Thursday, January 27, 2011

Two cosmology papes and the question of extra dimensions

First an advert, the two topics are unrelated, well, or may be not ;).

The first paper I would want to mention is this form Luis Alvarez Gaumé. Gaumé is one of the best known spanish string theorists. He is famous for the 80's paper with Witten about the (absence of) anomalies in string theory.

Now he has a paper in arxiv. A Minimal Inflation Scenario. This is the abstract:

We elaborate on a minimal inflation scenario based entirely on the general properties of supersymmetry breaking in supergravity models. We identify the inflaton as the scalar component of the Goldstino superfield. We write plausible candidates for the effective action describing this chiral superfield. In particular the theory depends (apart from parameters of O(1)) on a single free parameter: the scale of supersymmetry breaking. This can be fixed using the amplitude of CMB cosmological perturbations and we therefore obtain the scale of supersymmetry breaking to be 10^{12-14} GeV. The model also incorporates explicit R-symmetry breaking in order to satisfy the slow roll conditions. In our model the eta-problem is solved without extra fine-tuning. We try to obtain as much information as possible in a model independent way using general symmetry properties of the theory's effective action, this leads to a new proposal on how to exit the inflationary phase and reheat the Universe.

I am far from expert in the inflation technicalities but it looks like a quite natural proposal (well, maybe except for explicit R-parity breaking, which, otherwise, is good for some very specific models of dark matter tht were popular the last year trying to accommodate some now not too much discussed anomalies from PAMELA, ARTIC and others).

The other cosmology paper I wanted to mention is this: Holographic unification of dark matter and dark energy. The abstract is as follows: Using a new version of the holographic principle, a constant term was introduced, which conduces to the description of the standard cosmological LCDM model, and unifies under the same concept the dark matter and dark energy phenomena. The obtained model improves the results of previously considered holographic models based on local quantities. The inclusion of constant term is interpreted as a natural first approximation for the infrared cutoff which is associated with the vacuum energy, and the additional terms guarantee an appropriate evolutionary scenario that fits the astrophysical observations. The model allows to reproduce the standard LCDM model without explicitly introducing matter content, and using only geometrical quantities. It is also obtained that the model may describe the dark energy beyond the standard LCDM.

The idea of unifying dark matter and dark energy as one and the same phenomena looks interesting aesthetically. Against it it is the question of many independent observations leading to the existence of dark matter. Specially he galactic ones seem a little bit out of the sight of this paper. But I remember a paper a few time ago that could fit the rotation of galaxies by mean of an stringy phenomena without need of dark matter. Still there is a good reason for expecting the existence of dark matter: Supersymmetry. Specially if R-parity is conserved the LSP is stable and would be a component of dark matter. And one expect that supersymmetry would break near the electroweak scale, isn't it?

Well, maybe. But one of the main supports of low energy SSB is the resolution of the hierarchy problem. Fortunately there is another option for solving the hierarchy problem, the Randall-Sumdrum scenaries. Recently there were two results from the tevatron announcing two anomalies at more than 3 sigmas related to top quark physics. One is described in the article: Evidence for a Mass Dependent Forward-Backward Asymmetry in Top Quark Pair Production., Annalized in entries in the Lubos and Jester blog, among others.

The other, more recent one, was first announced in the Jester's blog: Another Intriguing Result from Tevatron's CDF that also was analysed in Lubos blog (I don' put the link because everybody reads Lubos blog, isn't it? ;) ).

Reading the entries, and the comments one sees that there is a main natural candidate for both phenomena, a KK (kaluza-klein) partner of the gluon. For that KK gluon being in the right track one must be in a Randall-Sumdrum scenario. That means that if the LHC, that restart it's operation in fabruary, confirms the anomalies we would have a (very likely) confirmation of the existence of extra dimensions which would be possibly the most amazing all times experimental discovery in physics.

But don't shout Eureka still. Recently the LHC published an article advertising that they had not seen evidence of micro black holes, which is a bad thing for the R-S sceneries. Well, one wouldn't care too much about it. Microscopic B-h are not the former prediction of R-S scenaries and the paper had some drawbacks also.

Yesterday there was a paper about this R-S braneworlds proposal and it's measure in the LHC: LHC bounds on large extra dimensions based on the analysis of 3.1 inverse picobarns of LHC data that fixed new limits in the possible extent of the extra dimensions. AS far as I understand that bounds don't forbid the KK gluon as the author of the top quarks anomalies. Of course there are another possible answers to that anomalies. For example Marco Frasca claims (The Tevatron affair and the “fat” gluon )that his model of "fat gluon" could do the job (although he acknowledges that the KK gluon fits better).

Well, the thing is that if braneworlds are proved valid the motivation for weak scale SSB would be lacking (because R-S would solve the hierarchy problem), and with it the best candidates for dark matter. Still one could think that another KK particle are candidates for dark matter. But maybe the theories that unifies dark matter and dark energy would gain some points, or maybe be not, cosmology you know ;).


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Matti Pitkanen said...

I see the large extra dimension based explanations as rather ad hoc. LHC already excludes them (you indeed mention mini black holes).

For TGD based explanation see the following short note at my blog:

Anyone willing to participate to the development of future theoretical physics should learn about twistors and forget M-theory;-). This is the approach which will replace QFT and strings with something much deeper and incredibly simpler and release us from the curse of path integrals.

After few decades theoreticians will just wonder how it was possible that rational human beings believed for decades that physics could be described by non-existing mathematics.

See this:

Anonymous said...

Artistically, I welcome you all again. After my prolonged presence in the forum is not, I could not find the watchword to your old profile and started a new one. I'm pleased as punch that I am again with you.

Javier said...

Hi Matti.

Sorry by the delay in the answer, but I wanted to have time to read a few more in the subjecto of twistor revolution before answering.

I haven't still read as much as I would like, but judging by what I have seen the twistor revolution is mainly a way to use twistor nanotechnology to show relations about amplitudes at different loop levels in scattering amplitudes in N=4 supersymmetry.

Ok,, I agree that relations among tree level n particle amplitudes an one loop level amplitudes, that even include different type of vertex is intriguing, and plenty worth of being investigated.

But, until the contrary is proved that is a way to investigate ordinary (and non physically realistic) supersymmetric QFT. I read in your blog that maybe changing the yangian symmetry by other similar ones one could get new physical theories, but as far as I see that is a suggestion that requires further details.

Anyway, although interesting and intriguing I don't see that this twistor revolution would replace in any way the string/M/F theories because, basically, it is, as developed today, just a calculational tool.