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Penedones J.,Perimeter Institute for Theoretical Physics | Penedones J.,Kavli Institute for Theoretical Physics
Journal of High Energy Physics

We explore the Mellin representation of conformal correlation functions recently proposed by Mack. Examples in the AdS/CFT context reinforce the analogy between Mellin amplitudes and scattering amplitudes. We conjecture a simple formula relating the bulk scattering amplitudes to the asymptotic behavior of Mellin amplitudes and show that previous results on the at space limit of AdS follow from our new formula. We find that the Mellin amplitudes are particularly useful in the case of conformal gauge theories in the planar limit. In this case, the four point Mellin amplitudes are meromorphic functions whose poles and their residues are entirely determined by two and three point functions of single-trace operators. This makes the Mellin amplitudes the ideal objects to attempt the conformal bootstrap program in higher dimensions. © SISSA 2011. Source

Dexter J.,University of California at Berkeley | Chris Fragile P.,College of Charleston | Chris Fragile P.,Kavli Institute for Theoretical Physics
Monthly Notices of the Royal Astronomical Society

High-resolution, multiwavelength and time-domain observations of the Galactic Centre black hole candidate, Sgr A*, allow for a direct test of contemporary accretion theory. Most models assume alignment between the accretion disc and black hole angular momentum axes, but this is not necessarily the case for geometrically thick accretion flows like that on to Sgr A*. Instead, we calculate images and spectra from a set of numerical simulations of accretion flows misaligned ('tilted') by 15° from the black hole spin axis and compare them with millimetre (mm) to near-infrared (NIR) observations. Non-axisymmetric standing shocks from eccentric fluid orbits dominate the emission, leading to a wide range of possible image morphologies. The strong effects of disc tilt lead to poorly constrained model parameters. These results suggest that previous parameter estimates from fitting aligned models, including estimates of the dimensionless black hole spin, likely only apply for small values of spin or tilt (upper limits of a < 0.3 or β < 15°). At 1.3 mm, the black hole images have crescent morphologies as in the aligned case, and the black hole shadow may still be accessible to future very long baseline interferometry (mm-VLBI) observations. Shock heating leads to multiple populations of electrons, some at high energies (Te > 1012 K). These electrons can naturally produce the observed NIR flux, spectral index and rapid variability ('flaring'). This NIR emission is uncorrelated with that in the mm, which also agrees with observations. These are the first numerical models to explain the time-variable mm to NIR emission of Sgr A*. Predictions of the model include significant structural changes observable with mm- VLBI on both the dynamical (hour) and Lense-Thirring precession (day-year) time-scales, and ≃30-50 μas changes in centroid position from extreme gravitational lensing events during NIR flares, detectable with the future VLT instrument GRAVITY.We further predict that multiwavelength monitoring should find no significant correlations between mm and NIR/X-ray light curves. The weak correlations reported to date are shown to be consistent with our model, where they are artefacts of the short light-curve durations. If the observed NIR emission is caused by shock heating in a tilted accretion disc, then this would require the Galactic Centre black hole to have a positive, non-zero spin parameter (a > 0) and may rule out a magnetically arrested state. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Cardy J.,University of Oxford | Cardy J.,All Souls College | Cardy J.,Kavli Institute for Theoretical Physics
Physical Review Letters

We consider a quantum quench in a finite system of length L described by a 1+1-dimensional conformal field theory (CFT), of central charge c, from a state with finite energy density corresponding to an inverse temperature Îâ‰L. For times t such that â.,"/2 Source

Cardy J.,University of Oxford | Cardy J.,All Souls College | Cardy J.,Kavli Institute for Theoretical Physics
Physical Review Letters

We show that block entanglement entropies in one-dimensional systems close to a quantum critical point can, in principle, be measured in terms of the population of low-lying energy levels following a certain type of local quantum quench. © 2011 American Physical Society. Source

Hebecker A.,University of Heidelberg | Kraus S.C.,German Electron Synchrotron | Westphal A.,Kavli Institute for Theoretical Physics
Physical Review D - Particles, Fields, Gravitation and Cosmology

Generically, the gravitational-wave or tensor-mode contribution to the primordial curvature spectrum of inflation is tiny if the field range of the inflaton is much smaller than the Planck scale. We show that this pessimistic conclusion is naturally avoided in a rather broad class of small-field models. More specifically, we consider models where an axionlike shift symmetry keeps the inflaton potential flat (up to nonperturbative cosine-shaped modulations), but inflation nevertheless ends in a waterfall regime, as is typical for hybrid inflation. In such hybrid natural inflation scenarios (examples are provided by Wilson line inflation and fluxbrane inflation), the slow-roll parameter Ïμ can be sizable during an early period (relevant for the cosmic microwave background spectrum). Subsequently, Ïμ quickly becomes very small before the tachyonic instability eventually terminates the slow-roll regime. In this scenario, one naturally generates a considerable tensor-mode contribution in the curvature spectrum, collecting nevertheless the required amount of e-foldings during the final period of inflation. While nonobservation of tensors by Planck is certainly not a problem, a discovery in the medium- to long-term future is realistic. © 2013 American Physical Society. Source

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