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Design of Experiments for Model Discrimination Hybridising Analytical and Data-Driven ApproachesFeb 12 2018Healthcare companies must submit pharmaceutical drugs or medical devices to regulatory bodies before marketing new technology. Regulatory bodies frequently require transparent and interpretable computational modelling to justify a new healthcare technology, ... More

Bayesian Optimization with Dimension Scheduling: Application to Biological SystemsNov 17 2015Bayesian Optimization (BO) is a data-efficient method for global black-box optimization of an expensive-to-evaluate fitness function. BO typically assumes that computation cost of BO is cheap, but experiments are time consuming or costly. In practice, ... More

The Marked Length Spectrum vs. The Laplace Spectrum on Forms on Riemannian NilmanifoldsOct 05 1995The subject of this paper is the relationship among the marked length spectrum, the length spectrum, the Laplace spectrum on functions, and the Laplace spectrum on forms on Riemannian nilmanifolds. In particular, we show that for a large class of three-step ... More

Cosmological Strucure Formation with Topological DefectsMar 23 2000Structure formation with topological defects is described. The main differences from inflationary models are highlighted. The results are compared with recent observations. It is concluded that all the defect models studied so far are in disagreement ... More

The three burials of Melquiades DGPJan 10 2008In this talk I review three fatal flaws of the DGP braneworld model, which has been put forward as a possible model for late time acceleration without a cosmological constant: Ghosts, Cosmological Crashes, and Instability of the 5D vacuum. The talk is ... More

G1 Cosmologies with Gravitational and Scalar WavesDec 18 1998I present here a new algorithm to generate families of inhomogeneous massless scalar field cosmologies. New spacetimes, having a single isometry, are generated by breaking the homogeneity of massless scalar field $G_2$ models along one direction. As an ... More

Theory of the Interaction of Planetary Nebulae with the Interstellar MediumJan 02 2000The theory of the interaction of planetary nebulae with the interstellar medium is important for the interpretation of nebular morphologies that deviate from point symmetry. It can be used to probe the interstellar medium and its magnetic field. We emphasize ... More

An introduction to right-angled Artin groupsOct 23 2006Recently, right-angled Artin groups have attracted much attention in geometric group theory. They have a rich structure of subgroups and nice algorithmic properties, and they give rise to cubical complexes with a variety of applications. This survey article ... More

Gravitational waves from cosmological phase transitionsFeb 08 2010I discuss the generation of a stochastic background of gravitational waves during a first order phase transition. I present simple general arguments which explain the main features of the gravitational wave spectrum like the $k^3$ power law growth on ... More

Frontiers of the Universe: What do we know, what do we understand?May 07 2002In this talk I summarize the topics addressed during the conference. We have discussed the most relevant cosmological observations of the last say 10 years and their implications for our understanding of the Universe. My finding throughout this meeting ... More

Cosmic Magnetic Fields and the CMBSep 08 2006I describe the imprint of primordial magnetic fields on the CMB. I show that these are observable only if the field amplitude is of the order of $B\gsim 10^{-9}G$ on Mpc scale. I further argue that such fields are strongly constrained by the stochastic ... More

Nonsingular global stringsJun 04 1996Oct 17 1997We examine the possibility that time dependence might remove the singular nature of global string spacetimes. We first show that this time dependence takes a specific form -- a de-Sitter like expansion along the length of the string and give an argument ... More

Global field dynamics and cosmological tructure formationNov 02 1994We discuss gravitational effects of global scalar fields and, especially, of global topological defects. We first give an introduction to the dynamics of global fields and the formation of defects. Next we investigate the induced gravitational fields, ... More

Macroscopic open strings and gravitational confinementAug 14 2002We consider classical solutions for strings ending on magnetically charged black holes in four-dimensional Kaluza-Klein theory. We examine the classical superstring and the global vortex, which can be viewed as a nonsingular model for the superstring. ... More

Clarifying perturbations in the ekpyrotic universeDec 04 2001Mar 22 2002In this note I try to clarify the problem of perturbations in the ekpyrotic universe. I write down the most general matching conditions and specify the choices taken by the two debating sides. I also bring up the problem of surface stresses which always ... More

Holographic Superconductivity with Gauss-Bonnet gravityDec 07 2010I review recent work on holographic superconductivity with Einstein-Gauss-Bonnet gravity, and show how the critical temperature of the superconductor depends on both gravitational backreaction and the Gauss-Bonnet parameter, using both analytic and numerical ... More

Loop amplitudes in gauge theories: modern analytic approachesDec 20 2010Mar 24 2011This article reviews on-shell methods for analytic computation of loop amplitudes, emphasizing techniques based on unitarity cuts. Unitarity techniques are formulated generally but have been especially useful for calculating one-loop amplitudes in massless ... More

The theory of CMB anisotropiesSep 27 2001This is a review of the theory of CMB anisotropies, an updated version of a course given at the troisieme cycle de la Suisse Romande. An introduction to gauge invariant cosmological perturbation theory is given and the theory CMB anisotropies is develiped ... More

BraneworldsJul 01 2005This course is an introduction to the physics of braneworlds. We concentrate on braneworlds with only one extra-dimension and discuss their gravity. We derive the gravitational equations on the brane from the bulk Einstein equation and explore some limits ... More

Inflating p-branesApr 30 2003We look for solutions in Einstein gravity corresponding to inflating braneworlds of arbitrary dimension and co-dimension. These solutions correspond to isolated sources (no long range fields). Using dynamical systems techniques, we show that there exists ... More

Anisotropies in the Cosmic Microwave Background: Theoretical FoundationsOct 29 1996The analysis of anisotropies in the cosmic microwave background (CMB) has become an extremely valuable tool for cosmology. We even have hopes that planned CMB anisotropy experiments may revolutionize cosmology. Together with determinations of the CMB ... More

Cosmic p-branesOct 27 1995Jan 31 1997We consider the metrics for cosmic strings and $p$-branes in spacetime dimension $N>4$, that is, we look for solutions to Einstein-Maxwell-Dilaton gravity in $N$-dimensions with boost symmetry in the $p$-directions along the brane. Focussing first in ... More

Light Deflection in Perturbed Friedmann UniversesJan 18 1994In this letter a new formula for light deflection is derived using only physically observable concepts. The general result is specialized to cosmological perturbation theory and expressed in terms of gauge--invariant perturbation variables. The resulting ... More

Early Reionization in CosmologyNov 17 1993The cosmic microwave background (CMB) anisotropies have turned out to represent one of the most stringent 'bottle necks' for scenarios of large scale structure formation. As a possibility to relax this constraint, it has been proposed that early reionization ... More

Nonsingular global string compactificationsNov 03 1999Nov 17 1999We consider an exotic `compactification' of spacetime in which there are two infinite extra dimensions, using a global string instead of a domain wall. By having a negative cosmological constant we prove the existence of a nonsingular static solution ... More

The Cosmic Microwave Background: The history of its experimental investigation and its significance for cosmologyJun 05 2015This review describes the discovery of the cosmic microwave background radiation in 1965 and its impact on cosmology in the 50 years that followed. This discovery has established the Big Bang model of the Universe and the analysis of its fluctuations ... More

Geometrical constraints on dark energy modelsOct 15 2007This contribution intends to give a pedagogical introduction to the topic of dark energy (the mysterious agent supposed to drive the observed late time acceleration of the Universe) and to various observational tests which require only assumptions on ... More

The spatial structure of cell signaling systemsAug 03 2013The spatial structure of the cell is highly organized at all levels: from small complexes and assemblies, to local nano- and micro-clusters, to global, micrometer scales across and between cells. We suggest that this multiscale spatial cell organization ... More

Physics of Cosmic Microwave Background anisotropies and primordial fluctuationsSep 17 2001The physics of the origin and evolution of CMB anisotropies is described. I explain the idea and status of cosmic parameter estimation and follow it up with critical comments on its dependence on model assumptions and initial conditions.

Gauge Invariant Cosmological Perturbation TheoryNov 17 1993After an introduction to the problem of cosmological structure formation, we develop gauge invariant cosmological perturbation theory. We derive the first order perturbation equations of Einstein's equations and energy momentum ``conservation''. Furthermore, ... More

A New Construction of Isospectral Riemannian Nilmanifolds with ExamplesFeb 10 1995We present a new construction for obtaining pairs of higher-step isospectral Riemannian nilmanifolds and compare several resulting new examples. In particular, we present new examples of manifolds that are isospectral on functions, but not isospectral ... More

Braneworld black holesApr 16 2008In these lectures, I give an introduction to and overview of braneworlds and black holes in the context of warped compactifications. I first describe the general paradigm of braneworlds, and introduce the Randall-Sundrum model. I discuss braneworld gravity, ... More

What do we really know about Dark Energy?Mar 28 2011Sep 25 2011In this paper I discuss what we truly know about dark energy. I shall argue that up to date our single indication for the existence of dark energy comes from distance measurements and their relation to redshift. Supernovae, CMB anisotropies and observations ... More

Rigidity of cosmic acceleration in a class of k-essence cosmologiesOct 05 2004We study the structural stability of a cosmic acceleration (inflation) in a class of k-essence cosmologies against changes in the shape of the potential. Those models may be viewed as generalized tachyon cosmologies and this analysis extends previous ... More

Thick ideals in equivariant and motivic stable homotopy categoriesMar 29 2015We study thick ideals in the stable motivic homotopy category SH(k) and in its subcategories of compact and of finite cellular objects. If k is a subfield of the complex or even the real numbers, then using comparison functors we find thick ideals corresponding ... More

Continuous Families of Riemannian manifolds, isospectral on functions but not on 1-formsFeb 13 1997The purpose of this paper is to present the first continuous families of Riemannian manifolds isospectral on functions but not on 1-forms, and simultaneously, the first continuous families of Riemannian manifolds with the same marked length spectrum but ... More

The Gregory-Laflamme instabilityJul 28 2011I give a brief review of the Gregory-Laflamme instability as originally found for a 5-dimensional black string. This is a chapter of the book "Black Holes in Higher Dimensions" to be published by Cambridge University Press (editor: G. Horowitz)

The Marked Length Spectrum Versus the Laplace Spectrum on Forms on Riemannian NilmanifoldsOct 13 1995The subject of this paper is the relationship among the marked length spectrum, the length spectrum, the Laplace spectrum on functions, and the Laplace spectrum on forms on Riemannian nilmanifolds. In particular, we show that for a large class of three-step ... More

A Note on the Finiteness of Curvature at Cosmic String CuspsDec 05 1994We examine a Nambu-Goto string trajectory in the neighbourhood of a cusp and determine the extrinsic curvature invariants. These are demonstrated to be finite, in contradiction with naive expectation. Thus the Nambu action is a valid approximation for ... More

Cosmological perturbation theoryFeb 05 2004Sep 21 2004This is a review on cosmological perturbation theory. After an introduction, it presents the problem of gauge transformation. Gauge invariant variables are introduced and the Einstein and conservation equations are written in terms of these variables. ... More

Black string instabilities in anti-de Sitter spaceApr 13 2000Sep 25 2000We show how to extend the usual black string instability of vacuum or charged black p-branes to the anti-de Sitter background. The string fragments in an analogous fashion to the $\Lambda=0$ case, the main difference being that instead of a periodic array ... More

Signatures of Topological Defects in the Microwave Sky: An IntroductionFeb 28 1997An introduction to topological defects in cosmology is given. We discuss their possible relevance for structure formation. Especial emphasis is given on the signature of topological defects in the spectrum of anisotropies in the cosmic microwave background. ... More

Symmetries and assisted inflation in the DGP braneworld modelFeb 03 2004Oct 20 2004We present here the general transformation that leaves unchanged the form of the field equations for perfect fluid cosmologies in the DGP braneworld model. Specifically, a prescription for relating exact solutions with different equations of state is ... More

Discussion of "Multiple Testing for Exploratory Research" by J. J. Goeman and A. SolariAug 16 2012Goeman and Solari [Statist. Sci. 26 (2011) 584-597, arXiv:1208.2841] have addressed the interesting topic of multiple testing for exploratory research, and provided us with nice suggestions for exploratory analysis. They defined properties that an inferential ... More

Dark Energy and Modified GravityNov 25 2008Explanations of the late-time cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are mostly based on a dark energy field with fine-tuned, unnatural properties. There is a great variety ... More

Recent Progress in Regge CalculusFeb 03 1997While there has been some advance in the use of Regge calculus as a tool in numerical relativity, the main progress in Regge calculus recently has been in quantum gravity. After a brief discussion of this progress, attention is focussed on two particular, ... More

Gravitational wave production: A strong constraint on primordial magnetic fieldsJun 13 2001Oct 01 2001We compute the gravity waves induced by anisotropic stresses of stochastic primordial magnetic fields. The nucleosynthesis bound on gravity waves is then used to derive a limit on the magnetic field amplitude as function of the spectral index. The obtained ... More

Kalb-Ramond axion production in anisotropic string cosmologiesMar 13 2000We compute the energy spectra for massless Kalb-Ramond axions in four-dimensional anisotropic string cosmological models. We show that, when integrated over directions, the four-dimensional anisotropic model leads to infra-red divergent spectra similar ... More

Testing Superstring Theories with Gravitational WavesMay 26 2011Oct 24 2011We provide a simple transfer function that determines the effect of an early matter dominated era on the gravitational wave background and show that a large class of compactifications of superstring theory might be tested by observations of the gravitational ... More

Microwave Background Anisotropies from Scaling Seed PerturbationsFeb 03 1997Jul 28 1997We study microwave background anisotropies induced by scaling seed perturbations in a universe dominated by cold dark matter. Using a gauge invariant linear perturbation analysis, we solve the perturbation equations on super-horizon scales, for CMB anisotropies ... More

Selection rules for splitting stringsMay 01 1997Oct 17 1997It has been pointed out that Nielsen-Olesen vortices may be able to decay by pair production of black holes. We show that when the abelian Higgs model is embedded in a larger theory, the additional fields may lead to selection rules for this process - ... More

Cosmological constraints combining H(z), CMB shift and SNIa observational dataApr 20 2007Jul 27 2007Recently H(z) data obtained from differential ages of galaxies have been proposed as a new geometrical probe of dark energy. In this paper we use those data, combined with other background tests (CMB shift and SNIa data), to constrain a set of general ... More

A new method for the Alcock-Paczynski testJun 15 2012Aug 07 2012We argue that from observations alone, only the transverse power spectrum $C_\ell(z_1,z_2)$ and the corresponding correlation function $\xi(\theta,z_1,z_2)$ can be measured and that these contain the full three dimensional information. We determine the ... More

The Effect of Heteroscedasticity on Regression TreesJun 16 2016Regression trees are becoming increasingly popular as omnibus predicting tools and as the basis of numerous modern statistical learning ensembles. Part of their popularity is their ability to create a regression prediction without ever specifying a structure ... More

Single Cut IntegrationNov 10 2010Nov 11 2010We present an analytic technique for evaluating single cuts for one-loop integrands, where exactly one propagator is taken to be on shell. Our method extends the double-cut integration formalism of one-loop amplitudes to the single-cut case. We argue ... More

Spin Properties of Supermassive Black Holes with Powerful OutflowsJan 13 2016Relationships between beam power and accretion disk luminosity are studied for a sample of 55 HERG, 13 LERG, and 29 RLQ with powerful outflows. The ratio of beam power to disk luminosity tends to be high for LERG, low for RLQ, and spans the full range ... More

Vortices and black holes in dilatonic gravityJun 25 1999Apr 11 2000We study analytically black holes pierced by a thin vortex in dilatonic gravity for an arbitrary coupling of the vortex to the dilaton in an arbitrary frame. We show that the horizon of the charged black hole supports the long-range fields of the Nielsen-Olesen ... More

Unitarity Cuts with Massive Propagators and Algebraic Expressions for CoefficientsDec 07 2006Jun 13 2007In the first part of this paper, we extend the d-dimensional unitarity cut method of hep-ph/0609191 to cases with massive propagators. We present formulas for integral reduction with which one can obtain coefficients of all pentagon, box, triangle and ... More

A new Garside structure for braid groups of type $(e,e,r)$Jan 06 2009Feb 04 2009We describe a new presentation for the complex reflection groups of type $(e,e,r)$ and their braid groups. A diagram for this presentation is proposed. The presentation is a monoid presentation which is shown to give rise to a Garside structure. A detailed ... More

Molybdenum, Ruthenium, and the Heavy r-process Elements in Moderately Metal-Poor Main-Sequence Turnoff StarsApr 03 2013The ratios of elemental abundances observed in metal-poor stars of the Galactic halo provide a unique present-day record of the nucleosynthesis products of its earliest stars. While the heaviest elements were synthesized by the r- and s-processes, dominant ... More

The Nature of the Controversy over Time-Symmetric Quantum CounterfactualsJan 22 2014It is proposed that the recent controversy over "time-symmetric quantum counterfactuals" (TSQCs), based on the Aharonov-Bergmann-Lebowitz Rule for measurements of pre- and post-selected systems, can be clarified by taking TSQCs to be counterfactuals with ... More

The $k$-Dominating GraphSep 24 2012Mar 01 2013Given a graph $G$, the $k$-dominating graph of $G$, $D_k(G)$, is defined to be the graph whose vertices correspond to the dominating sets of $G$ that have cardinality at most $k$. Two vertices in $D_k(G)$ are adjacent if and only if the corresponding ... More

On Habiro's cyclotomic expansions of the Ohtsuki invariantJan 31 2005Feb 07 2005We give a self-contained treatment of Le and Habiro's approach to the Jones function of a knot and Habiro's cyclotomic form of the Ohtsuki invariant for manifolds obtained by surgery around a knot. On the way we reproduce a state sum formula of Garoufalidis ... More

Curvature Corrections to Dynamics of Domain WallsOct 13 1994The most usual procedure for deriving curvature corrections to effective actions for topological defects is subjected to a critical reappraisal. A logically unjustified step (leading to overdetermination) is identified and rectified, taking the standard ... More

Probabilistic inference of basic stellar parameters: application to flickering starsJul 04 2016Jul 16 2016The relations between observable stellar parameters are usually assumed to be deterministic. That is, given an infinitely precise measurement of independent variable, `$x$', and some model, the value of dependent variable, `$y$' can be known exactly. ... More

The field-of-norms functor and the Hilbert symbol for higher local fieldsOct 04 2010The field-of-norms functor is applied to deduce explicit reciprocity formulae for the Hilbert symbol in the mixed characteristic case from the explicit formula for the Witt symbol in characteristic p > 2 in the context of higher local fields. Is is shown ... More

Non-crossing partitions of type (e,e,r)Mar 23 2004Apr 21 2004We investigate a new lattice of generalised non-crossing partitions, constructed using the geometry of the complex reflection group $G(e,e,r)$. For the particular case $e=2$ (resp. $r=2$), our lattice coincides with the lattice of simple elements for ... More

Automorphism groups of some affine and finite type Artin groupsAug 30 2004We observe that, for each positive integer n > 2, each of the Artin groups of finite type A_n, B_n=C_n, and affine type \tilde A_{n-1} and \tilde C_{n-1} is a central extension of a finite index subgroup of the mapping class group of the (n+2)-punctured ... More

Gravitational waves from stochastic relativistic sources: primordial turbulence and magnetic fieldsMar 17 2006Sep 15 2006The power spectrum of a homogeneous and isotropic stochastic variable, characterized by a finite correlation length, does in general not vanish on scales larger than the correlation scale. If the variable is a divergence free vector field, we demonstrate ... More

Nested braneworlds and strong brane gravityApr 30 2001We find the gravitational field of a `nested' domain wall living entirely within a brane-universe, or, a localised vortex within a wall. For a vortex living on a critical Randall-Sundrum brane universe, we show that the induced gravitational field on ... More

External leg corrections in the unitarity methodSep 23 2011Feb 11 2012Unitarity cuts diverge in the channel of a single massive external fermion. We propose an off-shell continuation of the momentum that allows a finite evaluation of the unitarity cuts. If the cut is taken with complete amplitudes on each side, our continuation ... More

Axisymmetric metrics in arbitrary dimensionsJun 16 2003Jul 10 2004We consider axially symmetric static metrics in arbitrary dimension, both with and without a cosmological constant. The most obvious such solutions have an SO(n) group of Killing vectors representing the axial symmetry, although one can also consider ... More

Tachyonic perturbations in AdS$_5$ orbifoldsSep 28 2004May 02 2005We show that scalar as well as vector and tensor metric perturbations in the Randall-Sundrum II braneworld allow normalizable tachyonic modes, i.e., possible instabilities. These instabilities require nonvanishing initial anisotropic stresses on the brane. ... More

Radio Galaxies and the Acceleration of the Universe Beyond a Redshift of OneDec 04 2002Radio galaxies provide a means to determine the coordinate distance,the luminosity distance, the dimensionless luminosity distance, or the angular size distance to sources with redshifts as large as two. Dimensionless coodinate distances for 55 supernovae ... More

Black Hole Spins of Radio SourcesNov 12 2008Dec 16 2008A new approach to constraining or determining the spin of a massive black hole is proposed. A key parameter in the analysis is the dimensionless ratio, r, of the energy released to the mass of the hole. It is shown that the black hole spin, j, may be ... More

Cosmology from cross correlation of CMB lensing and galaxy surveysNov 04 2013In recent years cross correlation of lensing of the Cosmic Microwave Background (CMB) with other large scale structure (LSS) tracers has been used as a method to detect CMB lensing. Current experiments are also becoming sensitive enough to measure CMB ... More

Assessing replicability of findings across two studies of multiple featuresApr 02 2015Replicability analysis aims to identify the findings that replicated across independent studies that examine the same features. We provide powerful novel replicability analysis procedures for two studies for FWER and for FDR control on the replicability ... More

Replicability analysis for genome-wide association studiesSep 13 2012Apr 29 2014The paramount importance of replicating associations is well recognized in the genome-wide associaton (GWA) research community, yet methods for assessing replicability of associations are scarce. Published GWA studies often combine separately the results ... More

N=1/2 Wess-Zumino model is renormalizableJul 17 2003Nov 21 2003The Wess-Zumino model on N=1/2 nonanticommutative superspace, which contains the dimension-6 term F^3, is shown to be renormalizable to all orders in perturbation theory, upon adding F and F^2 terms to the original Lagrangian. The renormalizability is ... More

Maximum likelihood estimation of mark-recapture-recovery models in the presence of continuous covariatesNov 05 2012Nov 29 2013We consider mark-recapture-recovery (MRR) data of animals where the model parameters are a function of individual time-varying continuous covariates. For such covariates, the covariate value is unobserved if the corresponding individual is unobserved, ... More

Semi-Markov Arnason-Schwarz modelsMay 18 2015We consider multi-state capture-recapture-recovery data where observed individuals are recorded in a set of possible discrete states. Traditionally, the Arnason-Schwarz model has been fitted to such data where the state process is modeled as a first-order ... More

Random groups arising as graph productsJun 17 2010In this paper we study the hyperbolicity properties of a class of random groups arising as graph products associated to random graphs. Recall, that the construction of a graph product is a generalization of the constructions of right-angled Artin and ... More

Classical 3-loop 2-body diagramsJun 13 2013Jul 30 2013As part of the study of the two-body problem in Einstein's gravity, the fourth post-Newtonian order (4PN) of the two-body effective action is being computed presently by both effective field theory (EFT) methods and others. Diagrams with 3 (or 4) classical ... More

Color structures and permutationsMar 26 2014Nov 10 2014Color structures for tree level scattering amplitudes in gauge theory are studied in order to determine the symmetry properties of the color-ordered sub-amplitudes. We mathematically formulate the space of color structures together with the action of ... More

The Oscillating Universe: an Alternative to InflationOct 19 1995Jan 05 1996The aim of this paper is to show, that the 'oscillating universe' is a viable alternative to inflation. We remind that this model provides a natural solution to the flatness or entropy and to the horizon problem of standard cosmology. We study the evolution ... More

Computing the Bergsma Dassios sign-covarianceMay 27 2016Bergsma and Dassios (2014) introduced an independence measure which is zero if and only if two random variables are independent. This measure can be naively calculated in $O(n^4)$. Weihs et al. (2015) showed that it can be calculated in $O(n^2 \log n)$. ... More

The Extreme Overabundance of Molybdenum in Two Metal-Poor StarsAug 17 2011Nov 09 2011We report determinations of the molybdenum abundances in five mildly to extremely metal-poor turnoff stars using five Mo II lines near 2000A. In two of the stars, the abundance of molybdenum is found to be extremely enhanced, as high or higher than the ... More

A lexicographic shellability characterization of geometric latticesAug 09 2011Oct 16 2013Geometric lattices are characterized in this paper as those finite, atomic lattices such that every atom ordering induces a lexicographic shelling given by an edge labeling known as a minimal labeling. Equivalently, geometric lattices are shown to be ... More

Contracting Boundaries of CAT(0) SpacesAug 29 2013As demonstrated by Croke and Kleiner, the visual boundary of a CAT(0) group is not well-defined since quasi-isometric CAT(0) spaces can have non-homeomorphic boundaries. We introduce a new type of boundary for a CAT(0) space, called the contracting boundary, ... More

Subgroups and quotient groups of automorphism groups of RAAGsSep 13 2009Mar 23 2010We study subgroups and quotients of outer automorphism groups of right-angled Artin groups (RAAGs). We prove that for all RAAGS, the outer automorphism group is residually finite and, for a large class of RAAGs, it satisfies the Tits alternative. We also ... More

Massive particles and unitarity cutsFeb 11 2012We present an extension of the spinor integration formalism of one loop amplitudes from the double-cut to the single-cut case. This technique can be applied for the computation of the tadpole coefficients. Moreover we describe an off-shell continuation ... More

Estimates of Black Hole Spin Properties of 55 SourcesMar 04 2011Studies of black hole spin and other parameters as a function of redshift provide information about the physical state and merger and accretion histories of the systems. One way that black hole spin may be estimated is through observations of extended ... More

A Decade of Dark Energy: 1998 - 2008Jan 18 2009The years 1998 to 2008 were very exciting years for cosmology. It was a pleasure to accept this invitation to describe my contributions to the development of our knowledge and understanding of the universe over the course of the past decade. Here, I begin ... More

Information Rates of Minimal Non-Matroid-Related Access StructuresJan 23 2008In a secret sharing scheme, shares of a secret are distributed to participants in such a way that only certain predetermined sets of participants are qualified to reconstruct the secret. An access structure on a set of participants specifies which sets ... More

Primordial Magnetic Fields and CausalityMay 05 2003Oct 29 2003We discuss the implications of causality on a primordial magnetic field. We show that the residual field on large scales is much more suppressed than usually assumed and that a helical component is even more reduced. Due to this strong suppression, even ... More

Dark Energy and Dark GravityNov 01 2007Dec 05 2007Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations ... More

Dynamical Casimir effect for gravitons in bouncing braneworldsApr 05 2007Nov 15 2007We consider a two-brane system in a five-dimensional anti-de Sitter spacetime. We study particle creation due to the motion of the physical brane which first approaches the second static brane (contraction) and then recedes from it(expansion). The spectrum ... More

CMB anisotropies caused by gravitational waves: A parameter studyFeb 26 1997May 26 1998Anisotropies in the cosmic microwave background radiation due to gravity waves are investigated. An initial spectrum of gravity waves may have been induced during an epoch of inflation. We study the propagation of such a spectrum in a Friedmann background ... More

Braneworld InstantonsJul 12 2001Nov 23 2001We derive the general equations of motion for a braneworld containing a (localized) domain wall. Euclideanization gives the geometry for braneworld false vacuum decay. We explicitly derive these instantons and compute the amplitude for false vacuum decay. ... More

Convexity of parabolic subgroups in Artin groupsJan 29 2014We prove that any standard parabolic subgroup of any Artin group is convex with respect to the standard generating set.