Here we have another paper doing the cosmological constant from backreaction. This time the authors use the suberHubble fluctuations in the scalar field rather than gradient terms of energy momentum tensor used previously, so it is a fundamentally different approach.
In the conclusions the authors address the problem of causality by saying it is easy to refute, because "Our formalism is based entirely on the equations of GR, which are generally covariant...", which still leaves me slighlty nervous (previous approaches were also based on covariant GR and turned out to have many problems), but the proposed mechanism is a very interesting possiblity anyway...
[astroph/0510523] BackReaction: A Cosmological Panacea
Authors:  P. Martineau, R. Brandenberger 
Abstract:  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 backreaction of long wavelength (superHubble) fluctuation modes on the background metric. Our results indicate that, following preheating, the energy density associated with backreaction is subdominant and behaves as a tracker during the radiation era. At the onset of matter domination, however, the effects of backreaction 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 backreaction and their relation to our work. 
[PDF] [PS] [BibTex] [Bookmark] 

 Posts: 183
 Joined: September 24 2004
 Affiliation: Brookhaven National Laboratory
 Contact:

 Posts: 1
 Joined: October 21 2004
 Affiliation: Rice University
 Contact:
[astroph/0510523] BackReaction: A Cosmological Panacea
A recent paper by R.Wald [grqc/0509108] seems to claim you cannot do this...

 Posts: 49
 Joined: December 17 2004
 Affiliation: Perimeter Institute/ University of Waterloo
 Contact:
[astroph/0510523] BackReaction: A Cosmological Panacea
I finally got to read through this paper.
At first look, the backreaction jargon of this paper can be simply translated to a quintessence with its initial condition set by inflation.
However, in closer reading, the treatment seems to have more serious problems:
1 [tex]\varphi^4[/tex] theory studied here, undergoes coherent oscillations after the end of inflation, and finally decoheres into a radiation fluid, rather than a uniform field with small inhomogeneities, as assumed here. Eventually, [tex]\varphi^4[/tex] theory can only act as a radiation fluid and not quintessence.
2 [tex]\varphi[/tex] field, which is assumed to be the inflaton, at least partially decays into radiation at reheating, which is not considered at all.
3 The first order terms cannot be simply averaged to zero at the 2nd order level.
4 Finally, G_{0i} equation cannot be used to relate metric to field fluctuations, when the scalar field is subdominant, as is the case in the matter era.
All this should be added to the fact that WMAP seems to have ruled out [tex]\varphi^4[/tex] as a possible inflationary scenario.
At first look, the backreaction jargon of this paper can be simply translated to a quintessence with its initial condition set by inflation.
However, in closer reading, the treatment seems to have more serious problems:
1 [tex]\varphi^4[/tex] theory studied here, undergoes coherent oscillations after the end of inflation, and finally decoheres into a radiation fluid, rather than a uniform field with small inhomogeneities, as assumed here. Eventually, [tex]\varphi^4[/tex] theory can only act as a radiation fluid and not quintessence.
2 [tex]\varphi[/tex] field, which is assumed to be the inflaton, at least partially decays into radiation at reheating, which is not considered at all.
3 The first order terms cannot be simply averaged to zero at the 2nd order level.
4 Finally, G_{0i} equation cannot be used to relate metric to field fluctuations, when the scalar field is subdominant, as is the case in the matter era.
All this should be added to the fact that WMAP seems to have ruled out [tex]\varphi^4[/tex] as a possible inflationary scenario.