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Lubos Motl science forum beginner
Joined: 02 May 2005
Posts: 38

Posted: Mon May 29, 2006 3:52 am Post subject:
Hepth papers on Monday



http://motls.blogspot.com/2006/05/hepthpapersonmonday.html
Hepth papers on Monday
***********************
Oriol Pujolas looks at the DGP model. If you don't know, the DGP model is
a popular model among phenomenologists in which you have a
fivedimensional gravity with a 3brane in it, and the action contains
both the bulk 5dimensional EinsteinHilbert term ("integral R.sqrt(g)")
as well as an additional 4dimensional EinsteinHilbert term that is stuck
on the 3brane. The latter term is unlikely to appear in string theory, I
think. At distances shorter than a crossover scale  usually assumed to be
the Hubble scale without a good reason  gravity looks 4dimensional. At
longer distances, the bulk term starts to dominate and gravity is
5dimensional. Pujolas studies quantum fluctuations in this model: the
author computes some Green's functions, and a oneloop effective potential
arising from the quantum fluctuations.
J.C. Bueno Sanchez and K. Dimopoulos study a new kind of inflation based
on a rolling scalar field  a quintessence. Although the picture is
stringinspired, it does not seem that they are talking about a specific
welldefined background in string theory. Their description of the
"trapped quintessential inflation" has many stages and it reads like a
novel. The first stage of their inflation starts in a steep potential,
hypothetically generated by nonperturbative effects in string theory.
Then you reach an ESP  enhanced symmetry point  where you produce a lot
of light particles. They argue that this leads to a period of inflation
which I don't quite understand; is a strong particle production compatible
with inflation? At any rate, the scalar fields eventually leave the ESP
point and go into another steep region, which gives you reheating. Then
the scalars freeze because of cosmological friction. It is fixed for a
long time until the present, when it starts to behave as a quintessence
and roll towards vanishing vacuum energy. A pretty complicated picture
combining many pieces that are popular. It seems that we don't have enough
data to check each wheel and gear of the construction (and similar
constructions).
Rudnei Ramos and Marcus Pinto investigate phase transitions in certain
nonrelativistic field theories with a lot of scalar fields. They argue
that they have proved a nogo theorem: the models of this kind, describing
things like hard spheres, can exhibit neither inverse symmetry breaking
(which means that a symmetry is broken at high temperatures instead of the
usual low temperatures) nor symmetry nonrestoration (which describes a
situation in which the symmetry is not restored even at very high
temperatures). Finally they focus on a specific model that includes a
BoseEinstein condensate in the phase diagram. There is no reentrant phase
(an intermediate phase with a partial symmetry breaking) in this model,
but they argue that other models might have it.
Canoura, Edelstein, and Ramallo investigate the AdS/CFT correspondence
with SasakiEinstein manifolds and additional Dbranes. As you can see,
that combines two concepts. One of them is the class of SasakiEinstein
manifolds L^{a,b,c}. These manifolds have topology of "S2 x S3". The
metric is rather complicated but for a choice of three positive integers
"a,b,c", you can follow Cvetic, Lu, Page, Pope, as well as Martelli and
Sparks, and construct a manifold whose topology is "S2 x S3" such that the
cone constructed above this base is a CalabiYau space. The second
ingredient of the paper are additional Dbranes (referred to as "Dbrane
probles") added into the bulk. They add bulk open string degrees of
freedom on the AdS side, and flavors on the CFT side. The paper may be
viewed as Witten's construction of dibaryonic operators implemented in
the case of the complicated SasakiEinstein manifolds replacing Witten's
simple fivesphere.
Alfonso Ramallo has another paper about a similar topic. He also adds
Dbranes in the AdS bulk. In fact, he wants to consider intersecting
Dbranes. Such Dbranes generate new fields  flavors or "quarks"  in the
dual gauge theory. Such "quarks" can form bound states  mesons. Ramallo
claims to be able to compute the spectrum of such mesons in a rather
general case where the Dbranes can have rather general dimensions. He
uses the quenched approximations  the Dbranes are treated as probes.
Anacleto, Nascimento, and Petrov study noncommutative field theories,
especially their UV behavior. The particular theory they consider has a
real scalar field with a quartic potential and a Dirac field coupled by a
Yukawa term. They choose the "coherent state approach". It seems that it
means that this allows them to make the propagators suppressed by
"exp(theta.p^2/2)" relatively to the propagators in the "ordinary"
approach that are the same as in the commutative counterpart of the theory
(because the kinetic term is the same). Using the Schwinger
parameterization, they end up with an integral for the oneloop effective
potential that clearly converges for nonzero values of the
noncommutativity "theta": at high values of "t", the Schwinger parameter,
the integrand is exponentially suppressed. Also, there is a cancellation
whenever the quartic coupling and the Yukawa coupling are related to each
other in a way that resembles the supersymmetric relation (even though in
their particular theory, there is just one real boson field, and no SUSY).
At any rate, I think that the nontrivial divergence structure only occurs
at the twoloop level where you can see the UVIR mixing and similar
things, but they don't get that far, so I doubt that the experts in
noncommutative field theory will be thrilled.
Raphael Bousso tries to define a better probabilistic distribution for the
landscape than anyone else, and he is approaching this problem in a
typically Boussian i.e. holographic way. Recently we discussed Vilenkin's
approach to the question of the probabilities on the landscape. They end
up with a pile of ambiguous and contradictory mess, and Raphael is
rightfully dissatisfied with these anthropic results. His first principle
is that only one causal diamond should be looked at when you calculate the
probabilities. This sounds correct to me because regions of spacetime
behind a particular diamond could be complementary to the diamond itself:
they may be described by the same degrees of freedom and you should not
doublecount them.
His ultimate definition of the probability distribution is relatively
simple. Consider the set of stable and unstable vacua that you want to
include into your landscape game. Some of them are "terminal" (usually
denoted "Z" in Bousso's paper)  and they don't decay. Others are unstable
(labeled "A,B" etc.) and they do decay. Raphael looks at genealogy trees
of these vacua. Each mother vacuum can decay to the daughter vacua, and
Raphael only uses the "branching ratios" i.e. the probabilities that "A"
decays to "B" or "Z". I hope that he knows how to calculate these
dimensionless branching ratios from the instanton actions and vacuum
energies but I don't see the rule in the paper. Instead, he focuses on the
ultimate probability distribution for the vacua calculated from the
branching ratios for the decay. It is virtually identical to Google's
PageRank algorithm, as long as you replace hyperlinks between web pages by
decay channels between vacua, and Raphael can obtain finite results for
the probabilities (PageRank) of different vacua whenever there exists at
least one available terminal vacuum. If it exists, small trees dominate
Raphael's ensemble. Raphael does not say which vacua of string theory
ultimately end up with the highest PageRank, but someone else could have
an answer. I agree that Raphael's calculation is more justified than the
random ad hoc anthropic prescriptions that various people want to apply to
the eternal inflation, but I think that much more work is needed to
extract useful information from Raphael's approach even if it is correct.
______________________________________________________________________________
Email: lumo@matfyz.cz fax: +1617/4960110 Web: http://lumo.matfyz.cz/
eFax: +1801/4541858 work: +1617/3849488 home: +1617/8684487 (call)
Webs: http://schwinger.harvard.edu/~motl/ http://motls.blogspot.com/
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