New study shows Dark Energy isn't needed? Relativty explains it?

[Garth]

Following close on the heels of new study shows Dark Matter isn't needed? Relativty explains it? we have Back-Reaction: A Cosmological Panacea which does for Dark Energy what General Relativity Resolves Galactic Rotation Without Exotic Dark Matter tries to do for Dark Matter.

We present a solution to the dark energy problem in terms of the Effective Energy Momentum Tensor (EMT) of cosmological perturbations. The approach makes use of the gravitational back-reaction of long wavelength (super-Hubble) fluctuation modes on the background metric. Our results indicate that, following preheating, the energy density associated with back-reaction is sub-dominant and behaves as a tracker during the radiation era. At the onset of matter domination, however, the effects of back-reaction begin to grow relative to the matter density and the associated equation of state quickly approaches that of a cosmological constant. Using standard values for the preheating temperature and the amplitude of the inflaton following preheating, we show that this mechanism leads to a very natural explanation of dark energy. We comment on other recent attempts to explain the dark energy using back-reaction and their relation to our work.

It now seems that both Dark Energy and Dark Matter are artifacts of incomplete application of General Relativity and nothing further is required.

Comments?

Garth

 

[pervect]

#include <smartass.h>

I thought the universe was freely coasting - where did this cosmological constant stuff come from?

"Your Honor, I will show first, that my client never borrowed the Ming vase from the plaintiff; second, that he returned the vase in perfect condition; and third, that the crack was already present when he borrowed it."

(just kidding).

To be serious, I'll have to read the paper more closely. I've seen effective energy momentum tensors used for high frequency gravitational waves, but I don't understand how they apply to
"long wavelength metric pertubations" as per the paper.

 

[EL]

Following close on the heels of new study shows Dark Matter isn't needed? Relativty explains it? we have Back-Reaction: A Cosmological Panacea which does for Dark Energy what General Relativity Resolves Galactic Rotation Without Exotic Dark Matter tries to do for Dark Matter.
It now seems that both Dark Energy and Dark Matter are artifacts of incomplete application of General Relativity and nothing further is required.
Comments?
Garth

But Garth, seriously, since Cooperstock & Tieu has been debunked, what indicates that GR may solve the dark matter problem? I know you hope GR would do it, but it's not helping your point by referring to an article which is incorrect.
Regarding Dark Energy, if there really is a true cosmological constant (which I really doubt), GR could in fact solve the problem.

 

[Garth]

#include <smartass.h>

I thought the universe was freely coasting - where did this cosmological constant stuff come from?

This is nothing to do with SCC or the Freely Coasting models, I make comment because I am interested in all attempts to explain the observations of what we call Dark Energy and Dark Matter. Until they are identified in a laboratory we need to keep all options open and have an open mind.

I know you hope GR would do it, but it's not helping your point by referring to an article which is incorrect.

I don't actually - I have been playing 'devil's advocate', my own position is that of Self Creation Cosmology - An Alternative Gravitational Theory
My complaint is not against the Mainstream model as such, just the over-confidence placed in it that is unprepared to counter any such concordant and testable alternatives.
Garth

 

[EL]

This is nothing to do with SCC or the Freely Coasting models, I make comment because I am interested in all attempts to explain the observations of what we call Dark Energy and Dark Matter. Until they are identified in a laboratory we need to keep all options open and have an open mind.
My complaint is not against the Mainstream model as such, just the over-confidence placed in it that is unprepared to counter any alternatives.
Garth

Sure, I just thought you claimed that C&T had been kind of successful in their attempt.

 

[Garth]

Furthermore, it seems the 'back-reaction' explanation for DE has also been debunked! Can the Acceleration of Our Universe Be Explained by the Effects of Inhomogeneities? by no less a person than Robert Wald! Answer:

No. It is simply not plausible that cosmic acceleration could arise within the context of general relativity from a back-reaction effect of inhomogeneities in our universe, without the presence of a cosmological constant or ``dark energy.''

So that's that sorted!

Garth

 

[EL]

Furthermore, it seems the 'back-reaction' explanation for DE has also been debunked! Can the Acceleration of Our Universe Be Explained by the Effects of Inhomogeneities? by no less a person than Robert Wald! Answer:
So that's that sorted!
Garth

Nice, I had also heard that acceleration from inhomogeneites was kind of outruled, but never seen it on paper.

BTW, what do people here think the DE is?

 

[Chronos]

If the universe is homogenous at large scales, and dark energy is only apparent at large scales, I'm very confused by the inhomogenity thing. I have this happy meal vision of dark energy being this tiny, gravitational backreaction. If gravity curves nearby space, perhaps it negatively curves space at great distances.

 

[Garth]

BTW, what do people here think the DE is?

We have to be careful to distinguish between curvature of space, positive, negative or flat and space-time, which is gravitation by another name. In the Milne empty model space is curved, negatively, while space-time is flat - all the Riemannian components are zero.

It seems something is required to adjust the curvature of space to make the GR theoretical model fit the present observations. The universe appears flat (WMAP) while only having between a quarter and a third of the necessary density.

The cosmological constant is a degree of freedom (analogous to an integration constant), which affects the curvature of space, allowing it to vary without altering the material content. GR requires an observation to 'fix the constant', perhaps DE is nothing more than that.

On the other hand you can change the spatial curvature by adding extra unknown content of various speculative types with various equations of state to achieve the same effect, such as zero point energy (w = -1 which mirrors Lambda), quintessence, leaky branes etc. etc.

Unless these possibilities are confirmed by laboratory experiment to actually exist and to have the appropriate properties, great humility is required - the more we know the more we are aware of what we do not know.

The problem is GR is underdetermined as far as DE is concerned and therefore not falsifiable, a determined and therefore falsifiable alternative gravitational theory would have the advantage.

Garth

 

[SpaceTiger]

Nice, I had also heard that acceleration from inhomogeneites was kind of outruled, but never seen it on paper.

This was probably the strongest rebuttal to the initial idea (superhorizon perturbations):

http://lanl.arxiv.org/abs/astro-ph/0503582"

Since then, the Kolb et al. group have shifted their focus from superhorizon to subhorizon perturbations. The Wald paper attempts to tackle both.

 

[Chronos]

Thanks for the link, ST! I sort of feel vindicated by Hirata et al... "Can superhorizon cosmological perturbations explain the acceleration of the universe?" Not that I have an alternative model, but, I've always been firmly opposed to the superhorizon model

 

[EL]

This was probably the strongest rebuttal to the initial idea (superhorizon perturbations):
http://lanl.arxiv.org/abs/astro-ph/0503582"
Since then, the Kolb et al. group have shifted their focus from superhorizon to subhorizon perturbations. The Wald paper attempts to tackle both.

Ah, great. Thanks!