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bill alsept
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Marcus, I believe I'm interested in the subject of your last three postings but it’s a little beyond me. Do you have the patience to explain it one more time in a simpler way? Thanks
Patrick,PatrickPowers said:I have read that Albert Einstein declared his introduction of the the cosmological constant greatest blunder of his life.
marcus said:"Dark energy" is a phony idea. "Dark energy problem" is hype. Case closed. So that's simple enough.
"No need to get it confused with the QFT Vacuum Energy. That is QFT's problem, they calculate something in a fixed flat geometry context (quite alien to GR) and it comes out ridiculously wrong. So they should deal with it."
==quote FLS http://arxiv.org/abs/1103.4841 ==marcus said:The Finazzi-Liberati-Sindoni (FLS) paper could be something of a game-changer, so I want to back up and reconsider what I was saying. Here is an excerpt from their conclusions...
atyy said: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"
Harv said:Does anyone here doubt that quantum vacuum energy exists?
marcus said:The tendency in observational cosmology in recent years has been to confirm and accept that Lambda is in fact simply a constant and not necessarily connected with the naive QFT calculation of vacuum energy (which is after all based on a non-quantum static flat Minkowski geometry.) To some extent this is a matter of one's background and opinions---I'm not talking about diehard QFT-ers, this is the trend I see in observational cosmology. Here are some illustrative links:
That is really really close to -1. As time goes on the constraints seem to tighten and I hear less and less about Lambda considered as an actual "energy". We may be getting closer to accepting it simply as a small constant amount of curvature.
i.e. You can cancel the QFT vacuum energy, and account for the observed dark energy, by supposing that the cosmological constant = "dark energy - QFT vacuum energy".Haelfix said:It is not that we have a theory that gives a wrong prediction. We can make our theories give the right value.
mitchell porter said:But doesn't the QFT vacuum energy depend on the high-energy cutoff? (except when it's always exactly zero at all scales). In which case, the value of the cosmological constant required by the strategy above, will depend on the cutoff.
I can see two ways around this.
marcus said:Be that as it may, I think it would be a good idea if people who want to discuss in this thread would simply READ the relevant section of the paper.
http://arxiv.org/pdf/1002.3966
It starts on page 5. The relevant section is:
IV. THE VACUUM ENERGY IN QUANTUM FIELD THEORY
Perhaps it would help focus discussion if I were to paste some excerpts in. Then those who have read section IV (relevant to our discussion) could refer to some immediately visible text.
1002.3966v3 said:Does this large energy exist for real? That is, does it have observable eects? In particular: does it act as a source for the gravitational eld, as all forms of energy are known to do? Does it have a gravitational mass (and therefore an inertial mass)?
...
In fact, simple physical arguments indicate that the vacuum energy, by itself, cannot be \real" in the sense of gravitating: if it did, any empty box containing a quantum eld would have a huge mass, and we could not move it with a force, since gravitational mass is also inertial mass.
...
On physical grounds, vacuum energy does not gravitate. A shift in vacuum energy does gravitate.
mitchell porter said:i.e. You can cancel the QFT vacuum energy, and account for the observed dark energy, by supposing that the cosmological constant = "dark energy - QFT vacuum energy".
But doesn't the QFT vacuum energy depend on the high-energy cutoff? (except when it's always exactly zero at all scales). In which case, the value of the cosmological constant required by the strategy above, will depend on the cutoff.
Haelfix said:In short, properly understood, the cosmological constant problem is essentially an *infrared* problem, not an ultraviolet one. It is another example of a hierarchy problem in physics, except this time the relevant scales are the difference in size between the Hubble scale and particle physics (as opposed to particle physics and the Planck scale)...
For starters here's a clarifying passage from page 6.marcus said:...
iii. The enormous difference between the small value of the cosmological constant revealed by the cosmic acceleration and the large value that can be derived from quantum field theory.
I think we are mainly concerned with point iii here. A person steeped in QFT viewpoint may view Lambda as a classical fudge or lifeline, to correct for the stupendous ZPE calculated from non-QGR-based QFT.
That is he may think of the embarrassing 120-order-of-magnitude QFT vacuum energy discrepancy as in some sense "correct" but just needing to be "canceled" by some Lambda lifepreserver that the other people are responsible for.
As this points out, there is another possible perspective on the embarrassing QFT discrepancy. That is: it is a QFT problem---probably showing that QFT needs some foundational work. One might for example speculate that the embarrassing vacuum energy might go away if QFT would simply stop using Minkowski geometry, and ground itself in quantum relativistic geometry.
Be that as it may, I think it would be a good idea if people who want to discuss in this thread would simply READ the relevant section of the paper.
http://arxiv.org/pdf/1002.3966
It starts on page 5. The relevant section is:
IV. THE VACUUM ENERGY IN QUANTUM FIELD THEORY
Perhaps it would help focus discussion if I were to paste some excerpts in. Then those who have read section IV (relevant to our discussion) could refer to some immediately visible text.
Bianchi and Rovelli said:We think that the origin of the confusion is that there are two distinct ways of viewing the cosmological term in the action. The first is to assume that this term is nothing else than the effect of the quantum fluctuations of the vacuum. Namely that λ = 0 in (21) and the observed acceleration is entirely due to the radiative corrections Λ (in the above notation). The second view is that there is a term λ in the bare gravitational lagrangian, which might (or might not) be renormalized by radiative corrections. The two points of view are physically different. We think that the common emphasis on the first point of view is wrong.
In other words, it is a mistake to identify the cosmological constant λ with the zero point energy Λ of a QFT, for the same reason one should not a priori identify the charge of the electron with its radiative corrections.
Haelfix said:Now the separate confusion is that there is absolutely no problem whatsoever in moving the cosmological constant term from the left side to the right side of the Einstein field equations in general. You can always do that!
That does not change the predictions or physics in any way, in particular whether the term is renormalized or not!
mitchell porter said:smoit #56, are you saying that for the 10-dimensional SO(16) x SO(16) heterotic string, you can get the observed cosmological constant by assuming a physically reasonable supersymmetry scale? If so, could you then look for a way to compactify six dimensions without adding to the vacuum energy?
marcus said:There is no known natural way to derive the tiny cosmological constant that plays a role in cosmology from particle physics
...
it means that there is something we do not understand yet in particle physics. What could this be?[/INDENT]
mitchell porter said:smoit #56, are you saying that for the 10-dimensional SO(16) x SO(16) heterotic string, you can get the observed cosmological constant by assuming a physically reasonable supersymmetry scale? If so, could you then look for a way to compactify six dimensions without adding to the vacuum energy?
marcus said:In other words, it is a mistake to identify the cosmological constant λ with the zero point energy Λ of a QFT, for the same reason one should not a priori identify the charge of the electron with its radiative corrections
marcus said:They say it is a mistake to identify the cosmo constant with the QFT zero point energy, and you obviously agree since you claim that no sensible particle theorist would confuse the two.
marcus said:An effect commonly put forward to support the “reality” of such a vacuum energy is the Casimir effect. But the Casimir effect does not reveal the existence of a vacuum energy: it reveals the effect of a “change” in vacuum energy, and it says nothing about where the zero point value of this energy is. In fact, simple physical arguments indicate that the vacuum energy, by itself, cannot be “real” in the sense of gravitating: if it were, any empty box containing a quantum field would have a huge mass, and we could not move it with a force, since gravitational mass is also inertial mass. On physical grounds, vacuum energy does not gravitate. A shift in vacuum energy does gravitate. This is nicely illustrated by an example discussed by Polchinski in [3]:...
marcus said:So far we don't see it getting fixed by those means, however.
smoit said:We? Meaning all the retired mathematicians who post on the physics forum?
Dark energy is a hypothetical form of energy that is believed to make up about 68% of the universe. It is thought to be responsible for the observed accelerating expansion of the universe. However, some scientists argue that dark energy is a fake problem because it is based on assumptions and has not been directly observed or measured.
Scientists study dark energy through observations of the universe's expansion and the distribution of galaxies. They also use mathematical models and simulations to understand its effects on the universe. However, since dark energy has not been directly detected, these methods are still being refined and debated.
Some alternative theories to dark energy include modified theories of gravity, such as MOND (Modified Newtonian Dynamics), which suggest that our understanding of gravity may need to be revised. Other theories propose that the universe is not expanding at all, but rather that the observed effects can be explained by other factors.
The cosmological constant is a term in Einstein's theory of general relativity that represents a constant energy density in space. Some scientists argue that dark energy can be explained by the cosmological constant, while others believe that the two concepts are not equivalent and that dark energy is a separate phenomenon.
While there is no direct evidence for dark energy, there is observational evidence that suggests the expansion of the universe is accelerating. This evidence comes from studies of distant supernovae, the cosmic microwave background, and the large-scale structure of the universe. However, the interpretation of this evidence and its relationship to dark energy is still a subject of debate among scientists.