe's paper does not mention what might be called the topological Casimir effect. In this case the fields are confined by the topology of compact space instead of by any "perfectly conducting sheets" and so forth. Naively, it seems that there is no coupling dependence in such a situation.

Are you saying that although observation of the "normal" Casimir effect isn't proof that the vacuum energy exists, an observation of the topological Casimir effect could demonstrate it?

 

[marcus]

Returning to main topic of thread and e.g. RUTA's recent comments the gist seems to be that we probably have an interesting cosmological scale to understand, that being (depending on whether one writes it as a time or a length)

9.3 billion years, or 9.3 billion lightyears

You could imagine this as a limit on angular resolution---that nature has a smallest distinguishable angle. Personally I (almost) never heard of such a thing and my head spins slightly when I think of it. If I knew of some body of conventional physics research that incorporates a smallest measurable angle of the right magnitude it would not seem so farfetched.

But as it is, if I want to think of this as a "Planck angle" or extreme minimal angle I have to kind of grit my teeth and swallow hard.

Anyway here is a "Planck angle" or θ_min = (Planck length)/(9.3 billion light years) radian.

See post #103 for more about this angle--which comes up in the quantum-group version of the rotation group.

 

[marcus]

Or you could just think of Λ as the zero-point curvature that is intrinsic to nature's geometry.

And as curvature that would be an inverse area, so that 1/Λ is an area.
And therefore 1/√Λ is a length.

Which length we believe to be 9.3 billion LY based on the large amount of supernova data which has accumulated.

This zero point curvature of nature's geometry is very small, and therefore its reciprocal, the area, is very large---and therefore the corresponding natural distance scale, the squareroot of that area, is large. But still it is kind of in the same ballpark with other cosmology distance scales, so perhaps easier to assimilate as "curvature-related" idea than it is in the guise of an angle.

I calculated this zero point curvature constant back in post #10
https://www.physicsforums.com/showthread.php?p=2633720#post2633720
so if you want you can see how to get it, and update the estimate as needed. If H is the current value of the Hubble parameter then:

Λ = 3Ω_ΛH²

So 1/√Λ must be

1/sqrt( 3Ω_ΛH²) and we can try this in google calculator:

1/(sqrt(3*.73)*(71 km/s per Mpc))

When I put that into the googlebox I get 9.306 billion years...thanks google for this nice constant of nature

 

[marcus]

Or you could just think of Λ as the zero-point curvature that is intrinsic to nature's geometry.

And as curvature that would be an inverse area, so that 1/Λ is an area.
And therefore 1/√Λ is a length.

Which length we believe to be 9.3 billion LY based on the large amount of supernova data which has accumulated.
...

Today's paper by Tony Padilla and co-author picked up on the vacuum curvature idea and how you could empirically isolate it from contributions from particle physics vacuum energy. (Currently grossly overestimated using flat-geometry QFT particle physics.)
Padilla is at Nottingham, same department as John Barrett and Kirill Krasnov. Nottingham hosted the main QG conference in 2008.

http://arxiv.org/abs/1203.1040
Cleaning up the cosmological constant
Ian Kimpton, Antonio Padilla
(Submitted on 5 Mar 2012)
We present a novel idea for screening the vacuum energy contribution to the overall value of the cosmological constant, thereby enabling us to choose the bare value of the vacuum curvature empirically, without any need to worry about the zero-point energy contributions of each particle. The trick is to couple matter to a metric that is really a composite of other fields, with the property that the square-root of its determinant is the integrand of a topological invariant, and/or a total derivative. This ensures that the vacuum energy contribution to the Lagrangian is non-dynamical. We then give an explicit example of a theory with this property that is free from Ostrogradski ghosts, and is consistent with solar system physics and cosmological tests.
4 pages
Padilla's paper depends on prior work http://arxiv.org/abs/1106.2000 by Copeland et al. Copeland is also at Nottingham. This was published earlier this year in Physical Review Letters.

It would be nice if relativists could cleanse the cosmological curvature constant of contribution from QFT vacuum energy, which particle physicists are having such a hard time determining. Then instead of a "cosmological constant problem" it could be seen for what it is: simply a flatspace QFT zeropoint energy problem. Terms like "dark energy" could be dropped and the volume of hype could be turned down a notch.

 

[marcus]

This paper is something of a conundrum to me. Tony Padilla is a particle theorist by group membership and training.
How will the paper be received by relativists? Or will it not even be received---because wild and over the top? My impression after re-reading is that Padilla has a lot of nerve.
http://www.nottingham.ac.uk/~ppzphy7/webpages/people/antonio_padilla/
This may be good. Or may not. He has a highly personal style. See video clips here:
http://www.nottingham.ac.uk/~ppzap4/

On further investigation I find I'm halfway to becoming a fan of the guy. A native of Liverpool BTW, and a soccer player. BA and Masters in Math at Cambridge. Here's a YouTube about the paper I referred to earlier [ http://arxiv.org/abs/1106.2000 ]that was published in Physics Review Letters January 2012 (after the reference to the Beatles was removed). It is by Edmund Copeland, Tony Padilla, and a couple of others.
Copeland (older guy) also appears in the YouTube clip.
Tony begins discussing the cosmological constant at minute 4:50 of the clip.

Here's something about Brady Haran, the guy who made this video and a bunch more:
http://www.bradyharan.com/

 

[marcus]

As a reminder of the main topic here, this quote from Bianchi and Rovelli's piece in Nature sums up what the thread is about:

==quote B&R's piece in Nature July 15, 2010==
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===

More back a few posts in post #97 https://www.physicsforums.com/showthread.php?p=3626952#post3626952
Sorry to say I do not have a working link to the B&R piece in Nature. If you have a subscription, it's
Cosmology forum: Is dark energy really a mystery?
Eugenio Bianchi, Carlo Rovelli, and Rocky Kolb
Nature 466, 321–322 (15 July 2010)
However they make the same arguments at greater length and in more detail in this article:
Google "bianchi prejudices constant" and get http://arxiv.org/abs/1002.3966

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The new element, which I first learned of yesterday, and which is discussed in the previous two posts #111 and #112, is
this paper by two physicists in the particle theory group at Nottingham (Nottinghamsters? ):

http://arxiv.org/abs/1203.1040
Cleaning up the cosmological constant
Ian Kimpton, Antonio Padilla
(Submitted on 5 Mar 2012)
We present a novel idea for screening the vacuum energy contribution to the overall value of the cosmological constant, thereby enabling us to choose the bare value of the vacuum curvature empirically, without any need to worry about the zero-point energy contributions of each particle. The trick is to couple matter to a metric that is really a composite of other fields, with the property that the square-root of its determinant is the integrand of a topological invariant, and/or a total derivative. This ensures that the vacuum energy contribution to the Lagrangian is non-dynamical. We then give an explicit example of a theory with this property that is free from Ostrogradski ghosts, and is consistent with solar system physics and cosmological tests.
4 pages
Padilla's paper depends on prior work http://arxiv.org/abs/1106.2000 by Copeland et al. Copeland is also at Nottingham. This was published earlier this year in Physical Review Letters.

 

[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"

So far I think this is the most relevant on-target response. We shouldn't even be surprised by what B&R are saying. The observational data, of which there are massive amounts, indicate we are dealing with a curvature constant of nature. Like the speed constant c, it does not change over space and time. Or there are no indications that it does. Just a constant term in the Einstein equations governing the universe's geometry. The data that has come in since Atyy's quote was written (2007) only serves to confirm this.

The new feature, at this point, is that some people think they have a way to separate out and measure the bare L, a way to somehow discount any possible contributions of QFT "vacuum energy". That is what Tony Padilla's paper is about.

Interestingly, he has a YouTube clip talking about this and related matters for general audience. It has Padilla and a the senior author Edmund Copeland talking about a paper they published in Physical Review Letters in January 2012, that the March 2012 paper is based on.

To just get the part about the cosmological constant, skip to minute 4:50.

Copeland and Padilla are particle theorists in the HEP theory group at Nottingham---their starting point differs from that of Bianchi and Rovelli, who of course are quantum relativists. The conclusions, though, are remarkably compatible.

What they are talking about in the YouTube is their January paper where they developed a mathematical technique to "degravitate" the QFT vacuum energy, to zero-out its effect. So that left the road clear for Padilla's March 2012 paper where he presents this as a way to determine the clean bare curvature constant Lambda (devoid of of QFT contribution).
See the abstract and link for Cleaning Up the Cosmological Constant, in the preceding post.

 

[atyy]

Dark energy is of course not a "real" problem, it's a question of fine tuning, like the hierarchy problem. There's no inconsistency, so if our theories were ultimate theories, there'd have no problem. But if we believe our theories are just temporary, then fine tuning suggests new physics. Of course since we don't know if our theories are by amazing good luck ultimate theories, solving a fine tuning problem is a matter of taste. Anyway, in line with the Fab Four, how about:

Ellis, Inhomogeneity effects in Cosmology

DGP 4D Gravity on a Brane in 5D Minkowski Space

Nicolis, Rattazzi, Trincherini The galileon as a local modification of gravity

 

[marcus]

Hi Atyy, I shall argue that Padilla has an unusually interesting modification of gravity here and one that can be tested. It is a variant of ordinary GR that reproduces the solar system tests of GR to satisfactory precision. IOW it is just as good as GR as far as we are able to tell.
So far! (There is more testing work to be done.)

In this variation on GR, the coupling to matter is mediated by a scalar field (not the Higgs field but something distantly analogous.)

I invite you to watch the YouTube of Copeland and Padilla, or perhaps you already have!
It's amusing in spots, and enlightening, I think. It was made BEFORE Padilla came around to the "cleaning up" idea to get a pure constant curvature.

BTW Padilla gave a presentation of this last July at PASCOS 2011 (Cambridge DAMPT)
http://www.damtp.cam.ac.uk/research/gr/workshops/PASCOS/2011/presentations/Padilla-pascos2011.pdf
PASCOS 2011 = The 17th annual symposium on Particles, Strings, and Cosmology.
http://www.damtp.cam.ac.uk/research/gr/workshops/PASCOS/2011/

If you happen to glance at the program, he gave his talk Wednesday 6 July in Session 3: Modified Gravity. So I guess we have to call what he is talking about "modified gravity" though it is different from, and to me more interesting than all the other modified gravities that I am used to seeing. This scalar field that mediates the coupling of matter to geometry is quite elegant, I think. And it makes the curvature impervious to QFT vacuum energy, so no wonder we don't see a gravitating vacuum energy effect!

This opens the door for relativists to offer a geometrical explanation for why the curvature constant Lambda is what it is.

 

[atyy]

Padilla mentions the Galileon stuff in the introduction of his PRL paper, and says that it is closely related. The Galileon stuff was originally motivated by DGP - which passed First-Year Sloan Digital Sky Survey-II (SDSS-II) Supernova Results: Constraints on Non-Standard Cosmological Models.

 

[marcus]

Padilla mentions the Galileon stuff in the introduction of his PRL paper, and says that it is closely related. The Galileon stuff was originally motivated by DGP - which passed First-Year Sloan Digital Sky Survey-II (SDSS-II) Supernova Results: Constraints on Non-Standard Cosmological Models.

Good point! What about his current paper? The March 2012 one. My impression is that by the March paper he is on to a fairly unique approach, one he calls "novel". At that point comparison with other non-standard variants of GR is no longer appropriate. You may find evidence to the contrary--I don't see any.

 

[atyy]

Good point! What about his current paper? The March 2012 one.

Hmmm, seems unrelated to the Fab Four idea.

 

[marcus]

Hmmm, seems unrelated to the Fab Four idea.

You know, I think you're right! He cites the Fab Four paper when he really doesn't need to; just in passing to make a minor point.