Speaker
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Title
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Abstract
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| Ophelia Jade Philippine Fabre
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Testing anisotropic universes with the Cosmic Microwave Background
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Some features in the Cosmic Microwave Background (CMB) data are not
statistically consistent at large scale with the Lambda-CDM model of the
Universe. These statistical anomalies point towards a possible violation of
statistical isotropy and are the reasons underlying the search of a new
cosmic model. Einstein's equations of relativity only constrain local
geometry, leaving global topology undetermined. Nonetheless, some special
topologies could explain the signature of large scale anisotropy detected
in CMB data.
In this talk, I will present flat multi-connected topologies, which are
models leading to a breaking of the global isotropy. The last scattering
surface, from which the CMB is released, represents the most distant source
of photon in the Universe and thus, its diameter is the largest accessible
scale of our Universe. I will estimate how far we can detect a topological
signature beyond the last scattering surface with the help of
Kullback-Leibler divergence.
|
Swastik Bhattacharya
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Fluctuations and Transport Phenomena in Black Hole Membranes
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It is well-known that the Black Hole horizon obeys the equation of motion
for a viscous fluid. A statistical model of this fluid can help us obtain
some insight into the thorny problems of the microscopic Black Hole degrees
of freedom and Black Hole entropy. In this talk, I shall describe a model,
where the fluid on the Schwarzschild black hole horizon is a condensate .
We shall use Mean Field Theory to describe this system. Here I shall focus
on the fluctuations of the mean field away from thermal equilibrium. In
particular, I shall show that the Langevin equation governing the energy
transported from outside into the horizon-fluid corresponds to the
Raychaudhuri equation for the null congruences on the Black Hole horizon.
We shall also briefly outline a method, that uses the Green-Kubo formula to
compute the coefficient of Bulk Viscosity from the consideration of the
fluctuations. Finally, I shall discuss how these results together lend
further support to the paradigm that Gravity is emergent.
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| Abhishek Basak
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Unimodular gravity in cosmology
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The smallness of the cosmological constant is still an unsolved problem. In Einstein's theory of
General Relativity the cosmological constant is related to the vacuum energy density. This gives
rise to the theoretical value of cosmological constant much higher than the observation (60-120
orders of magnitude). Unimodular gravity is one possible modification of Einstein's theory where
the cosmological constant appears as an integration constant.
In the context of scalar fields non-minimally coupled to gravity, the cosmological constant
appearing in the Unimodular gravity can be small. I will discuss the consequences of FLRW
cosmology within the context of non-minimal coupling in Unimodular gravity.
I will also construct the cosmological perturbation theory for Unimodular gravity during inflation
and compare the physical consequences with General Relativity. |
| Arpan Bhattacharyya
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Second Law , Entropy functional and "C" theorem
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Using the second law of black holes one can derive the entropy
functionals for various higher curvature theories of gravity by fixing
uniquely the ambiguities that enter in the Noether charge method proposed
by Iyer and Wald. Using these entropy functionals and linearized second
law we will construct the holographic "c"-functions in the context of
AdS/CFT and propose that the boundary RG flow is dual to the
thermodynamics of the causal horizons in Poincare AdS. |
| Apratim Kaviraj
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Analytic results from conformal bootstrap at large spin
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We use analytic conformal bootstrap methods to determine the
anomalous dimensions and OPE coefficients for large spin operators in
general conformal field theories containing a scalar operator . It is
known that such theories will contain an infinite sequence of large spin
operators. By considering the case where such operators are separated by a
twist gap from other operators at large spin, we analytically determine
the anomalous dimensions at large spin. To do this we extract an
approximate expression for the conformal blocks in any dimension. We find
that the anomalous dimensions are negative if the twists satisfy
unitarity bound, thus extending the Nachtmann theorem to non-zero n. In
the large twist limit we find that the anomalous dimension becomes
universal, with our result in perfect agreement with results from two
different holographic calculations.
|
| Karthik Inbasekhar
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2 to 2 scattering in supersymmetric matter Chern-Simons theories
at large N
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Non-abelian Chern-Simons theories are very rich. When coupled to matter in
the fundamental representation of U(N), these theories are exactly solvable
in the large N limit. We study the most general renormalizable
N=1, U(N) Chern-Simons gauge theory coupled to a single (generically
massive) fundamental matter multiplet. At leading order in the t' Hooft
large N limit we present computations and conjectures for the 2 X 2
S matrix in these theories; our results apply at all orders in the
t'Hooft coupling and the matter self interaction. Our S matrices are
in perfect agreement with the recently conjectured strong weak coupling
self duality of this class of theories. The consistency of our results with
unitarity requires a modification of the usual rules of crossing symmetry
in precisely the manner anticipated in arXiv:1404.6373, lending substantial
support to the conjectures of that paper. In a certain range of coupling
constants our S matrices have a pole whose mass vanishes on a self dual
co-dimension one surface in the space of couplings.
|
| Menika Sharma
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CFT families and Higher-Spin tribes
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Two-dimensional "minimal-model" CFTs are known to be dual to
higher spin theories on AdS3. This duality can be verified in detail
by matching the partition function of the CFT with that of the higher
spin theory. However, this check has, so far, has been performed only
for the diagonal modular-invariant partition function. In this talk, I
will construct other partition functions for the minimal-model CFT.
Then I will consider whether a match can be found for them on the
higher-spin AdS side.
|
| Nilay Kundu
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Non-relativistic Solutions in Lovelock and
Chern-Simons Gravity Theories
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In this talk we will discuss non-relativistic solutions, such as
Lifshitz and Schrodinger solutions, in Lovelock and Chern-Simons Gravity
Theories. Firstly, We will understand the relation between these two
theories, namely Lovelock and Chern-Simons Gravity Theories. Then, we will
try to study specific conditions under which the above mentioned
non-relativistic theories exist in those theories |
| Abhishek Chowdhury
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Hilbert Series and Black Hole Microstate Counting
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Exact results for the BPS index are known for a class of BPS
dyons in type II string theory compactified on a six dimensional torus. In
a previous paper we had set up the problem of counting the same BPS states
in a duality frame in which the states carry only Ramond-Ramond charges. We
explicitly counted the number of states carrying the lowest possible
charges and find agreement with the result obtained in other duality
frames. Furthermore, we found that after factoring out the supermultiplet
structure, each of these states carry zero angular momentum. We are now
trying to generalize the systematics to other charges for which the
configurations are non-abelian. It all boils down to solving multivariate
polynomial equations and Hilbert series provides a way to classify the
building blocks (the Monomials).
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| Taniya Mandal
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Multiple Attractors in String Theory.
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Attractor mechanism provides macroscopic description of
entropy for extremal black holes. As a consequence of this property, the
moduli fields of these extremal black holes are fixed at their horizon
and are given by the black hole charges irrespective of their asymptotic
values. Hence the entropy of these black holes are solely dependent on
the black hole charges. However, this is the case only when the moduli
space is connected. If the moduli space contains several disjoint
branches, black holes possess multiple attractors. The entropy and the
attractor solution are unique in each branch. In this talk, we will
discuss some examples of multiple attractors with suitable charge
configurations in the context of four dimensional N=2 supergravity
theory coupled to n vector multiplets. We will also consider the case of
axionic black holes for which the attractors undergo a phase transition
as we change the values of charges across a domain in the charge
lattice.
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| Srijit Bhattacharya
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Entropy Functional and Second Law in Curvature Squared Gravity
Theories
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Standard methods for calculating the black hole entropy beyond general
relativity are ambiguous when the horizon is non-stationary. We fix these
ambiguities in all quadratic curvature gravity theories, by demanding that
the entropy be increasing at every time, for linear perturbations to a
stationary black hole. Our result matches with the entropy formula found
previously in holographic entanglement entropy calculations. We explicitly
calculate the entropy increase law for Vaidya-like solution in Ricci square
gravity to show that unlike the Wald entropy the holographic entropy obeys
a Second Law. We also derive bounds on the higher curvature couplings
demanding the validity of the second law for higher order perturbations. |
| Prof T R Govindarajan
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Fermionic Edge states and Moving
Boundaries
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TBA |
| Bidisha Chakrabarty
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On orbifolded fuzzball geometry
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Non-supersymmetric black hole microstates are of great interest in the
context of the black hole information paradox. We identify the holographic
description of the general class of non-supersymmetric orbifolded D1-D5-P
supergravity solutions found by JMaRT. This class includes both completely
smooth solutions and solutions with conical defects, and in the
near-decoupling limit these solutions describe degrees of freedom in the
cap region. The CFT description involves a general class of states obtained
by fractional spectral flow in both left-moving and right-moving sectors,
generalizing previous work which studied special cases in this class. We
compute the massless scalar emission spectrum and emission rates in both
gravity and CFT and find perfect agreement, thereby providing strong
evidence for our proposed identification. We also investigate the physics
of ergoregion emission as pair creation for these orbifolded solutions. Our
results represent the largest class of non-supersymmetric black hole
microstate geometries with identified CFT duals presently known. |
| Soumyabrata Chatterjee
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Ads Cosmology and Gauge Theory Correlator
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Using Ads/CFT prescription,we compute two point Yang-Mills correlator on a
constant time slice for the Kasner background.Pushing the surface close to
the initial singularity,we find,in some cases,the correlator does not
develop pole.We compute the similar correlator numerically where the bulk
is a Kasner Ads soliton.We find that the qualitative behaviour of the
correlator remains unchanged.We further investigate the case,using
Ads/CFT,where the spacetime is sourced by a perfect fluid stress tensor. |
| Suman Ganguli
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Collapse of Charged Null Fluid and Energy Conditions
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In this talk a detailed study of gravitational collapse of
charged null fluid in asymptotically flat and AdS spacetimes of arbitrary
dimensions will be presented. The solutions, which are generalization of
Vaidya solution, always have critical hypersurfaces, "beyond" which the
energy momentum tensor of charged null fluid violates energy conditions.
Both the locations and causal properties of these hypersurfaces depend on
distribution profile of charge and energy in the collapsing shells. It was
shown by Ori* that, when the effects of repulsive Lorentz force are
considered, the four momenta of constituent particles vanish on these
critical hypersurfaces and the fluid "bounces off" it by reversing its
direction of momentum and therefore does not get into the region where
energy conditions are violated. In this talk it will be shown that if the
distribution profile of charge and energy in the collapsing shell obey
certain conditions, then the fluid indeed bounces back and the energy
conditions are satisfied in both asymptotically flat and AdS spacetime of
arbitrary dimensions. Further, it will be shown that for the simplest
distribution of charge and energy critical, the hypersurface is spacelike
and lies inside the trapped spacetime region. The apparent violation in
this type of configurations due to inevitability of collapse "beyond" the
critical hypersurface will be addressed. |
| Sudipto Paul Chowdhury
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Finite temperature effects on stability of intersecting D-branes
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TBA |