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Sandipan Kundu

Publications and source records attributed to Sandipan Kundu.

47 records · Page 3Linked to original sources

Holographic Space-time and Black Holes: Mirages As Alternate Reality

We revisit our investigation of the claim of [1] that old black holes contain a firewall, i.e. an in-falling observer encounters highly excited states at a time much shorter than the light crossing time of the Schwarzschild radius. We used the formalism of Holographic Space-time (HST) where there is no dramatic change in particle physics inside the horizon until a time of order the Schwarzschild radius. We correct our description of the interior of the black hole . HST provides a complete description of the quantum mechanics along any time-like trajectory, even those which fall through the black hole horizon. The latter are described as alternative factorizations of the description of an external observer, turning the mirage of the interior provided by that observer's membrane paradigm on the stretched horizon, into reality.

hep-th↗

Holographic Entanglement in a Noncommutative Gauge Theory

In this article we investigate aspects of entanglement entropy and mutual information in a large-N strongly coupled noncommutative gauge theory, both at zero and at finite temperature. Using the gauge-gravity duality and the Ryu-Takayanagi (RT) prescription, we adopt a scheme for defining spatial regions on such noncommutative geometries and subsequently compute the corresponding entanglement entropy. We observe that for regions which do not lie entirely in the noncommutative plane, the RT-prescription yields sensible results. In order to make sense of the divergence structure of the corresponding entanglement entropy, it is essential to introduce an additional cut-off in the theory. For regions which lie entirely in the noncommutative plane, the corresponding minimal area surfaces can only be defined at this cut-off and they have distinctly peculiar properties.

hep-th↗

Non-Gaussianity Consistency Relations, Initial States and Back-reaction

We explore the consistency relations for the three-point functions, in the squeezed limit, of scalar and tensor perturbations in single-field inflation with general initial conditions for the perturbations. For slow-roll inflation, we find that all the three-point functions of scalar and tensor perturbations with a coherent state as the initial state are identical to the three-point functions with the Bunch-Davies initial state. On the other hand, there is an apparent violation of some of the consistency relations for initial states that are related to the Bunch-Davies state by Bogoliubov transformations and we identify the reason for this violation. The back-reaction calculations indicate that the three-point functions for these states can be large enough to violate the consistency relations, however, they are too small to be observed in the near future.

astro-ph.CO↗

The Membrane Paradigm and Firewalls

Following the Membrane Paradigm, we show that the stretched horizon of a black hole retains information about particles thrown into the hole for a time of order the scrambling time m ln(m/M_P), after the particles cross the horizon. One can, for example, read off the proper time at which a particle anti-particle pair thrown into the hole, annihilates behind the horizon, if this time is less than the scrambling time. If we believe that the Schwarzschild geometry exterior to the horizon is a robust thermodynamic feature of the quantum black hole, independent of whether it is newly formed, or has undergone a long period of Hawking decay, then this classical computation shows that the "firewall" resolution of the AMPS paradox is not valid.

hep-th↗

Dynamics of Non-supersymmetric Flavours

We investigate the effect of the back-reaction by non-supersymmetric probes in the Kuperstein-Sonnenschein model. In the limit where the back-reaction is small, we discuss physical properties of the back-reacted geometry. We further introduce additional probe flavours in this back-reacted geometry and study in detail the phase structure of this sector when a constant electromagnetic field or a chemical potential are present. We find that the Landau pole, which serves as the UV cut-off of the background geometry, also serves as an important scale in the corresponding thermodynamics of the additional flavour sector. We note that since these additional probe flavours are indistinguishable from the back-reacting flavours, the results we obtain point to a much richer phase structure of the system.

hep-th↗

Holographic Mutual Information at Finite Temperature

Using the Ryu-Takayanagi conjectured formula for entanglement entropy in the context of gauge-gravity duality, we investigate properties of mutual information between two disjoint rectangular sub-systems in finite temperature relativistic conformal field theories in d-spacetime dimensions and non-relativistic scale-invariant theories in some generic examples. In all these cases mutual information undergoes a transition beyond which it is identically zero. We study this transition in detail and find universal qualitative features for the above class of theories which has holographic dual descriptions. We also obtain analytical results for mutual information in the specific regime of the parameter space. This demonstrates that mutual information contains the quantum entanglement part of the entanglement entropy, which is otherwise dominated by the thermal entropy at large temperatures.

hep-th↗

Waveform Design for Secure SISO Transmissions and Multicasting

Wireless physical-layer security is an emerging field of research aiming at preventing eavesdropping in an open wireless medium. In this paper, we propose a novel waveform design approach to minimize the likelihood that a message transmitted between trusted single-antenna nodes is intercepted by an eavesdropper. In particular, with knowledge first of the eavesdropper's channel state information (CSI), we find the optimum waveform and transmit energy that minimize the signal-to-interference-plus-noise ratio (SINR) at the output of the eavesdropper's maximum-SINR linear filter, while at the same time provide the intended receiver with a required pre-specified SINR at the output of its own max-SINR filter. Next, if prior knowledge of the eavesdropper's CSI is unavailable, we design a waveform that maximizes the amount of energy available for generating disturbance to eavesdroppers, termed artificial noise (AN), while the SINR of the intended receiver is maintained at the pre-specified level. The extensions of the secure waveform design problem to multiple intended receivers are also investigated and semidefinite relaxation (SDR) -an approximation technique based on convex optimization- is utilized to solve the arising NP-hard design problems. Extensive simulation studies confirm our analytical performance predictions and illustrate the benefits of the designed waveforms on securing single-input single-output (SISO) transmissions and multicasting.

cs.CR↗

Strongly Coupled Gauge Theories: High and Low Temperature Behavior of Non-local Observables

We explore the high and low temperature behavior of non-local observables in strongly coupled gauge theories that are dual to AdS. We develop a systematic expansion for equal time two-point correlation, spatial Wilson loops and entanglement entropy at finite temperature using the AdS/CFT correspondence, leading to analytic expressions for these observables at high and low temperature limits. This approach enables the identification of the contributions of different regions of the bulk geometry to these gauge theory observables.

hep-th↗

Back-reaction of Non-supersymmetric Probes: Phase Transition and Stability

We consider back-reaction by non-supersymmetric D7/anti-D7 probe branes in the Kuperstein-Sonnenschein model at finite temperature. Using the smearing technique, we obtain an analytical solution for the back-reacted background to leading order in N_f/N_c. This back-reaction explicitly breaks the conformal invariance and introduces a dimension 6 operator in the dual field theory which is an irrelevant deformation of the original conformal field theory. We further probe this back-reacted background by introducing an additional set of probe brane/anti-brane. This additional probe sector undergoes a chiral phase transition at finite temperature, which is absent when the back-reaction vanishes. We investigate the corresponding phase diagram and the thermodynamics associated with this phase transition. We also argue that additional probes do not suffer from any instability caused by the back-reaction, which suggests that this system is stable beyond the probe limit.

hep-th↗

Inflation with General Initial Conditions for Scalar Perturbations

We explore the possibility of a single field quasi-de Sitter inflationary model with general initial state for primordial fluctuations. In this paper, first we compute the power spectrum and the bispectrum of scalar perturbations with coherent state as the initial state. We find that a large class of coherent states are indistinguishable from the Bunch-Davies vacuum state and hence consistent with the current observations. In case of a more general initial state built over Bunch-Davies vacuum state, we show that the constraints on the initial state from observed power spectrum and local bispectrum are relatively weak and for quasi-de Sitter inflation a large number of initial states are consistent with the current observations. However, renormalizability of the energy-momentum tensor of the fluctuations constraints the initial state further.

astro-ph.CO↗

Constraining the Inflationary Equation of State

We explore possible constraints on the inflationary equation state: p=wρ. While w must be close to -1 for those modes that contribute to the observed power spectrum, for those modes currently out of experimental reach, the constraints on w are much weaker, with only w<-1/3 as an a priori requirement. We find, however, that limits on the reheat temperature and the inflationary energy scale constrain w further, though there is still ample parameter space for a vastly different (accelerating) equation of state between the end of quasi-de Sitter inflation and the beginning of the radiation-dominated era. In the event that such an epoch of acceleration could be observed, we review the consequences for the primordial power spectrum.

astro-ph.CO↗