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Why all these prejudices against a constant? ( dark energy is a fake probem) 
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#109
Dec3111, 11:42 AM

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PF Gold
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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 resolutionthat 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 anglewhich comes up in the quantumgroup version of the rotation group. 


#110
Dec3111, 11:59 AM

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PF Gold
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Or you could just think of Λ as the zeropoint 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 largeand 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 "curvaturerelated" 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_{Λ}H^{2} So 1/√Λ must be 1/sqrt( 3Ω_{Λ}H^{2}) 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 


#111
Mar612, 10:10 PM

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PF Gold
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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 zeropoint 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 squareroot 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 nondynamical. 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. 


#112
Mar712, 12:47 PM

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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 receivedbecause wild and over the top? My impression after rereading 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/ 


#113
Mar712, 02:11 PM

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PF Gold
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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 illunderstood effect of QFT’s vacuum energy — or that of any other mysterious substance. Λ is a sort of ‘zeropoint curvature’; it is a repulsive force caused by the intrinsic dynamics of spacetime. ... ... 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 merrygoround 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 zeropoint 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 squareroot 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 nondynamical. 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. 


#114
Mar812, 07:59 PM

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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 Nottinghamtheir 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 zeroout 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. 


#115
Mar812, 09:20 PM

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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 


#116
Mar812, 10:13 PM

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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. 


#117
Mar812, 10:40 PM

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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 FirstYear Sloan Digital Sky SurveyII (SDSSII) Supernova Results: Constraints on NonStandard Cosmological Models.



#118
Mar812, 11:00 PM

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PF Gold
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#119
Mar812, 11:46 PM

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#120
Mar912, 12:10 AM

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PF Gold
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#121
Mar912, 12:51 AM

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PF Gold
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Here's the link to Padilla's article again:
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 zeropoint 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 squareroot 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 nondynamical. 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 ==a key quote== In summary, then, we have proposed a novel way to clean up the cosmological constant problem. By coupling matter to a composite metric, g ̃ab(φ, ∂φ, . . .), satisfying the property (1), we have been able to eliminate the troublesome vacuum energy from contributing to the dynamics of the system. Thus one ought to be able to choose the vacuum curvature to take on an empirical value, as dictated by observation, with a clean conscience. This is the take home message of this paper. ==endquote== ===============EDIT TO REPLY TO NEXT POST================ Hi Paulibus, since I can still edit I'll reply here. I simply agree. It makes sense to me too. So much for "dark energy". I doubt we're ever going to be able to use it to run the family car or grind the corn to make the tortillas! But I wanted to say that I relished your reference to that perceptive French expression for nonBaconian speculations (la haute poppicocquerie) and was glad to hear about Helge Kragh's book "Higher Speculations  Grand Theories and Failed revolutions in Physics and Cosmology" O.U.P, 2011. 


#122
Mar912, 01:28 AM

P: 175

technique would also “deinertialise ” the QFT vacuum energy? Seems reasonable to me. I can’t see how either the gravitational or inertial mass of the entire universe’s vacuum energy could be detected and measured; spring balances and accelerating rockets are useful only for relatively local stuff. 


#123
Mar912, 08:21 PM

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PF Gold
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Hi Paulibus, beyond what I said in post #121 by way of reply
I want to emphasize the testabiliity angle which is one of the possibilities that makes this proposal exciting: ==quote page 4, http://arxiv.org/pdf/1203.1040v1.pdf == As we have seen, any solution to GR, with arbitrary cosmological constant, is a solution to our theory. However, it is clear that the reverse is not true. Our theory is expected to permit solutions that are not present in GR. This opens up the possibility of finding some interesting and potentially testable new features. Work is under way to study the impact of these new features, beginning with cosmological solutions of the specific model presented here. ==endquote== Also to reiterate the main result for clarity ==quote page 2== Actually, we can go even further. Any solution of GR, vacuum or otherwise, is also a solution to our theory, whatever the value of the vacuum curvature. As the vacuum energy drops out of the dynamics, we are free to choose the vacuum curvature with a clean conscience. Indeed, one can straightforwardly check that the field equations are satisfied by the choice, G ̃_{ab} = −Λ ̃g ̃_{ab} + τ_{ab}, T ̃_{ab} = −σg ̃_{ab} + τ_{ab} ( 9 ) where τ_{ab} describes the matter excitations above the vacuum, σ is the vacuum energy, and Λ ̃ is the vacuum curvature. This follows from the fact that the equations of motion are linear in E ̃_{ab}, with constant contributions dropping out completely. In particular, this means that the standard ΛCDM cosmology, with Λ chosen empirically without any concern, is a perfectly good solution to our theory, and does not suffer from the same fine tuning issues as the corresponding solution in GR. ==endquote== Assuming this initiative goes thru, the ball is now in the relativist's court. It is they who must explain the value of the constant vacuum curvature, if it needs explaining. Does it need any more explanation than, for example, the value of Newton G? Perhaps, perhaps not. We'll see. 


#124
Apr912, 12:32 PM

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PF Gold
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Posts about "exotic matter and the Casimir engine" and "exotic matter and the Casimir effect" wouldn't really belong in this thread.
What's being discussed in the thread is the widely (but not universally!) shared view that the observed expansion speedup is best explained as simply due to a constant Λ which occurs naturally (along with Newton G) in the Einstein equation. These two constants G and Λ are those allowed by the symmetry of the theory and so *must* appear in the equation. Either or both could, of course, have turned out to be zero, but in fact neither did: both are positive. This is the (classical) equation which to the best of our knowledge governs the evolution of geometry. So it's inappropriate to talk about "dark energy" or "zero point energy" in this context, any more than one would drag in such stuff in connection with Newton G. There need be no mention of "exotic matter" fields to explain the value of Λ any more than one needs such inventions to explain the value of G. Seen from this perspective, since today's quantum field theory is based on a rigid flat geometry in which dynamic expanding geometry necessarily cannot be captured, it can have little or no relevance to explaining Λ: whatever oversized value of "vacuum energy" QFT theoreticians might come up with has no bearing on the observed value of the cosmological constant. They just need to go back and get their flatspace vacuum energy right. For an interesting recent contribution to this discussion see Kimpton Padilla's paper. 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 zeropoint 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 squareroot 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 nondynamical. 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 


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