Results for "J. H. Roberts"

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Long-range interactions and the sign of natural amplitudes in two-electron systemsJun 19 2013Sep 12 2013In singlet two-electron systems the natural occupation numbers of the one-particle reduced density matrix are given as squares of the natural amplitudes which are defined as the expansion coefficients of the two-electron wave function in a natural orbital ... More
Searching for Signatures of Cosmic Superstrings in the CMBOct 30 2009Because cosmic superstrings generically form junctions and gauge theoretic strings typically do not, junctions may provide a signature to distinguish between cosmic superstrings and gauge theoretic cosmic strings. In cosmic microwave background anisotropy ... More
Recent developments for the optical model of nucleiNov 07 2018A brief overview of various approaches to the optical-model description of nuclei is presented. A survey of some of the formal aspects is given which links the Feshbach formulation for either the hole or particle Green's function to the time-ordered quantity ... More
Constraints on the Effects of Locally-biased Galaxy FormationDec 15 1997While it is well-known that "biased galaxy formation" can increase the strength of galaxy clustering, it is less clear whether straightforward biasing schemes can change the shape of the galaxy correlation function on large scales. Here we consider "local" ... More
Glueballs in a Hamiltonian Light-Front Approach to Pure-Glue QCDAug 19 1999We calculate a renormalized Hamiltonian for pure-glue QCD and diagonalize it. The renormalization procedure is designed to produce a Hamiltonian that will yield physical states that rapidly converge in an expansion in free-particle Fock-space sectors. ... More
From the Kähler-Ricci flow to moving free boundaries and shocksApr 12 2016We show that the twisted K\"ahler-Ricci flow on a complex manifold X converges to a flow of moving free boundaries, in a certain scaling limit. This leads to a new phenomenon of singularity formation and topology change which can be seen as a complex ... More
Revealing short-range non-Newtonian gravity through Casimir-Polder shieldingNov 13 2018We carry out a realistic, yet simple, calculation of the Casimir-Polder interaction in the presence of a metallic shield in order to aid the design of experiments to test non-Newtonian gravity. In particular, we consider a rubidium atom near a movable ... More
Canny Algorithm, Cosmic Strings and the Cosmic Microwave BackgroundNov 12 2008Apr 04 2010We describe a new code to search for signatures of cosmic strings in cosmic microwave anisotropy maps. The code implements the Canny Algorithm, an edge detection algorithm designed to search for the lines of large gradients in maps. Such a gradient signature ... More
Systematic Renormalization in Hamiltonian Light-Front Field TheoryApr 21 1998Nov 18 1998We develop a systematic method for computing a renormalized light-front field theory Hamiltonian that can lead to bound states that rapidly converge in an expansion in free-particle Fock-space sectors. To accomplish this without dropping any Fock sectors ... More
Pseudo-rip: Cosmological models intermediate between the cosmological constant and the little ripDec 13 2011Apr 05 2012If we assume that the cosmic energy density will remain constant or strictly increase in the future, then the possible fates for the universe can be divided into four categories based on the time asymptotics of the Hubble parameter H(t): the cosmological ... More
The Little RipJun 24 2011We examine models in which the dark energy density increases with time (so that the equation-of-state parameter w satisfies w < -1), but w approaches -1 asymptotically, such that there is no future singularity. We refine previous calculations to determine ... More
Phase diagram of a Heisenberg spin-Peierls model with quantum phononsDec 29 1998May 31 1999Using a new version of the density-matrix renormalization group we determine the phase diagram of a model of an antiferromagnetic Heisenberg spin chain where the spins interact with quantum phonons. A quantum phase transition from a gapless spin-fluid ... More
Factorization of percolation density correlation functions for clusters touching the sides of a rectangleNov 19 2008We consider the density at a point z = x + i y of critical percolation clusters that touch the left [P_L(z)], right [P_R(z)], or both [P_{LR}(z)] sides of a rectangular system, with open boundary conditions on the top and bottom. The ratio C(z) = P_{LR}(z) ... More
Determining the Lensing Fraction of SDSS Quasars: Methods and Results from the EDRJan 23 2003We present an algorithm for selecting gravitational lens candidates from amongst Sloan Digital Sky Survey (SDSS) quasars. In median Early Data Release (EDR) conditions, the algorithm allows for the recovery of pairs of equal flux point sources down to ... More
Stabilizing moduli with thermal matter and nonperturbative effectsFeb 12 2008Apr 16 2008Even with recent progress, it is still very much an open question to understand how all compactification moduli are stabilized, since there are several mechanisms. For example, it is possible to generate a scalar potential either classically or through ... More
Design of three dimensional isotropic microstructures for maximized stiffness and conductivityDec 19 2007The level-set method of topology optimization is used to design isotropic two-phase periodic multifunctional composites in three dimensions. One phase is stiff and insulating whereas the other is conductive and mechanically compliant. The optimization ... More
Quantum gravitational decoherence of matter wavesMar 29 2006Jul 26 2006One of the biggest unsolved problems in physics is the unification of quantum mechanics and general relativity. The lack of experimental guidance has made the issue extremely evasive, though various attempts have been made to relate the loss of matter ... More
Systematic Renormalization in Hamiltonian Light-Front Field Theory: The Massive GeneralizationDec 09 1998Jul 06 1999Hamiltonian light-front field theory can be used to solve for hadron states in QCD. To this end, a method has been developed for systematic renormalization of Hamiltonian light-front field theories, with the hope of applying the method to QCD. It assumed ... More
New Catalogs of Compact Radio Sources in the Galactic PlaneJan 27 2005May 19 2005Archival data have been combined with recent observations of the Galactic plane using the Very Large Array to create new catalogs of compact centimetric radio sources. The 20 cm source catalog covers a longitude range of -20 deg < l < 120 deg; the latitude ... More
A new approach to the Sachs-Wolfe effectMar 14 2005Jan 08 2013We present a new approach to the Sachs-Wolfe effect, which is based on the dynamics of photons in a space and time varying gravitational field. We consider the influence of plasma dispersion effects on photon propagation, and establish the limits of validity ... More
Factorization of correlations in two-dimensional percolation on the plane and torusNov 04 2010Jan 07 2011Recently, Delfino and Viti have examined the factorization of the three-point density correlation function P_3 at the percolation point in terms of the two-point density correlation functions P_2. According to conformal invariance, this factorization ... More
Anisotropic magnetoresistance contribution to measured domain wall resistances of in-plane magnetised (Ga,Mn)AsNov 30 2006We demonstrate the presence of an important anisotropic magnetoresistance contribution to the domain wall resistance recently measured in thin-film (Ga,Mn)As with in-plane magnetic anisotropy. Analytic results for simple domain wall orientations supplemented ... More
Regularity of weak minimizers of the K-energy and applications to properness and K-stabilityFeb 09 2016Mar 07 2016Let $(X,\omega)$ be a compact K\"ahler manifold and $\mathcal H$ the space of K\"ahler metrics cohomologous to $\omega$. If a cscK metric exists in $\mathcal H$, we show that all finite energy minimizers of the extended K-energy are smooth cscK metrics, ... More
Compressibility of random walker trajectories on growing networksNov 22 2018Jan 28 2019We find that the simple coupling of network growth to the position of a random walker on the network generates a traveling wave in the probability distribution of nodes visited by the walker. We argue that the entropy of this probability distribution ... More
Gravitational-wave sensitivity curvesAug 04 2014Dec 11 2014There are several common conventions in use by the gravitational-wave community to describe the amplitude of sources and the sensitivity of detectors. These are frequently confused. We outline the merits of and differences between the various quantities ... More
The Last of FIRST: The Final Catalog and Source IdentificationsJan 06 2015Jan 09 2015The FIRST survey, begun over twenty years ago, provides the definitive high-resolution map of the radio sky. This VLA survey reaches a 20cm detection sensitivity of 1 mJy over 10,575 deg**2 largely coincident with the SDSS area. Images and a catalog containing ... More
Regularity of weak minimizers of the K-energy and applications to properness and K-stabilityFeb 09 2016Jul 16 2018Let $(X,\omega)$ be a compact K\"ahler manifold and $\mathcal H$ the space of K\"ahler metrics cohomologous to $\omega$. If a cscK metric exists in $\mathcal H$, we show that all finite energy minimizers of the extended K-energy are smooth cscK metrics, ... More
A Signature of Cosmic Strings Wakes in the CMB PolarizationMar 03 2010We calculate a signature of cosmic strings in the polarization of the cosmic microwave background (CMB). We find that ionization in the wakes behind moving strings gives rise to extra polarization in a set of rectangular patches in the sky whose length ... More
Exact factorization of correlation functions in 2-D critical percolationJun 27 2007Sep 24 2007By use of conformal field theory, we discover several exact factorizations of higher-order density correlation functions in critical two-dimensional percolation. Our formulas are valid in the upper half-plane, or any conformally equivalent region. We ... More
Quasars: What turns them off?Jun 08 2006(Abridged) We explore the idea that the anti-hierarchical turn-off observed in the quasar population arises from self-regulating feedback, via an outflow mechanism. Using a detailed hydrodynamic simulation we calculate the luminosity function of quasars ... More
Compressibility of random walker trajectories on growing networksNov 22 2018Apr 04 2019We find that the simple coupling of network growth to the position of a random walker on the network generates a traveling wave in the probability distribution of nodes visited by the walker. We argue that the entropy of this probability distribution ... More
A Radio Counterpart for the Unidentified TeV Source HESS J1813-178: The Radio-Gamma-Ray ConnectionMay 18 2005We discovered independently the shell-type supernova remnant G12.82-0.02, recently reported by Brogan et al. (2005), which is coincident with the unidentified TeV gamma-ray source revealed in the HESS survey of the Galactic plane. Estimating the ambient ... More
Efficient Monte Carlo for high excursions of Gaussian random fieldsMay 05 2010Oct 08 2012Our focus is on the design and analysis of efficient Monte Carlo methods for computing tail probabilities for the suprema of Gaussian random fields, along with conditional expectations of functionals of the fields given the existence of excursions above ... More
Percolation crossing probabilities in hexagons: a numerical studyJul 30 2014Dec 12 2014In a recent article, one of the authors used $c=0$ logarithmic conformal field theory to predict crossing-probability formulas for percolation clusters inside a hexagon with free boundary conditions. In this article, we verify these predictions with high-precision ... More
Percolation Crossing Formulas and Conformal Field TheoryMay 14 2007Jun 03 2007Using conformal field theory, we derive several new crossing formulas at the two-dimensional percolation point. High-precision simulation confirms these results. Integrating them gives a unified derivation of Cardy's formula for the horizontal crossing ... More
Probing spacetime fluctuations using cold atom trapsFeb 16 2010Jan 03 2013We investigate the position oscillation of a particle that models the center of mass quantum state of a trapped Bose-Einstein condensate coupled to the zero-point fluctuations of the gravitational field. A semiclassical analysis is performed that allows ... More
Towards non-local density functionals by explicit modeling of the exchange-correlation hole in inhomogeneous systemsOct 05 2012Feb 20 2013We put forward new approach for the development of a non-local density functional by a direct modeling of the shape of exchange-correlation (xc) hole in inhomogeneous systems. The functional is aimed at giving an accurate xc-energy and an accurate corresponding ... More
Measuring AGN Feedback with the Sunyaev-Zel'dovich EffectSep 06 2007One of the most important and poorly-understood issues in structure formation is the role of outflows driven by active galactic nuclei (AGN). Using large-scale cosmological simulations, we compute the impact of such outflows on the small-scale distribution ... More
Collisional Model for Granular Impact DynamicsOct 11 2013Dec 16 2013When an intruder strikes a granular material from above, the grains exert a stopping force which decelerates and stops the intruder. Many previous studies have used a macroscopic force law, including a drag force which is quadratic in velocity, to characterize ... More
Fracture resistance via topology optimisationAug 11 2006The fracture resistance of structures is optimised using the level-set method. Fracture resistance is assumed to be related to the elastic energy released by a crack propagating in a normal direction from parts of the boundary which are in tension, and ... More
A random walker's view of networks whose growth it shapesNov 22 2018Jan 28 2019We study a simple model in which the growth of a network is determined by the location of one or more random walkers. Depending on walker speed, the model generates a spectrum of structures situated between well-known limiting cases. We demonstrate that ... More
Dark Energy, with SignaturesMay 17 2010We propose a class of simple dark energy models which predict a late-time dark radiation component and a distinctive time-dependent equation of state $w(z)$ for redshift $z < 3$. The dark energy field can be coupled strongly enough to Standard Model particles ... More
Convexity of the extended K-energy and the large time behaviour of the weak Calabi flowOct 05 2015Feb 07 2016Let $(X,\omega)$ be a compact connected K\"ahler manifold and denote by $(\mathcal E^p,d_p)$ the metric completion of the space of K\"ahler potentials $\mathcal H_\omega$ with respect to the $L^p$-type path length metric $d_p$. First, we show that the ... More
pi- and rho-mesons, and their diquark partners, from a contact interactionFeb 22 2011We present a unified Dyson-Schwinger equation treatment of static and electromagnetic properties of pseudoscalar and vector mesons, and scalar and axial-vector diquark correlations, based upon a vector-vector contact-interaction. A basic motivation for ... More
Angular dependence of domain wall resistivity in artificial magnetic domain structuresJun 23 2006We exploit the ability to precisely control the magnetic domain structure of perpendicularly magnetized Pt/Co/Pt trilayers to fabricate artificial domain wall arrays and study their transport properties. The scaling behaviour of this model system confirms ... More
Investigation of artificial domains realised by local gallium focused ion beam (FIB) modification of Pt/Co/Pt trilayer structuresFeb 05 2008We present the results of experimental investigations of magnetic switching and magnetotransport in a new generation of magnetic devices containing artificially patterned domains. Our devices are realised by locally reducing the coercive field of a perpendicularly ... More
Comparing deep neural networks against humans: object recognition when the signal gets weakerJun 21 2017Dec 11 2018Human visual object recognition is typically rapid and seemingly effortless, as well as largely independent of viewpoint and object orientation. Until very recently, animate visual systems were the only ones capable of this remarkable computational feat. ... More
Flux Enhancement of Slow-moving Particles by Sun or Jupiter: Can they be Detected on Earth?May 10 2013Slow-moving particles capable of interacting solely with gravity might be detected on Earth as a result of the gravitational lensing induced focusing action of the Sun. The deflection experienced by these particles are inversely proportional to the square ... More
Beat Cepheids as Probes of Stellar and Galactic MetallicityNov 27 2006Jan 12 2007The mere location of a Beat Cepheid model in a Period Ratio vs. Period diagram (Petersen diagram) puts very tight constraints on its metallicity Z. The Beat Cepheid Peterson diagrams are revisited with linear nonadiabatic turbulent convective models, ... More
Note on Forced Burgers TurbulenceJan 21 1999Aug 12 1999A putative powerlaw range of the probability density of velocity gradient in high-Reynolds-number forced Burgers turbulence is studied. In the absence of information about shock locations, elementary conservation and stationarity relations imply that ... More
The Geometry of Cycles in the Cayley Diagram of a GroupNov 02 1993A study of triangulations of cycles in the Cayley diagrams of finitely generated groups leads to a new geometric characterization of hyperbolic groups.
Inflation, Topological Defects and Baryogenesis: Selected Topics at the Interface between Particles & Fields and CosmologyFeb 02 1997Modern cosmology provides a connection between the physics of particles and fields and observational cosmology. Making use of this link, a wealth of new observational data can be utilized to explore and constrain theories of fundamental physics. Inflationary ... More
Implementing Markov's Limiting Curvature HypothesisOct 01 1995The effective action for gravity at high curvatures is likely to contain higher derivative terms. These corrections may have profound consequences for the singularity structure of space-time and for early Universe cosmology. In this contribution, recent ... More
Moduli Stabilization in String Gas CosmologySep 20 2005Sep 25 2005String gas cosmology is an approach towards studying the effects of superstring theory on early universe cosmology which is based on new symmetries and new degrees of freedom of string theory. Within this context, it appears possible to stabilize the ... More
Looking Beyond Inflationary CosmologySep 11 2005Sep 25 2005In spite of the phenomenological successes of the inflationary universe scenario, the current realizations of inflation making use of scalar fields lead to serious conceptual problems which are reviewed in this lecture. String theory may provide an avenue ... More
NONSINGULAR COSMOLOGY AND PLANCK SCALE PHYSICSMar 01 1995New Planck scale physics may solve the singularity problems of classical general relativity and may lead to interesting consequences for very early Universe cosmology. Two approaches to these questions are reviewed in this article. The first is an effective ... More
String Gas Cosmology after PlanckMay 10 2015We review the status of String Gas Cosmology after the 2015 Planck data release. String gas cosmology predicts an almost scale-invariant spectrum of cosmological perturbations with a slight red tilt, like the simplest inflationary models. It also predicts ... More
Principles, Progress and Problems in Inflationary CosmologyAug 05 2002Inflationary cosmology has become one of the cornerstones of modern cosmology. Inflation was the first theory within which it was possible to make predictions about the structure of the Universe on large scales, based on causal physics. The development ... More
CCpi0 Event Reconstruction at MiniBooNESep 07 2009We describe the development of a fitter to reconstruct $\nu_\mu$ induced Charged-Current single $\pi^0$ (CC$\pi^0$) events in an oil Cherenkov detector (CH$_2$). These events are fit using a generic muon and two photon extended track hypothesis from a ... More
Information travels in massless fields in 1+1 dimensions where energy cannotDec 16 2015Apr 06 2016It has been demonstrated that, in (1+1) spacetime dimensions, massless fields can be used for information transmission even between parties that are time-like rather than light-like separated - and even without the receiver obtaining any energy from the ... More
Lectures on the Theory of Cosmological PerturbationsJun 09 2003The theory of cosmological perturbations has become a cornerstone of modern quantitative cosmology since it is the framework which provides the link between the models of the very early Universe such as the inflationary Universe scenario (which yield ... More
Back Reaction of Cosmological PerturbationsApr 04 2000The presence of cosmological perturbations affects the background metric and matter configuration in which the perturbations propagate. This effect, studied a long time ago for gravitational waves, also is operational for scalar gravitational fluctuations, ... More
Processing of Cosmological Perturbations in a Cyclic CosmologyMay 10 2009The evolution of the spectrum of cosmological fluctuations from one cycle to the next is studied. It is pointed out that each cycle leads to a reddening of the spectrum. This opens up new ways to generate a scale-invariant spectrum of curvature perturbations. ... More
Alternatives to Cosmological InflationFeb 27 2009The inflationary paradigm, although very successful phenomenologically, suffers from several conceptual problems which motivate the search for alternative scenarios of early universe cosmology. Here, two possible alternatives will be reviewed. - "string ... More
Inflationary Cosmology: Progress and ProblemsOct 20 1999These lecture notes intend to form a short pedagogical introduction to inflationary cosmology, highlighting selected areas of recent progress such as reheating and the theory of cosmological perturbations. Problems of principle for inflationary cosmology ... More
Topological Defects and the Formation of Structure in the UniverseApr 06 1996Topological defects, in particular cosmic strings, give rise to an interesting mechanism for generating the primordial perturbations in the early Universe which are required to explain the present structure. An overview of the cosmic string model will ... More
Information travels in massless fields in 1+1 dimensions where energy cannotDec 16 2015Nov 29 2016It has been demonstrated that, in (1+1) spacetime dimensions, massless fields can be used for information transmission even between parties that are time-like rather than light-like separated - and even without the receiver obtaining any energy from the ... More
Automatic Quotients of Free GroupsMay 04 2003Mar 20 2005Automatic groups admitting prefix closed automatic structures with uniqueness are characterized as the quotients of free groups by normal subgroups possessing sets of free generators satisfying certain language-theoretic conditions.
A Finitely presented group whose word problem has sampleable hard instancesFeb 07 2016Hard instances of natural computational problems are often elusive. In this note we present an example of a natural decision problem, the word problem for a certain finitely presented group, whose hard instances are easy to find. More precisely the problem ... More
Challenges for String Gas CosmologySep 14 2005Sep 25 2005In spite of the phenomenological successes of the inflationary universe scenario, the current realizations of inflation making use of scalar fields lead to serious conceptual problems which are reviewed in this lecture. String theory may provide an avenue ... More
Trans-Planckian Physics and Inflationary CosmologyOct 20 2002Nov 01 2002Due to the quasi-exponential red-shifting which occurs during an inflationary period in the very early Universe, wavelengths which at the present time correspond to cosmological lengths are in general sub-Planckian during the early stages of inflation. ... More
Inflation and the Theory of Cosmological PerturbationsNov 11 1997The hypothesis that the Universe underwent a period of exponential expansion at very early times has become the most popular theory of the early Universe. Not only does it solve some of the problems of standard big bang cosmology, but it also provides ... More
Is the Spectrum of Gravitational Waves the "Holy Grail" of Inflation?Apr 18 2011It is often said that detecting a spectrum of primordial gravitational waves via observing B-mode polarization of the Cosmic Microwave Background is the "Holy Grail" of inflation. The purpose of this short note is to point out that it is indeed of immense ... More
Modern Cosmology and Structure FormationNov 11 1994An introduction to modern cosmology is given, focusing on theories for the origin of structure in the Universe. After a brief review of the theory of growth of cosmological perturbations and a summary of some important observational results, most of the ... More
Topological Defects and Structure FormationOct 24 1993Topological defects are produced during phase transitions in the very early Universe. They arise in most unified theories of strong, weak and electromagnetic interactions. These lectures focus on the role of topological defects in cosmology, with particular ... More
Probing Particle Physics from Top Down with Cosmic StringsJan 18 2014Making use of the wealth of new observational data coming from the sky it is possible to constrain particle physics theories beyond the Standard Model. One way to do this is illustrated in this article: a subset of theories admits cosmic string solutions, ... More
String Gas Cosmology: Progress and ProblemsMay 17 2011May 21 2011String Gas Cosmology is a model of the evolution of the very early universe based on fundamental principles and key new degrees of freedom of string theory which are different from those of point particle field theories. In String Gas Cosmology the universe ... More
Cosmology of the Very Early UniverseMar 08 2010In these lectures I focus on early universe models which can explain the currently observed structure on large scales. I begin with a survey of inflationary cosmology, the current paradigm for understanding the origin of the universe as we observe it ... More
Quantum Signaling in Relativistic Motion and Across Acceleration HorizonsFeb 22 2017Aug 07 2017The quantum channel between two particle detectors provides a prototype framework for the study of wireless quantum communication via relativistic quantum fields. In this article we calculate the classical channel capacity between two Unruh-DeWitt detectors ... More
Algorithmic Search in Group TheoryDec 19 2018A method of random search based on Kolmogorov complexity is proposed and applied to two search problems in group theory. The method is provably effective but not practical, so the applications involve heuristic approximations. Perhaps surprisingly, these ... More
Notes on Lattice-Reduction-Aided MMSE EqualizationAug 12 2010Over the last years, novel low-complexity approaches to the equalization of MIMO channels have gained much attention. Thereby, methods based on lattice basis reduction are of special interest, as they achieve the optimum diversity order. In this paper, ... More
Back Reaction of Cosmological Perturbations and the Cosmological Constant ProblemOct 17 2002The presence of cosmological fluctuations influences the background cosmology in which the perturbations evolve. This back-reaction arises as a second order effect in the cosmological perturbation expansion. The effect is cumulative in the sense that ... More
Dynamical Breaking of CPT and BaryogenesisJan 14 1999The asymmetry between matter and antimatter in the Universe indicates that there was a period in the very early Universe when the CPT symmetry was broken. The conservative interpretation is that the breaking was dynamical, induced by the distinguished ... More
Topological Defects and CosmologyJun 24 1998Many particle physics models of matter admit solutions corresponding to stable or long-lived topological defects. In the context of standard cosmology it is then unavoidable that such defects will form during phase transitions in the very early Universe. ... More
Topological Defect Models of Structure Formation After the COBE Discovery of CMB AnisotropiesOct 24 1992The COBE satellite has discovered anisotropies in the cosmic microwave background (CMB) consistent with a scale invariant spectrum of density perturbations. As reviewed in this lecture, topological defect models of structure formation generically produce ... More
Theory of Cosmological Perturbations and Applications to Superstring CosmologyJan 06 2005The theory of cosmological perturbations is the main tool which connects theories of the early universe (based on new fundamental physics such as string theory) with cosmological observations. In these lectures, I will provide an introduction to this ... More
A Status Review of Inflationary CosmologyJan 11 2001The first aim of this lecture is to highlight two areas of recent progress in inflationary cosmology, namely reheating and the quantum theory of cosmological perturbations. The second aim is to discuss important conceptual problems for the current realizations ... More
Particle Physics Aspects of Modern CosmologyJan 13 1997Modern cosmology has created a tight link between particle physics / field theory and a wealth of new observational data on the structure of the Universe. These lecture notes focus on some of the most important aspects concerning the connection between ... More
Some Key Issues Confronting Inflationary CosmologySep 06 1996Does inflation provide a compelling explanation for why the universe is so large, so flat, and so old, and a predictive theory of density perturbations? In this brief contribution (based on the role of the author as moderator of the discussion session ... More
Physics of the Very Early UniverseOct 01 1995In this report of the workshop on `Physics of the Very Early Universe' held at GR14, the recent status and open problems in a selected number of major areas of interest are reviewed, focusing on attempts to develop a superstring cosmology and on progress ... More
Formation of Structure in the UniverseSep 01 1995Sep 01 1995An introduction to modern theories for the origin of structure in the Universe is given. After a brief review of the growth of cosmological perturbations in an expanding Universe and a summary of some important observational results, the lectures focus ... More
A Nonsingular UniverseOct 24 1992We construct an effective action for gravity in which all homogeneous solutions are nonsingular. In particular, there is neither a big bang nor a big crunch. The action is a higher derivative modification of Einstein's theory constructed in analogy to ... More
Introduction to Early Universe CosmologyMar 11 2011Observational cosmology is in its "golden age" with a vast amount of recent data on the distribution of matter and light in the universe. This data can be used to probe theories of the very early universe. It is small amplitude cosmological fluctuations ... More
Beyond Standard Inflationary CosmologySep 13 2018The inflationary scenario is not the only paradigm of early universe cosmology which is consistent with current observations. General criteria will be presented which any successful early universe model must satisfy. Various ways, including inflation, ... More
The Fermi Paradox is Neither Fermis Nor a ParadoxApr 02 2016The so-called Fermi paradox claims that if technological life existed anywhere else, we would see evidence of its visits to Earth-and since we do not, such life does not exist, or some special explanation is needed. Enrico Fermi, however, never published ... More
Granular circulation in a cylindrical pan: simulations of reversing radial and tangential flowsAug 03 2006Granular flows due to simultaneous vertical and horizontal excitations of a flat-bottomed cylindrical pan are investigated using event-driven molecular dynamics simulations. In agreement with recent experimental results, we observe a transition from a ... More
Observational evidence for temporary planetary wave forcing of the MLT during fall equinoxJul 04 2016We present direct observations of zonal wave numbers 1 and 2 planetary wave activity in the mesopause region derived from a longitudinal chain of high-latitude Northern Hemisphere (51-66$^{\circ}$N) Super Dual Auroral Radar Network radars. Over a 9 year ... More
Models for Little Rip Dark EnergyJul 30 2011Jan 16 2012We examine in more detail specific models which yield a little rip cosmology, i.e., a universe in which the dark energy density increases without bound but the universe never reaches a finite-time singularity. We derive the conditions for the little rip ... More
Automating Ellerman bomb detection in ultraviolet continuaJan 23 2019Ellerman bombs are transient brightenings in the wings of H-alpha 6563 {\AA} that pinpoint photospheric sites of magnetic reconnection in solar active regions. Their partial visibility in the 1600 {\AA} and 1700 {\AA} continua registered routinely by ... More
Preparation contextuality powers parity-oblivious multiplexingMay 10 2008Jan 08 2009In a noncontextual hidden variable model of quantum theory, hidden variables determine the outcomes of every measurement in a manner that is independent of how the measurement is implemented. Using a generalization of this notion to arbitrary operational ... More