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

smoit

We? Meaning all the retired mathematicians who post on the physics forum?

marcus

Yes Mr. Smolin-and-Woit and despite a bt of sarcastic snarling and grumbling we and our like are legion.

But in fact when I said "we" I was interpreting from the passage from Bianchi and Rovelli that I quoted in post #65:

"Why does standard QFT have so much trouble adjusting to this straightforward physical fact? We do not know the answer, but ..."

marcus

Since we are discussing the argument on page 7 of the Bianchi Rovelli paper, I should give the link again:
http://arxiv.org/abs/1002.3966
Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
(Submitted on 21 Feb 2010)
The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.
9 pages, 4 figures

An easy way to get the paper is simply to google "constant prejudices"
The arxiv link should turn up as the first or second hit.
Anyone coming in new would be well-advised to read the paper. It is easy to understand and puts the discussion here in a clearer light.

smoit

FYI, after almost 2 years, the authors have so far failed to publish it in a refereed journal.

Harv

But we can't verify the coupling of gravity directly to the individual loops involved in these quantum contributions.

smoit

The shift in the electrostatic energy due to vacuum polarization (experimentally measured in the Lamb shift) contributes an amount large enough that, in case if it did not gravitate, would violate the equivalence principle to a precision of one part in a million. However, we can experimentally verify the equivalence principle to a precision of one part in 10¹² and therefore these loops must couple to gravity.

Fra

I'm not sure if you refer to some new experiments I'm unaware of but I think it's still important to note which domains to theory space where certain "principles" are tested. Ingoring that is one of the things I find most disturbing, extrapolations of "evidence" into new domains are often made without much argument. This is exactly what people also do with things that are well tested for PARTICLE physics which usually means the observer is in a classical laboratory and the system is a very small subsystem. Inferences from such situations just don't generalize to cosmological scenarios. This fact is often ignored on grounds that "such extrapolations worked in the past".

I suspect the 10^12 test you refer to is the classical mechanics test of the torsion pendulum, right?

If there really experiments made that verifies the equivalence principle for actual lamb shift, that would be news for me. I think it's in principle testable, but the problem is as far a I know that the gravitational field on earth is too weak to yield much of a significant possibility to test it here?

But I do not follow all new experiemtns, if someone konws of an actual test of the equivalence principle for lamb shifted systems I would be interested to read about how the experiment was conducted. I have seen some old papers where it was "in principle testable" was devicded, but the conclusion was that in practice it wasn't becaues hte gravity on earth is so weak.

I'm not sure if some astronomic observations of lambshifted systems near more massive bodies is possible? I'm not sure how that would be done.

/Fredrik

smoit

@Fra, Here is a reference that is the most referred in the literature but you'd have to go to your library to actually read it: http://slac.stanford.edu/spires/find...ww?irn=6818293

Please, read very carefully what I said in my previous comment b/c your response indicated that you had misunderstood it.

Harv

I looked at Polchinski's paper and understand now why you're right. Thanks.

smoit

@Harv, no problem! Joe's article is good and pretty much sums up the current situation. I saw his talk on this topic at KITP back in 2006, you can find it here if you're curious: http://online.itp.ucsb.edu/online/st...06/polchinski/

Haelfix

Yea tis true, thats why I qualified my statement. For the nonexperts, what this means in practise is that you need to know all the fields and matter all the way up to the Planck scale, as they will all contribute (naively increasing in magnitude, not necessarily in sign) contributions to the total constant. You can't ignore them. This is also why no first principles solution to the problem exists and probably never will exist (absent the discovery of some highly constraining mechanism or symmetry).

However I think I am correct in pointing out that the apparent magnitude of the problem occurs b/c of the scales mismatch, which is essentially set by the infrared physics.
Yep thats interesting, but I don't quite understand these constructions. What is MSusy specifically here (is it arbitrary)? Also, why wouldn't the mechanism that generates these nonperturbative corrections at the low energy scale (in the language of the effective field theory) not also generate unwanted and observable KK states?

As far as numerology goes. I like the following two observations as well. If you take the cutoff to be the mass of the lightest neutrino, the scales match. Another weird coincidence... The supersymmetry breaking scale seems to be exactly halfway (on a logarithmic scale) between the vacuum energy scale and the Planck scale. Why?

smoit

By M_SUSY I just denoted a generic scale of level spacing. Its value should be related to the string scale, I think, since SUSY is broken at the string scale in this construction.
That's why I did not really want to identify M_SUSY with the scale obtained in some 4D EFT from the soft breaking but the numerology looks cute
You probably mean perturbative instead on non-perturbative, right? This computation is in 10D, so there are no KK states involved. That said, at such extremely short distances even in a compactified 4D vacuum all the KK modes become light and the theory does effectively become 10 or maybe 11 dimensional. I think that what one really needs here for a realistic computation is to translate soft SUSY breaking in some 4D EFT into the splittings in the entire string spectrum and then compute the one-loop partition function. My hunch is that the scale of the boson-fermion splitting in the string levels would be related to the gravitino mass scale instead of the string scale and the result of the computation may actually give the correct order of magnitude. What was really neat for me to learn was that the finiteness of the CC in a non-tachyonic non-SUSY string theory is guaranteed by the modular invariance, despite the presence of an infinite tower of contributions in the UV. Here is a nice reference where you can read about these ideas: http://arxiv.org/PS_cache/hep-th/pdf/9503/9503055v2.pdf

Fra

Smoit thanks for the link. I haven't read it but that indeed looks like the classical paper I also thought you meant. It could well be that I didn't get your point at all, in that case I'm sorry.

Anyway my point was this: That paper tests WEP to one part in 10^12, but it's all classical mechanics (torsion balance) and relatively speaking macroscopic classical systems (which is dominated by baryonic mass) with classical measurements.

Thus I question the validity of that test when applied to situations where the classical mechanics framework just don't hold. Also just as a ballpark number it seems the contribution of lamb shift to the classical level mass is the order of 1 in 10^15 or so? Which seems to be beyond hte level os current experimental tests?

So I didn't quite get how that classical mechanics test of WEP for 1 part in 10^12 gives any information about the the nature loop corrections in general (which then of course goes outside classical mechanics)?

Perhasp I'm missing something, could you explain?

/Fredrik

Fra

Trying to make sure I understand the logic, let me know if I get it wrong:
Are you suggesting that (when considering the origin of mass) since the idea is that the actual classical masses (such as those in the 1971 torsion balance experiment) are largely made up of confined energy such as confined virtual gluons etc, therefore the conclusion is that all such "virtual energies" as infered by all observers(?) must contribute equally to both inertial and gravitatonal mass?

/Fredrik

smoit

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.

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.

smoit

I said nothing about virtual gluons and their contributions to the rest mass of baryons b/c I didn't want to get into discussing lattice QCD results for hardon masses but yes, virtual gluon loops do contribute a significant portion of a baryon's inertial mass and such effects are much more significant than the virtual photon loops Polchinski talks about.

RUTA

I read the Bianchi & Rovelli paper as well as Chap VIII.2 “The Cosmological Constant Problem and the Cosmic Coincidence Problem” in Zee, Quantum Field Theory in a Nutshell, Princeton Univ Press, 2003 (p 434). I do not find any disagreement regarding the facts, only their reactions thereto. For example:

B&R write, “But to claim that dark energy represents a profound mystery, is, in our opinion, nonsense.” In contrast, Zee introduces this subject by saying, “I now come to the most egregious paradox of present day physics.”

Regarding the coincidence problem, B&R write, “it is quite reasonable that humans exist during those 10 or so billions years (sic) when [tex]\Omega_b[/tex] and [tex]\Omega_\Lambda[/tex] are within a few orders of magnitude from each other.” In contrast, Zee writes, “the epoch when [tex]\rho_M \sim \Lambda[/tex] happens to be when galaxy formation has been largely completed. Very bizarre!”

Both agree that (per B&R) “There is no known way to derive the tiny cosmological constant that plays a role in cosmology from particle physics. And there is no understanding of why this constant is not renormalized to a high value.” B&R’s reaction to this fact is, “But this does not means (sic) that there is something mysterious in the cosmological constant itself: it means that there is something we do not understand yet in particle physics.” While Zee writes, “But Nature has a big surprise for us. While theorists racked their brains trying to come up with a convincing argument that [tex]\Lambda = 0[/tex] observational cosmologists steadily refined their measurements and recently changed their upper bound to an approximate equality [tex]\Lambda \sim (10^{-3} \mbox{ev})^4!!![/tex] The cosmological constant paradox deepens.”

In short, the B&R paper merely argues for a particular emotional reaction to the situation regarding the cosmological constant in physics and cosmology. I would be surprised to see this paper published in a physics journal, since it does not expand upon our knowledge of physics. However, the paper’s use of a timely topic to highlight one aspect of our failure to unify the Standard Model with gravity is not without value. I could see this included in the proceedings for a philosophy of science symposium, for example. It would also be appropriate for a pedagogical journal in physics. It certainly motivated me to look more deeply into the issue. Thanks for posting this, marcus.

Edit: I'm trying to figure out how to use TeX in PF. Obviously, I haven't found an "in line" tex command yet.