Is there an unambiguous 'Dark Energy' signal in the relevant astronomical data?

[Nereid]

This poll is a companion to the one Wallace posted recently: https://www.physicsforums.com/showthread.php?t=168166"

Just as there is a certain ambiguity in that poll, so too there is ambiguity in this one.

For example, you may feel that the relevant astronomical data themselves are clear - after all, that instrument X attached to telescope Y detected Z photons between t1 and t2, while pointed in direction [RA, Dec] is pretty darn unambiguous, is it not? - but that the processing (flatfielding, calibration, transforms, ...) or contextualisation or characterisation (t days after maximum of a 1a SNe at redshift z with galactic absorption of {a} and intrinsic reddening of {b} ...) are iffy.

FWIW, the astronomical data that most unambiguously says 'dark energy' are sets of observations of high-z supernovae, with support coming from observations of the CMB and large-scale structure. The consistency between these three sets of quite independent astronomical observations is also generally regarded as indicating the 'DE signal' is real.

 

[Wallace]

In the broadest definition of DE as a being signal that something is wrong with a GR+Matter Universe it's very clear that 'it' exists. What that 'it' is (a real energy, simple modification to GR such as a CC, or more serious modified gravity/string theory/Braneworld etc etc) is another matter.

It is however a done deal that the observations rule out a GR + some amount of regular (by which I mean w=0) matter Universe. The Milne model, Einsten-deSitter model etc are ruled out to enormous degrees of confidence by many types of independent observations.

The question of what the 'it' of DE (using the very broad definition of DE) is is far less certain, but it requires an amazing degree of theoretical prejudice to insist that the overwhelming body of evidence is all flawed in uniquely different ways as to conspire to give us a coherent but erroneous signal.

 

[marcus]

In the broadest definition of DE as a being signal that something is wrong with a GR+Matter Universe it's very clear that 'it' exists. What that 'it' is (a real energy, simple modification to GR such as a CC, or more serious modified gravity...

I agree, as an observer from the sidelines it looks to me also that there is a clear unambiguous signal that something is wrong with a model that is zero-CC General Relativity+Matter.

I am glad that Nereid put quotes on and called it a 'Dark Energy' signal.

That is what this unexplained gap in the model is often called so I use the term happily to repeat the point Wallace made namely that it isn't yet clear that this 'Dark Energy' actually is a real energy.
The gap may turn out to be attributable to the need to correct the law of gravity----to make an adjustment in the equation of GR.
There may be nothing there to call an energy---there may simply be an error in the 1915 version of the law that we are using and which shows the gap.

So I will answer YES in the poll---there is an unambiguous signal.

 

[smallphi]

The problem could be that we are using homogeneous FRW model to match observations while universe is obviously inhomogeneous on scalles smaller than 100 Mpc. Nobody knows how to treat inhomogeneous GR though.

 

[Wallace]

Ahh yes excellent point smallphi, you are right I think.

It is unlikely that the inhomogeneous nature of the Universe is solely responsible for the signal we call DE, but it hasn't been proven since as you say we havn't been able to do a full inhomogeneous GR treatment. Perturbation theory says this is not the case, but perturbation theory is not exact and GR is a funny thing...

 

[marcus]

The problem could be that we are using homogeneous FRW model to match observations while universe is obviously inhomogeneous on scalles smaller than 100 Mpc. Nobody knows how to treat inhomogeneous GR though.

Do you happen to know David Wiltshire's paper or papers about that. I only have a slight familiarity. I don't think the idea has caught on but I seem to recall some half-dozen people looking into that.

If you don't know Wiltshire's work please tell me and I will look up references.
I think he is a New Zealander

=========================
EDIT since i can still edit my post I think I will reply to your #7 here.
I didn't read the Wiltshire papers about this---and I think there were some by some Italians too. I am glad you had a look. Perhaps you should give a link to the Wiltshire paper you are talking about, so in case anyone wants to discuss it with you they can be sure which it is.

I think we had a thread or two about one or another Wiltshire paper sometime in the past year, at this forum.

 

[smallphi]

I've read it but I don't understan how rigorous his point it. As far as I could pick it up his idea is that we, as observers, can't be described as comoving observers in an averaged FRW model. There is difference between our local time and the cosmic time in FRW model. That's what I think he says.

People before him always assume that the difference between us and a comoving observer in FRW would be vanishingly small but who knows ...

Here is the link: http://arxiv.org/abs/gr-qc/0702082

 

[Wallace]

I think Rocky Kolb (or Edward Kolb in the literature) and some collaborators have done some work on this also.

 

[marcus]

I've read it but I don't understan how rigorous his point it. As far as I could pick it up his idea is that we, as observers, can't be described as comoving observers in an averaged FRW model. There is difference between our local time and the cosmic time in FRW model. That's what I think he says.

People before him always assume that the difference between us and a comoving observer in FRW would be vanishingly small but who knows ...

Here is the link: http://arxiv.org/abs/gr-qc/0702082

Thanks for the link. I had seen an earlier Wiltshire paper purporting to explain acceleration and was not aware of this one. Perhaps I saw the abstract earlier but didnt register how interesting it might be.

Your link gives a link to an expanded summary at Wiltshire's website http://www2.phys.canterbury.ac.nz/~dlw24/universe/

If OK with everybody I would like to copy the abridged abstract here, I personally can't even make a stab at assessing this, but Wiltshire is reputable and this approach is sufficiently different so I hope we can look at it. Maybe someone will find a critique that disposes of it. It might also be valid which would rattle a few windows.

Cosmic clocks, cosmic variance and cosmic averages
David L. Wiltshire
72 pages, 5 figures

"Cosmic acceleration is explained quantitatively, purely in general relativity, as an apparent effect due to quasilocal gravitational energy differences that arise in the decoupling of bound systems from the global expansion of the universe. "Dark energy" is recognised as a misidentification of those aspects of gravitational energy which by virtue of the equivalence principle cannot be localised, namely gradients in the energy associated with the expansion of space and spatial curvature variations in an inhomogeneous universe, as we observe. Gravitational energy differences between observers in bound systems, such as galaxies, and volume-averaged comoving locations within voids in freely expanding space can be so large that the time dilation between the two significantly affects the parameters of any effective homogeneous isotropic model one fits to the universe. A new approach to cosmological averaging is presented, which implicitly solves the Sandage-de Vaucouleurs paradox. When combined with a nonlinear scheme for cosmological evolution with back-reaction via the Buchert equations, a new observationally viable quantitative model of the universe is obtained. The expansion age is increased, allowing more time for structure formation. The baryon density fraction obtained from primordial nucleosynthesis bounds can be significantly larger, yet consistent with primordial lithium abundance measurements. The angular scale of the first Doppler peak in the CMB anisotropy spectrum fits the new model despite an average negative spatial curvature at late epochs, resolving the anomaly associated with ellipticity in the CMB anisotropies. A number of other testable consequences are discussed, with the potential to profoundly change the whole of theoretical and observational cosmology. [Abridged]

Ive started reading the paper. Impressed.

 

[Garth]

Would someone list the different signals of DE?

I say a signal does exist, but it is ambiguous.

Garth

 

[Chronos]

A clear choice exists, IMO. Rewrite GR or acknowledge a clear signal of something that resembles DE is evident.

 

[zankaon]

No long range gravity wave propagation?

'The emperor has no clothes'
The gravity wave observatories have no g.w.s.;
in spite of sufficient exquisite sensitivity and a large enough statistical volume. Hence consistent with a stiff pseudo-Riemannian spacetime manifold. Gravity waves would seem to be damped out quickly near sources. Hence the manifold would seem robust to perturbation. Such resistance to deformation would contribute energy to stress energy momentum tensor, and hence to left side of Einstein eq. i.e. Einstein tensor contraction, or curvature tensor. Hence hard qualitative evidence of at least one dark energy candidate (i.e. contributing to critical energy density sufficient for flatness). p.s. the vote answer was yes.
See previous more elaborate thread that follows:

09-16-2007 at 12:12 AM. cosmology thread, with minor edit

Do We Already Have Evidence Of Dark Energy - Our Manifold?

If LIGO I with it’s 10^-21 sensitivity, VIRGO etc. don’t detect gravity waves, might this then be interpreted as indicating that C_R pseudo-Riemannian spacetime continuum (i.e. manifold’s) stiffness is not INsignificant; rather than the assumption that g.w.s propagate long distance, and that it just requires a more sensitive detector? Statistically LIGO I seems to have a large enough volume and sample size for inclusion of compact objects in NS and BH binary systems in tight orbits at least, even if not catching any coalescing events. However even for binary coalescence of BHs, might generated {g.w.} decay very rapidly? So resistance to deformation (normal stress: extension and compression, and even any shear stress) might not be insignificant. Might such stiffness (resistance to deformation/distortion) be considered as like inertia of C_R manifold? That is, {g.w.} have non-localized energy, but such energy is associated with deformation of manifold. Hence such {g.w.} energy might be considered as trying to overcome resistance to deformation (i.e. stiffness) of C_R manifold. Hence such inertia of manifold (resistance to deformation) would seem to represent a contribution to stress energy momentum tensor and it’s matrix representation; thus contributing not insignificantly to overall curvature? So if long range g.w.s are not detected, then might LIGO I actually be exploring a qualitative assessment (not limits) as to stiffness of C_R manifold? Thus might C_R manifold be quite robust to perturbation? Any such robustness would seem consistent with such manifold not breaking up (i.e. so no ‘foam’?) for near to, and at C_p Planck scale; hence also consistent with no quantization of manifold C_R? Also then less likely to have leakage of g.w.s propagating out of a manifold into another dimension i.e. brane? Also wouldn’t any such significant stiffness of C_R manifold be less consistent with deformations associated with superstrings? Also if the concept of inertia of manifold is descriptive, then any entertained recent new acceleration (i.e. resulting then in a strain or elasticity of manifold) of such C_R manifold would seem less likely. Might energy associated with resistance to deformation of manifold represent a significant portion of energy required to approach flatness? That is, one significant contribution to dark energy is right before us, in the form of energy of manifold C_R; such stiffness of C_R manifold contributing to stress energy momentum tensor, and hence to curvature. How would one further explore such latter conjecture, other than any qualitative finding of no long range g.w. propagation? Perhaps one could consider all alternative possibilities of sources of energy sufficient for approach to flatness. Then to the extent that they can be found to be less probable and/or no supportive evidence, then the last standing definitive contributing source of such energy (i.e. energy of C_R manifold) might have to be in part (or in full) accepted. So have LIGO I, VIRGO already made a GREAT DISCOVERY - that is, the inertia of C_R manifold? So C_R manifold seems to have significant stiffness, and hence contributes a significant amount of energy to Tuv, and thus contributes significantly to curvature. SRM.