Monday, March 28, 2005 ... Deutsch/Español/Related posts from blogosphere

Superhorizon fluctuations and accelerating Universe

Several physicists and bloggers, e.g. Jacques Distler, Peter Woit and especially Sean Carroll who may be considered a true expert in these questions and who added a very new article after this article of mine was published, recently noticed a paper that claimed that the cosmological constant was not needed. Instead, the accelerating expansion was conjectured to be a consequence of fluctuations of a scalar field (and the associated stress energy tensor) whose wavelength was longer than the Hubble radius i.e. the size of the visible Universe, roughly speaking.

The paper has also been given a lot of attention in the media:
Everyone who is not a direct expert in these things should know that the community is highly skeptical about such a proposal - and in some cases, for example the case of Matias Zaldarriaga (a countrymate of most co-authors of that paper), stronger words than "skeptical" would be more appropriate. People seem to agree that whatever the very long wavelength fluctuations are, they are already included in the definition of the background itself. They cannot be a source of some additional local effects by locality - this very statement is, in my opinion, enough to reject the claim and more detailed calculations are unnecessary.

An additional influence of other, unobservable patches of a "greater Universe" would have to involve a very non-local mechanism - one that we can't quite rule out, but one that violates the rules of local field theory drastically and that would have to be explained by a better, non-local theory. It is conceivable that there is some new kind of non-locality at astronomical scales, but it is one of those extraordinary claims that require extraordinary evidence - for example a working theory of quantum gravity that implies such a new effect.

Moreover, it is natural to expect that the acceleration in this framework would not be isotropic (it would not be independent of the direction because of an extra "dipole moment" and other moments) - something that contradicts the observations. Also, Éanna É. Flanagan has a very different (and perhaps too technical) argument in his today's paper:
where he considers a hypothetical Universe where the normal, shorter-than-Universe fluctuations are absent. This allows him to use a different, local calculation of the deceleration parameter. He can show that he can't get the observed acceleration unless the additional strange velocity added at the Hubble scale is huge, which seems to violate other observations.
At any rate, Éanna assumes locality, and with this assumption, it seems clear that the paper of Kalb et al. cannot be correct without the need for complicated calculations such as those of Éanna.

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reader Anonymous said...

Hey Lubos, I just read Flannagans paper and im a little bothered by it.

It doesnt seem to me like he assumes locality at all, in fact he claims that second order effects can potentially give rise to order unity corrections to q0 in principle (though not sufficient in his gedanken universe), nor can departure from isotropy (again small effects). The disproof comes more in the apparent necessity for fluid velocity terms that strongly violate observational bounds. Good, so we all agree this can't happen.

What really bugs me is precisely the criteria that you mentioned. If second order effects aren't sufficient to change the sign of the deceleration parameter, what about third/fourth/all orders! Why isn't locality naturally manifesting itself in our equations. The whole model looks rather sick. What am I missing

reader Anonymous said...

Hi Lubos,
another paper (astro-ph/0503553) came out today by G. Geshnizjani, with some very negative conclusions: They showed that the gradient terms considered by Kolb can be viewed as a renormalization of the local spatial curvature and therfore they can not lead to an accelerated expansion (they are severly constraint by observation too). The arguments are quite simple and convincing (And Ghazal Geshnizjani is an expert on backreaction ;-).


reader Quantoken said...

It's too timid a response from research communities for being merely "skeptical". I think the whole thing is outright anti-science rubbish. The model, which suggests wavelengths far longer than the Hubble scale, rely on "stuff" that is clearly beyond the reach of our observation, that some how created effects that we can see in the observable universe.

The problem is, since it is beyond the reach of our observation, you can imagine any combination of anything you want beyond the event horizon of our visible universe, so you can always manage to fit the observed dat. And there is no feasible way of ever falsifying such theory, since there is no way of verifying whether the "stuff" really exist beyond our horizon or not.

If you accept such an un-scientific theory, you might as well accept a much simpler model where there is a supernatural being beyond the horizon of our universe, who takes care of everything we observe in this universe.


reader Anonymous said...


reader Luboš Motl said...

Dear Quantoken,

frankly speaking, I tend to agree with your description.

For me, the most important point is causality and locality - the things outside the Universe should not affect us - but even if we allowed these things to influence our Universe, it would not be quite scientific to blame some physical effects on influences that are beyond our ability to measure them.


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