241 research outputs found
A not so brief commentary on cosmological entropy bounds
There has been, quite recently, a discussion on how holographic-inspired
bounds might be used to encompass the present-day dark energy and
early-universe inflation into a single paradigm. In the current treatment, we
point out an inconsistency in the proposed framework and then provide a viable
resolution. We also elaborate on some of the implications of this framework and
further motivate the proposed holographic connection. The manuscript ends with
a more speculative note on cosmic time as an emergent (holographically induced)
construct.Comment: 12 pages and Revtex; (v2) reference added and a few cosmetic change
Inhomogeneous holographic thermalization
The sudden injection of energy in a strongly coupled conformal field theory
and its subsequent thermalization can be holographically modeled by a shell
falling into anti-de Sitter space and forming a black brane. For a homogeneous
shell, Bhattacharyya and Minwalla were able to study this process analytically
using a weak field approximation. Motivated by event-by-event fluctuations in
heavy ion collisions, we include inhomogeneities in this model, obtaining
analytic results in a long wavelength expansion. In the early-time window in
which our approximations can be trusted, the resulting evolution matches well
with that of a simple free streaming model. Near the end of this time window,
we find that the stress tensor approaches that of second-order viscous
hydrodynamics. We comment on possible lessons for heavy ion phenomenology.Comment: 53 pages, 10 figures; v2: references adde
Inhomogeneous Thermalization in Strongly Coupled Field Theories
To describe theoretically the creation and evolution of the quark-gluon
plasma, one typically employs three ingredients: a model for the initial state,
non-hydrodynamic early time evolution, and hydrodynamics. In this paper we
study the non-hydrodynamic early time evolution using the AdS/CFT
correspondence in the presence of inhomogeneities. We find that the AdS
description of the early time evolution is well-matched by free streaming. Near
the end of the early time interval where our analytic computations are
reliable, the stress tensor agrees with the second order hydrodynamic stress
tensor computed from the local energy density and fluid velocity. Our
techniques may also be useful for the study of far-from-equilibrium strongly
coupled systems in other areas of physics.Comment: 5 pages, 3 figures; v2: minor clarifications and reference adde
The Lorentz force between D0 and D6 branes in string and M(atrix) theory
We use different techniques to analyze the system formed by a D0 brane and a
D6 brane (with background gauge fields) in relative motion. In particular,
using the closed string formalism of boosted boundary states, we show the
presence of a term linear in the velocity, corresponding to the Lorentz force
experienced by the D0 brane moving in the magnetic background produced by the
D6 brane. This term, that was missed in previous analyses of this system, comes
entirely from the R-R odd spin structure and is also reproduced by a M(atrix)
theory calculation.Comment: 13 pages, plain LaTeX; some clarifying comments and a reference adde
Probing a D6 + D0 state with D6-branes: SYM - Supergravity correspondence at subleading level
We probe a non-supersymmetric D6 + D0 state with D6-branes and find agreement
at subleading order between the supergravity and super Yang-Mills description
of the long-distance, low-velocity interaction.Comment: LaTeX, 15 pages, no figure
Bayesian inference of physiologically meaningful parameters from body sway measurements
The control of the human body sway by the central nervous system, muscles, and conscious brain is of interest since body sway carries information about the physiological status of a person. Several models have been proposed to describe body sway in an upright standing position, however, due to the statistical intractability of the more realistic models, no formal parameter inference has previously been conducted and the expressive power of such models for real human subjects remains unknown. Using the latest advances in Bayesian statistical inference for intractable models, we fitted a nonlinear control model to posturographic measurements, and we showed that it can accurately predict the sway characteristics of both simulated and real subjects. Our method provides a full statistical characterization of the uncertainty related to all model parameters as quantified by posterior probability density functions, which is useful for comparisons across subjects and test settings. The ability to infer intractable control models from sensor data opens new possibilities for monitoring and predicting body status in health applications.Peer reviewe
Near Real-Time Data Labeling Using a Depth Sensor for EMG Based Prosthetic Arms
Recognizing sEMG (Surface Electromyography) signals belonging to a particular
action (e.g., lateral arm raise) automatically is a challenging task as EMG
signals themselves have a lot of variation even for the same action due to
several factors. To overcome this issue, there should be a proper separation
which indicates similar patterns repetitively for a particular action in raw
signals. A repetitive pattern is not always matched because the same action can
be carried out with different time duration. Thus, a depth sensor (Kinect) was
used for pattern identification where three joint angles were recording
continuously which is clearly separable for a particular action while recording
sEMG signals. To Segment out a repetitive pattern in angle data, MDTW (Moving
Dynamic Time Warping) approach is introduced. This technique is allowed to
retrieve suspected motion of interest from raw signals. MDTW based on DTW
algorithm, but it will be moving through the whole dataset in a pre-defined
manner which is capable of picking up almost all the suspected segments inside
a given dataset an optimal way. Elevated bicep curl and lateral arm raise
movements are taken as motions of interest to show how the proposed technique
can be employed to achieve auto identification and labelling. The full
implementation is available at https://github.com/GPrathap/OpenBCIPytho
Index Theorems and Loop Space Geometry
We investigate the evaluation of the Dirac index using symplectic geometry in
the loop space of the corresponding supersymmetric quantum mechanical model. In
particular, we find that if we impose a simple first class constraint, we can
evaluate the Callias index of an odd dimensional Dirac operator directly from
the quantum mechanical model which yields the Atiyah-Singer index of an even
dimensional Dirac operator in one more dimension. The effective action obtained
by BRST quantization of this constrained system can be interpreted in terms of
loop space symplectic geometry, and the corresponding path integral for the
index can be evaluated exactly using the recently developed localization
techniques.Comment: 15 pages, report CERN-TH-6471 and HU-TFT-92-1
Holography and thermalization in optical pump-probe spectroscopy
Using holography, we model experiments in which a 2 + 1D strange metal is pumped by a laser pulse into a highly excited state, after which the time evolution of the optical conductivity is probed. We consider a finite-density state with mildly broken translation invariance and excite it by oscillating electric field pulses. At zero density, the optical conductivity would assume its thermalized value immediately after the pumping has ended. At finite density, pulses with significant dc components give rise to slow exponential relaxation, governed by a vector quasinormal mode. In contrast, for high-frequency pulses the amplitude of the quasinormal mode is strongly suppressed, so that the optical conductivity assumes its thermalized value effectively instantaneously. This surprising prediction may provide a stimulus for taking up the challenge to realize these experiments in the laboratory. Such experiments would test a crucial open question faced by applied holography: are its predictions artifacts of the large N limit or do they enjoy sufficient UV independence to hold at least qualitatively in real-world systems?Peer reviewe
- âŠ