Wednesday, February 24, 2010

Entropic force goes nobel

Yeahm I know it is a somewhat misleading title ;).What I am meaning is that a nobel laureate -and a very famouse one, the cosmologist Geogre F. Smoot- wrote a paper (in collaboration with other people) in the subject of entropic force.

It is this: Entropic Accelerating Universe

This is the abstract:

To accommodate the observed accelerated expansion of the universe, one popu-
lar idea is to invoke a driving term in the Friedmann-Lema^ tre equation of dark
energy which must then comprise 70% of the present cosmological energy density.
We propose an alternative interpretation which takes into account the temperature
intrinsic to the information holographically stored on the screen which is the surface
of the universe. Dark energy is thereby obviated and the acceleration is due to an
entropic force naturally arising from the information storage on a surface screen.
We consider an additional quantitative approach based upon the entropy and sur-
face terms usually neglected in General Relativity and show that this leads to the
entropic accelerating universe

Certainly the claim is very ambitious. The authors pretend to explain the observed accelerated universe in terms of the entropic force. If truth it could perfectly be a noble prize winning article.

Certainly it is surprising to see such a respected physicist going into the risk of using a so criticized idea (the entropic force -as an origin of gravity- has received a lot of critics in the blogosphere and also some arxiv papers argumenting against it). Even thought reading the aper one sees that there are a few differences between the assumptions of Verlinde and the ones in this paper.

This authors accept the validity of general relativity. They don't say that is is a consequence of an entropic force in a holographic screen. What they do, instead, is to modify slightly the deduction of the Einstein equation from a conventional variational principle. Their only modifications goes into the usually discarded boundary surface term. It is there where they insert an "holographic screen entropic force" (I guess that this is the kind of terminology that a marketing department would suggest for a mass media successful theory).

It is the boundary term which is responsible for the accelerating universe. The content of the holographic entropic term is, according to the prescriptions of Verlinde, proportional to an Unrhu temperature to an horizon. In that point the authors make a discussion of possibilities of choice (the other alternatives being a Hawking or a de Sitter temperature).

The next section uses a purely entropic force argument to calculate the pressure due to the changes of entropy and the result agree with that of the dark energy.

The concluding section stablish definitively that the entropic force fits the data and wonders about further compatibilities, to be analysed in the future, of the idea related to big bang nucleosinthesis, cyclic models and inflation.

Well, certainly this paper relax the assumptions of Verlinde- which totally break with Einstein gravity- and with that relatively weak change they explain one of the mos puzzling issues of the actual cosmology. I think that still the introduction of the unruh temperature in the surface term is somewhat ad hoc (or at least that it would need further clarification). But the payback of it is very impressive.

Anyway, I am almost sure that this paper is going to be heavily discussed by the usual suspects so it is a good idea for the readers of this entry to keep an eye on them.

B.T.W. for today I had the idea to discus a line of papers which discuss how to extend the Higgs mechanism to gravity. he foundational paper of that topic seems to go back to t' Hooft and this month there have been two papers on the subject. Previously to find that recent papers I had wondered myself about that possibility (considering it with a mood of "what if?"), but clearly I arrived too late and another people have worked in that idea already. Still I think is a nice topic to read about and I will tell something about it in the future.

P.S. If I have to make a guess and it is accepted that the main content of this explanation for dark energy would be the anthropic arguments based on the landscape of string theory the idea of the paper discussed here looks better. As a plus, accepting general relativity it is not obvious that it is contrary to string theory. In fact possibly this article would rise a motivation to discuss "microscopic" implementations of the entropic force idea, as presented in this paper. Or I can be totally wrong. Certainly I had not thought too much about this topic before this paper, maybe it is time to consider it serioulsly.

P.S. 2 A second good point that I see is that the cosmological constant (or any variant explanation for dark energy) is a priori as ad hoc as an "entropic force", s introduced by these authors. And certainly it looks a more elegant solution. Sill I am reticent in some aspects,some kind of circularity "de sitter=>horizont=> temperature=>entropic force=>de sitter", but time, and further thinking, will say.

Monday, February 15, 2010

Let the entropy be with you

This new year "sensation" has been a paper by the well known string theorist Erick Verlinde about a possible entropic force like origin of gravity. The actual paper is this.

The article has been extensively discussed in the blogosphere and I will not add too much about it. The main reason of publishing about the topic is that I commented to a friend it's existence and he asked me for the link. A secondary reason is the possibility to paraphrase Obi Wan Kennoby (or possibly it's master, whose role was played by an actor who actually had a degree in Physics, which of course was the reason because he was a much better jedi that Obi one ; ).

About the paper itself all I can say is that the first paper about Physics was one by the two Verinde Brothers in the topic of Planckian scatering. This about entropic force is the first one I readed in the archos tabletPC that I comment in here. T wirte it I am using a B-move trackball remote mouse + keyboard which is wonderful way to get total control of your PC from a comfortable distance.

By the way, anyone with an undergraduate knowledge of physics which includes statistical mechanics and general relativity and who hs read about string theory in divulgaive books such as the elegant universe can read and understand the Verlinde paper by himself and make his own judgement of it's merits so n advise s needed.