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

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

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.

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

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.
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

Absolutely!

Physics Monkey

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

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
http://physicsforums.com/showthread....20#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

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...tonio_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.
http://www.youtube.com/watch?v=AlmVKJelaAk 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 http://physicsforums.com/showthread....52#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

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

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

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.
http://www.youtube.com/watch?v=AlmVKJelaAk
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/...pascos2011.pdf
PASCOS 2011 = The 17th annual symposium on Particles, Strings, and Cosmology.
http://www.damtp.cam.ac.uk/research/...s/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

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

Hmmm, seems unrelated to the Fab Four idea.