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Universiteit van Amsterdam
String Theory
Group
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Recent results |
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In
arXiv:1004.2521 (Exotic
branes and non-geometric backgrounds)
we show that if one applies
U-dualities to branes one can get very strange objects (which we
called exotic branes) especially if they have codimension two. We
show that from a higher dimensional point of view, these objects
should be interpreted as non-geometric backgrounds, which are
backgrounds where the metric does not come back to itself after
completing a closed loop. At the same time, black holes are often
dual to bound states of branes, and as one increases the string
coupling these bound states can start to look like excited versions
of a single brane with a compact geometry. The supertube effect is
the prototype of such phenomena. When studying black holes, U-dual
versions of the supertube effect are expected to be relevant, which
then inevitable gives rise to exotic branes. We thus speculate that
exotic branes are crucial in order to try to find geometric
descriptions of the underlying microscopic degrees of freedom of
black holes. |
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In
arXiv:1002.2429
(Holographic Brownian Motion and Time Scales in Strongly
Coupled Plasmas) we study Brownian
motion of a heavy quark in field theory plasma in the AdS/CFT setup
and discuss the time scales characterizing the interaction between
the Brownian particle and plasma constituents. In particular, the
mean-free-path time is related to the connected 4-point function of
the random force felt by the Brownian particle. By holographically
computing the 4-point function and regularizing the IR divergence
appearing in the computation, we write down a general formula for
the mean-free-path time, and apply it to the STU black hole which
corresponds to plasma charged under three U(1) R-charges. The result
indicates that the Brownian particle collides with many plasma
constituents simultaneously. The final result depends
logarithmically on the 't Hooft coupling and is somewhat reminiscent
of the perturbative result. There might be an interesting connection
to black hole thermalization. |
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In
arXiv:0912.1877 (Holographic
Lovelock Gravities and Black Holes)
we study holographic implications of
Lovelock gravities in AdS spacetimes. For a generic Lovelock gravity
in arbitrary spacetime dimensions we formulate the existence
condition for asymptotically AdS black holes. We consider small
fluctuations around these black holes and determine the constraint
on Lovelock parameters by demanding causality of the boundary
theory. For the case of cubic Lovelock gravity in seven spacetime
dimensions we compute the holographic Weyl anomaly and determine the
three point functions of the stress energy tensor in the boundary
CFT. Remarkably, these correlators happen to satisfy the same
relation as the one imposed by supersymmetry. We then compute the
energy flux; requiring it to be positive is shown to be completely
equivalent to requiring causality of the finite temperature CFT dual
to the black hole. These constraints are not stringent enough to
place any positive lower bound on the value of viscosity. Finally,
we conjecture an expression for the energy flux valid for any
Lovelock theory in arbitrary dimensions. |
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In
arXiv:0910.5347 (AdS_7/CFT_6,
Gauss-Bonnet Gravity, and Viscosity Bound)
we study the relation between the
causality and the positivity of energy bounds in Gauss-Bonnet
gravity in AdS_7 background and find a precise agreement. Requiring
the group velocity of metastable states to be bounded by the speed
of light places a bound on the value of Gauss-Bonnet coupling. To
find the positivity of energy constraints we compute the parameters
which determine the angular distribution of the energy flux in terms
of three independent coefficients specifying the three-point
function of the stress-energy tensor. We then relate the latter to
the Weyl anomaly of the six-dimensional CFT and compute the anomaly
holographically. The resulting upper bound on the Gauss-Bonnet
coupling coincides with that from causality and results in a new
bound on viscosity/entropy ratio.
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In
arXiv:0906.3272 (What is a chiral 2d CFT? And what
does it have to do with extremal black holes?)
We study backgrounds which have been
argued to be dual to a "chiral 2d CFT" with the aim to figure out
what exactly a chiral 2d CFT is supposed to be. We find that
generically these chiral 2d CFT's can be obtained from ordinary 2d
CFT's by performing a discrete light-cone quantization (DLCQ). We
also show that in many backgrounds that contain an AdS2 component,
it is not possible to excite degrees of freedom on this AdS2, in
agreement with previous results. If our ideas are correct it should
be possible to associate a 2d CFT to any extremal black hole. It is
presently unclear whether this can indeed be accomplished. |
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In
arXiv:0906.0011 (A bound on the entropy of
supergravity?) We study how
many degrees of freedom one can find in supergravity, in such a way
that those degrees of freedom carry the same quantum numbers as a
large BPS black hole. We find strong evidence that there are not
enough degrees of freedom. In it most dramatic (and unproven) form,
this would imply that any attempt at quantizing gravity in
its own right is necessarily flawed, and that one always needs
stringy degrees of freedom in order to properly quantize gravity and
in order to describe black holes in such a way that the usual laws
of quantum mechanics are not violated. |
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| Research
interests: |
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| ● String theory |
| ● Quantum
Gravity and Black Holes |
| ● AdS/CFT,
AdS/QCD,..... |
| ●
High-Energy Physics |
| ●
Mathematical Physics |
| ●
Condensed Matter Physics |
| ●
Theoretical Physics in General |
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