Black Hole Collapse in the 1/c Expansion

We present a first-principles CFT calculation corresponding to the spherical collapse of a shell of matter in three dimensional quantum gravity. In field theory terms, we describe the equilibration process, from early times to thermalization, of a CFT following a sudden injection of energy at time t=0. By formulating a continuum version of Zamolodchikov's monodromy method to calculate conformal blocks at large central charge c, we give a framework to compute a general class of probe observables in the collapse state, incorporating the full backreaction of matter fields on the dual geometry. This is illustrated by calculating a scalar field two-point function at time-like separation and the time-dependent entanglement entropy of an interval, both showing thermalization at late times. The results are in perfect agreement with previous gravity calculations in the AdS$_3$-Vaidya geometry. Information loss appears in the CFT as an explicit violation of unitarity in the 1/c expansion, restored by nonperturbative corrections.Comment: 39 pages, references added, corresponds with published versio

From Conformal Blocks to Path Integrals in the Vaidya Geometry

Correlators in conformal field theory are naturally organized as a sum over conformal blocks. In holographic theories, this sum must reorganize into a path integral over bulk fields and geometries. We explore how these two sums are related in the case of a point particle moving in the background of a 3d collapsing black hole. The conformal block expansion is recast as a sum over paths of the first-quantized particle moving in the bulk geometry. Off-shell worldlines of the particle correspond to subdominant contributions in the Euclidean conformal block expansion, but these same operators must be included in order to correctly reproduce complex saddles in the Lorentzian theory. During thermalization, a complex saddle dominates under certain circumstances; in this case, the CFT correlator is not given by the Virasoro identity block in any channel, but can be recovered by summing heavy operators. This effectively converts the conformal block expansion in CFT from a sum over intermediate states to a sum over channels that mimics the bulk path integral.Comment: 23 pages, 8 figure

Holography, Probe Branes and Isoperimetric Inequalities

In many instances of holographic correspondences between a d dimensional boundary theory and a d+1 dimensional bulk, a direct argument in the boundary theory implies that there must exist a simple and precise relation between the Euclidean on-shell action of a (d-1)-brane probing the bulk geometry and the Euclidean gravitational bulk action. This relation is crucial for the consistency of holography, yet it is non-trivial from the bulk perspective. In particular, we show that it relies on a nice isoperimetric inequality that must be satisfied in a large class of Poincar\'e-Einstein spaces. Remarkably, this inequality follows from theorems by Lee and Wang.Comment: 12 pages; v2: typos corrected, references added; v3: a few clarifying comments adde

The role of reading experience in atypical cortical tracking of speech and speech-in-noise in dyslexia

Available online 5 March 2022Dyslexia is a frequent developmental disorder in which reading acquisition is delayed and that is usually associ- ated with difficulties understanding speech in noise. At the neuronal level, children with dyslexia were reported to display abnormal cortical tracking of speech (CTS) at phrasal rate. Here, we aimed to determine if abnormal tracking relates to reduced reading experience, and if it is modulated by the severity of dyslexia or the presence of acoustic noise. We included 26 school-age children with dyslexia, 26 age-matched controls and 26 reading-level matched controls. All were native French speakers. Children’s brain activity was recorded with magnetoencephalography while they listened to continuous speech in noiseless and multiple noise conditions. CTS values were compared between groups, conditions and hemispheres, and also within groups, between children with mild and severe dyslexia. Syllabic CTS was significantly reduced in the right superior temporal gyrus in children with dyslexia com- pared with controls matched for age but not for reading level. Severe dyslexia was characterized by lower rapid automatized naming (RAN) abilities compared with mild dyslexia, and phrasal CTS lateralized to the right hemi- sphere in children with mild dyslexia and all control groups but not in children with severe dyslexia. Finally, an alteration in phrasal CTS was uncovered in children with dyslexia compared with age-matched controls in babble noise conditions but not in other less challenging listening conditions (non-speech noise or noiseless conditions); no such effect was seen in comparison with reading-level matched controls. Overall, our results confirmed the finding of altered neuronal basis of speech perception in noiseless and babble noise conditions in dyslexia compared with age-matched peers. However, the absence of alteration in comparison with reading-level matched controls demonstrates that such alterations are associated with reduced reading level, suggesting they are merely driven by reduced reading experience rather than a cause of dyslexia. Finally, our result of altered hemispheric lateralization of phrasal CTS in relation with altered RAN abilities in severe dyslexia is in line with a temporal sampling deficit of speech at phrasal rate in dyslexia.Florian Destoky, Julie Bertels and Mathieu Bourguignon have been supported by the program Attract of Innoviris (Grants 2015-BB2B-10 and 2019-BFB-110). Julie Bertels has been supported by a research grant from the Fonds de Soutien Marguerite-Marie Delacroix (Brussels, Bel- gium). Xavier De Tiège is Post-doctorate Clinical Master Specialist at the Fonds de la Recherche Scientifique (F.R.S.-FNRS, Brussels, Belgium). Mathieu Bourguignon has been supported by the Marie Sk ł odowska- Curie Action of the European Commission (Grant 743562). The MEG project at the CUB Hôpital Erasme and this study were financially supported by the Fonds Erasme (Research convention “Les Voies du Savoir ”, Brussels, Belgium). The PET-MR project at the CUB Hôpital Erasme is supported by the Association Vinçotte Nuclear (AVN, Brussels, Belgium)

Proprioceptive and tactile processing in individuals with Friedreich ataxia: an fMRI study

ObjectiveFriedreich ataxia (FA) neuropathology affects dorsal root ganglia, posterior columns in the spinal cord, the spinocerebellar tracts, and cerebellar dentate nuclei. The impact of the somatosensory system on ataxic symptoms remains debated. This study aims to better evaluate the contribution of somatosensory processing to ataxia clinical severity by simultaneously investigating passive movement and tactile pneumatic stimulation in individuals with FA.MethodsTwenty patients with FA and 20 healthy participants were included. All subjects underwent two 6 min block-design functional magnetic resonance imaging (fMRI) paradigms consisting of twelve 30 s alternating blocks (10 brain volumes per block, 120 brain volumes per paradigm) of a tactile oddball paradigm and a passive movement paradigm. Spearman rank correlation tests were used for correlations between BOLD levels and ataxia severity.ResultsThe passive movement paradigm led to the lower activation of primary (cSI) and secondary somatosensory cortices (cSII) in FA compared with healthy subjects (respectively 1.1 ± 0.78 vs. 0.61 ± 1.02, p = 0.04, and 0.69 ± 0.5 vs. 0.3 ± 0.41, p = 0.005). In the tactile paradigm, there was no significant difference between cSI and cSII activation levels in healthy controls and FA (respectively 0.88 ± 0.73 vs. 1.14 ± 0.99, p = 0.33, and 0.54 ± 0.37 vs. 0.55 ± 0.54, p = 0.93). Correlation analysis showed a significant correlation between cSI activation levels in the tactile paradigm and the clinical severity (R = 0.481, p = 0.032).InterpretationOur study captured the difference between tactile and proprioceptive impairments in FA using somatosensory fMRI paradigms. The lack of correlation between the proprioceptive paradigm and ataxia clinical parameters supports a low contribution of afferent ataxia to FA clinical severity

Emergent geometry from D-Branes

In this thesis, we explain and illustrate on several examples how to derive supergravity solutions by computing observables in the corresponding dual, lower-dimensional field theory.In particular, no a priori knowledge on the gravitational dual is assumed, including its dimensionality. The basic idea to construct the pre-geometric models is to consider the world-volume theory of probe D-branes in the presence of a large number N of higher-dimensional background branes. In the standard decoupling limit, the probes are moving only in the flat directions parallel to the background D-branes. We show however that the quantum effective action of the probe world-volume theory, obtained at large $N$ using standard vector model techniques, has the required field content to be interpreted as the action describing the probes in a higher-dimensional, curved and classical spacetime. The properties of the emerging supergravity solution are easily found by comparing the quantum effective action of the pre-geometric model with the non-abelian D-brane action. In all the examples we consider, this allows us to derive the metric, the dilaton and various form fields, overall performing exclusively field theoretic computations.The first part of the thesis consists of introductory chapters, where we review vector models at large N, aspects of brane physics in supergravity and string theory and the gauge/gravity correspondence. The second part contains the original contributions of this thesis, consisting of various explicit emergent geometry examples.Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

Introduction to conformal field theory

An elementary introduction to Conformal Field Theory is provided. We start by reviewing symmetries in classical and quantum field theories and the associated existence of conserved currents by Noether’s theorem. Next we describe general facts on conformal invariance in d dimensions and finish with the analysis of the special case d = 2, including an introduction to the operator formalism and a discussion of the trace anomaly.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

Gravity and on-shell probe actions

In any gravitational theory and in a wide class of background space-times, we argue that there exists a simple, yet profound, relation between the on-shell Euclidean gravitational action and the on-shell Euclidean action of probes. The probes can be, for instance, charged particles or branes. The relation is tightly related to the thermodynamic nature of gravity. We provide precise checks of the relation in several examples, which include both asymptotically flat and asymptotically AdS space-times, with particle, D-brane and M-brane probes. Perfect consistency is found in all cases, including in a highly non-trivial example including α ′ -corrections