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

by marcus
Tags: constant, dark energy, fake, prejudices, probem
 Astronomy Sci Advisor PF Gold P: 22,677 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 θmin = (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.
 Astronomy Sci Advisor PF Gold P: 22,677 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 = 3ΩΛH2 So 1/√Λ must be 1/sqrt( 3ΩΛH2) 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
Astronomy
PF Gold
P: 22,677
 Quote by 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
(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.
Astronomy
PF Gold
P: 22,677
 Quote by atyy 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.

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.
 Sci Advisor P: 7,917 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
 Astronomy Sci Advisor PF Gold P: 22,677 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.
 Sci Advisor P: 7,917 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.
Astronomy
PF Gold
P: 22,677
 Quote by 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.
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.
P: 7,917
 Quote by marcus Good point! What about his current paper? The March 2012 one.
Hmmm, seems unrelated to the Fab Four idea.
Astronomy
PF Gold
P: 22,677
 Quote by atyy 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.